PBT Publications

UCL HEP PBT Detector Development

Bateman, Joseph J. et al., “A novel approach for proton therapy pencil beam scanning patient specific quality assurance using an integrated detector system and 3D dose reconstruction”, Frontiers in Oncology 15 (2025)
DOI: 10.3389/fonc.2025.1677439
Keywords & Abstract

Abstract:
Purpose: Current proton beam therapy patient-specific quality assurance (PSQA) methods rely on time-intensive phantom measurements or machine-reported parameters without independent verification. This work presents an integrated detector system for phantom-less Pencil Beam Scanning (PBS) PSQA, providing independent spot-by-spot measurements of all critical beam parameters and 3D dose reconstruction. Methods: The integrated detector combines three separate systems: a scintillator range telescope for range and energy measurement; a CMOS pixel sensor for spot position and size verification; and a Transmission Calorimeter (TC) for beam intensity measurements. Measured parameters feed Monte Carlo simulations to reconstruct 3D dose distributions for comparison with treatment planning predictions. Validation was performed at UCLH using single spot position spread out Bragg Peak (SOBP) and 5×5×10 spot box field configurations. Results: Energy values obtained from range measurements showed strong correlation with DICOM values (R² > 0.998) with an accuracy of between 2.17 mm and 1.23 mm for different beam deliveries. CMOS pixel sensor measurements succeeded for single spot fields but experienced saturation at higher intensities and incomplete coverage for the larger box field. The TC demonstrated excellent dose linearity (R² = 1.000). Monte Carlo reconstructions agreed well with reference simulations for longitudinal profiles, though lateral reconstructions proved challenging with 77% gamma pass rates (2%/2 mm) for the box field. Discussion: This proof-of-concept demonstrates feasibility of independent beam parameter verification for PBS PSQA while maintaining patient geometry. The approach offers advantages over current methods but requires resolution of energy calibration offsets and detector limitations before clinical implementation. Future work will address these challenges and expand validation to clinical treatment plans.
Shaikh, Saad et al., “Range quality assurance measurements for clinical and FLASH proton beam therapy using the quality assurance range calorimeter”, Frontiers in Oncology 15 (2025)
DOI: 10.3389/fonc.2025.1622231
Keywords & Abstract

Abstract:
Objective: This work demonstrated the design and performance of a full-sized clinical prototype of the Quality Assurance Range Calorimeter (QuARC): a segmented large-volume scintillator-based detector for fast, accurate proton range quality assurance (QA) measurements. Materials & methods: The detector used 128 scintillator sheets of size 105×105×3 mm arranged into 4 modules of 32 sheets, where each sheet was directly coupled to a photodiode. Fast analogue-to-digital conversion facilitated measurement of scintillator sheet light output to 20-bit precision at 6 kHz, with a dynamic range of up to 350 pC. Results: Proton range measurements with the full-size detector were performed at the proton therapy research facility based at The Christie at clinical dose rates, corresponding to \sim1 nA nozzle current, where the range accuracy of the QuARC was found to be within 0.4 mm of facility reference across the full clinical energy range. The QuARC successfully performed range measurements of the 245 MeV beam at FLASH dose rate (\sim50 nA nozzle current), where the fitted range agreed with the clinical current measurement to 0.3 mm. Preliminary results show charge linearity of the detector to be within 3%. Conclusions: The QuARC has been shown to be a promising candidate for fast, accurate range QA at conventional clinical dose rates and thanks to its high precision and dynamic range, has been shown to also be viable at FLASH dose rates. Future work will investigate improving the accuracy and stability of the calibration process by optimising the scintillator sheet light output and mechanical setup.
Saad Shaikh et al., “Spread-out Bragg peak measurements using a compact quality assurance range calorimeter at the Clatterbridge cancer centre”, Physics in Medicine & Biology 69 (11) p.115015 (2024)
DOI: 10.1088/1361-6560/ad42fd
Keywords & Abstract

Keywords: Proton therapy, plastic scintillator, spread out Bragg peak, quality assurance

Abstract:
Objective. The superior dose conformity provided by proton therapy relative to conventional x-ray radiotherapy necessitates more rigorous quality assurance (QA) procedures to ensure optimal patient safety. Practically however, time-constraints prevent comprehensive measurements to be made of the proton range in water: a key parameter in ensuring accurate treatment delivery. Approach. A novel scintillator-based device for fast, accurate water-equivalent proton range QA measurements for ocular proton therapy is presented. Experiments were conducted using a compact detector prototype, the quality assurance range calorimeter (QuARC), at the Clatterbridge cancer centre (CCC) in Wirral, UK for the measurement of pristine and spread-out Bragg peaks (SOBPs). The QuARC uses a series of 14 optically-isolated 100×100×2.85 mm polystyrene scintillator sheets, read out by a series of photodiodes. The detector system is housed in a custom 3D-printed enclosure mounted directly to the nozzle and a numerical model was used to fit measured depth-light curves and correct for scintillator light quenching. Main results. Measurements of the pristine 60 MeV proton Bragg curve found the QuARC able to measure proton ranges accurate to 0.2 mm and reduced QA measurement times from several minutes down to a few seconds. A new framework of the quenching model was deployed to successfully fit depth-light curves of SOBPs with similar range accuracy. Significance. The speed, range accuracy and simplicity of the QuARC make the device a promising candidate for ocular proton range QA. Further work to investigate the performance of SOBP fitting at higher energies/greater depths is warranted.
Laurent Kelleter et al., “A scintillator-based range telescope for particle therapy”, Physics in Medicine and Biology 65 (16) p.165001 (2020)
DOI: 10.1088/1361-6560/ab9415
Inspire entry: Kelleter:2020qen
Keywords & Abstract

Keywords: Bragg curve, CMOS sensor, light quenching, particle therapy, pencil beam range, plastic scintillator, quality assurance

Abstract:
The commissioning and operation of a particle therapy centre requires an extensive set of detectors for measuring various parameters of the treatment beam. Among the key devices are detectors for beam range quality assurance. In this work, a novel range telescope based on a plastic scintillator and read out by a large-scale CMOS sensor is presented. The detector is made of a stack of 49 plastic scintillator sheets with a thickness of 2-3 mm and an active area of 100×100 mm^2, resulting in a total physical stack thickness of 124.2 mm. This compact design avoids optical artefacts that are common in other scintillation detectors. The range of a proton beam is reconstructed using a novel Bragg curve model that incorporates scintillator quenching effects. Measurements to characterise the performance of the detector were carried out at the Heidelberger Ionenstrahl-Therapiezentrum (HIT, Heidelberg, GER) and the Clatterbridge Cancer Centre (CCC, Bebington, UK). The maximum difference between the measured range and the reference range was found to be 0.41 mm at a proton beam range of 310 mm and was dominated by detector alignment uncertainties. With the new detector prototype, the water-equivalent thickness of PMMA degrader blocks has been reconstructed within 0.1 mm. An evaluation of the radiation hardness proves that the range reconstruction algorithm is robust following the deposition of 6,300 Gy peak dose into the detector. Furthermore, small variations in the beam spot size and transverse beam position are shown to have a negligible effect on the range reconstruction accuracy. The potential for range measurements of ion beams is also investigated.
Laurent Kelleter and Simon Jolly, “A mathematical expression for depth-light curves of therapeutic proton beams in a quenching scintillator”, Medical Physics 47 (5) p.2300-2308 (2020)
DOI: 10.1002/mp.14099
Keywords & Abstract

Abstract:
Purpose: Recently, there has been increasing interest in the development of scintillator-based detectors for the measurement of depth��dose curves of therapeutic proton beams (Beaulieu and Beddar [2016], Phys Med Biol., 61:R305��R343). These detectors allow the measurement of single beam parameters such as the proton range or the reconstruction of the full three-dimensional dose distribution. Thus, scintillation detectors could play an important role in beam quality assurance, online beam monitoring, and proton imaging. However, the light output of the scintillator as a function of dose deposition is subject to quenching effects due to the high-specific energy loss of incident protons, particularly in the Bragg peak. The aim of this work is to develop a model that describes the percent depth-light curve in a quenching scintillator and allow the extraction of information about the beam range and the strength of the quenching. Methods: A mathematical expression of a depth-light curve, derived from a combination of Birks' law (Birks [1951], Proc Phys Soc A., 64:874) and Bortfeld's Bragg curve (Bortfeld [1997], Med Phys., 24:2024��2033) that is termed a ``quenched Bragg'' curve, is presented. The model is validated against simulation and measurement. Results: A fit of the quenched Bragg model to simulated depth-light curves in a polystyrene-based scintillator shows good agreement between the two, with a maximum deviation of 2.5% at the Bragg peak. The differences are larger behind the Bragg peak and in the dose build-up region. In the same simulation, the difference between the reconstructed range and the reference proton range is found to be always smaller than 0.16 mm. The comparison with measured data shows that the fitted beam range agrees with the reference range within their respective uncertainties. Conclusions: The quenched Bragg model is, therefore, an accurate tool for the range measurement from quenched depth��dose curves. Moreover, it allows the reconstruction of the beam energy spread, the particle fluence, and the magnitude of the quenching effect from a measured depth-light curve.

UCL HEP PBT Publications

PBT - 2025

Bateman, Joseph J. et al., “A novel approach for proton therapy pencil beam scanning patient specific quality assurance using an integrated detector system and 3D dose reconstruction”, Frontiers in Oncology 15 (2025)
DOI: 10.3389/fonc.2025.1677439
Keywords & Abstract

Abstract:
Purpose: Current proton beam therapy patient-specific quality assurance (PSQA) methods rely on time-intensive phantom measurements or machine-reported parameters without independent verification. This work presents an integrated detector system for phantom-less Pencil Beam Scanning (PBS) PSQA, providing independent spot-by-spot measurements of all critical beam parameters and 3D dose reconstruction. Methods: The integrated detector combines three separate systems: a scintillator range telescope for range and energy measurement; a CMOS pixel sensor for spot position and size verification; and a Transmission Calorimeter (TC) for beam intensity measurements. Measured parameters feed Monte Carlo simulations to reconstruct 3D dose distributions for comparison with treatment planning predictions. Validation was performed at UCLH using single spot position spread out Bragg Peak (SOBP) and 5×5×10 spot box field configurations. Results: Energy values obtained from range measurements showed strong correlation with DICOM values (R² > 0.998) with an accuracy of between 2.17 mm and 1.23 mm for different beam deliveries. CMOS pixel sensor measurements succeeded for single spot fields but experienced saturation at higher intensities and incomplete coverage for the larger box field. The TC demonstrated excellent dose linearity (R² = 1.000). Monte Carlo reconstructions agreed well with reference simulations for longitudinal profiles, though lateral reconstructions proved challenging with 77% gamma pass rates (2%/2 mm) for the box field. Discussion: This proof-of-concept demonstrates feasibility of independent beam parameter verification for PBS PSQA while maintaining patient geometry. The approach offers advantages over current methods but requires resolution of energy calibration offsets and detector limitations before clinical implementation. Future work will address these challenges and expand validation to clinical treatment plans.
Shaikh, Saad et al., “Range quality assurance measurements for clinical and FLASH proton beam therapy using the quality assurance range calorimeter”, Frontiers in Oncology 15 (2025)
DOI: 10.3389/fonc.2025.1622231
Keywords & Abstract

Abstract:
Objective: This work demonstrated the design and performance of a full-sized clinical prototype of the Quality Assurance Range Calorimeter (QuARC): a segmented large-volume scintillator-based detector for fast, accurate proton range quality assurance (QA) measurements. Materials & methods: The detector used 128 scintillator sheets of size 105×105×3 mm arranged into 4 modules of 32 sheets, where each sheet was directly coupled to a photodiode. Fast analogue-to-digital conversion facilitated measurement of scintillator sheet light output to 20-bit precision at 6 kHz, with a dynamic range of up to 350 pC. Results: Proton range measurements with the full-size detector were performed at the proton therapy research facility based at The Christie at clinical dose rates, corresponding to \sim1 nA nozzle current, where the range accuracy of the QuARC was found to be within 0.4 mm of facility reference across the full clinical energy range. The QuARC successfully performed range measurements of the 245 MeV beam at FLASH dose rate (\sim50 nA nozzle current), where the fitted range agreed with the clinical current measurement to 0.3 mm. Preliminary results show charge linearity of the detector to be within 3%. Conclusions: The QuARC has been shown to be a promising candidate for fast, accurate range QA at conventional clinical dose rates and thanks to its high precision and dynamic range, has been shown to also be viable at FLASH dose rates. Future work will investigate improving the accuracy and stability of the calibration process by optimising the scintillator sheet light output and mechanical setup.
Yap, Jacinta S. L. et al., “LET measurements and simulation modelling of the charged particle field for the Clatterbridge ocular proton therapy beamline”, Journal of Instrumentation 20 (10) p.P10008 (2025)
DOI: 10.1088/1748-0221/20/10/P10008
ArXiv: 2501.17404
Inspire entry: Yap:2025zkt
Keywords & Abstract

Keywords: Instrumentation for hadron therapy, Hybrid detectors, Particle tracking detectors (Solid-state detectors), Relative Biological Effectiveness, Linear-Energy-Transfer, Monte-Carlo, Timepix, TOPAS
Zhang, Ye et al., “Enabling Gantry-less radiotherapy through upright patient positioning: key insights from the ESTRO Physics Workshop 2024”, In proceedings of “ESTRO Physics Workshop 2024” Radiotherapy and Oncology 206 (Supplemental 1) p.S2305-S2308 (2025)
DOI: 10.1016/S0167-8140(25)01223-X
Keywords & Abstract

Keywords: Science & Technology, Life Sciences & Biomedicine, Oncology, Radiology, Nuclear Medicine & Medical Imaging, image guidance, radiotherapy, upright positioning

Abstract:
Purpose/Objective: Recently, there is growing interest in gantry-less irradiation within the radiotherapy community. Rotating patients upright through a fixed RT beam is appealing, offering reduced treatment room costs, particularly for particle therapy, alongside potential anatomical and comfort benefits. Advances in upright imaging enable treatment planning and daily positioning verification in the same position as during radiation therapy, addressing the key concern of upright treatment in the past. This has led several radiotherapy centers worldwide to develop or adopt upright patient immobilization systems. However, many aspects of upright radiotherapy are yet to be addressed, before streamlined for clinical implementation. Material/Methods: The 2024 ESTRO physics workshop on ``Gantry-less Radiotherapy: Challenges and Opportunities'' brought together 22 participants for in-depth discussions on the open questions and existing solutions. Four invited speakers gave excellent presentations on past and present implementations of gantry-less radiotherapy, such as upright carbon ion RT (QST, Japan); the possibility of MR-guided proton and ion therapy leveraging either upright (Dresden, Germany) or horizontal patient rotation (Heidelberg, Germany); and the challenges of establishing any paradigm shift within modern radiotherapy (Wisconsin, USA). Representatives from industry presented current and future commercial solutions for upright radiotherapy. Results: Recommendations for early adopters of upright patient positioning emphasized necessary workflow adjustments alongside unchanged elements. Ongoing research projects showcase significant progress in addressing current challenges to successfully implement upright treatment. Consensus outlined (1) A wishlist-to-vendor for immediate clinical needs, and (2) long-term innovations enabled by upright positioning and gantry elimination was formulated. Limited upright imaging data (CT/MRI) and challenges in data sharing or generation were noted as major barriers to clinical assessment. Proposed solutions include developing physical/numerical phantoms simulating motion from supine to upright posture, alongside anatomically guided deformable registration algorithms to align supine and upright images, for which AI's potential was deemed critical. Virtual trials were identified as essential for evaluating suitable clinical indications for upright radiotherapy. Conclusion: Gantry-less radiotherapy with upright patient positioning holds significant promise, particularly for enhancing radiotherapy accessibility and enabling the development of innovative RT technologies. The workshop participants reached a consensus on the current challenges and technological opportunities associated with new workflows in upright radiotherapy. However, significant work remains. Realizing the potential of gantry-less treatments in the coming years will require continued collaborative efforts from academia, hospitals, and industry, ideally supported by patient representatives.

PBT - 2024

Saad Shaikh et al., “Spread-out Bragg peak measurements using a compact quality assurance range calorimeter at the Clatterbridge cancer centre”, Physics in Medicine & Biology 69 (11) p.115015 (2024)
DOI: 10.1088/1361-6560/ad42fd
Keywords & Abstract

Keywords: Proton therapy, plastic scintillator, spread out Bragg peak, quality assurance

Abstract:
Objective. The superior dose conformity provided by proton therapy relative to conventional x-ray radiotherapy necessitates more rigorous quality assurance (QA) procedures to ensure optimal patient safety. Practically however, time-constraints prevent comprehensive measurements to be made of the proton range in water: a key parameter in ensuring accurate treatment delivery. Approach. A novel scintillator-based device for fast, accurate water-equivalent proton range QA measurements for ocular proton therapy is presented. Experiments were conducted using a compact detector prototype, the quality assurance range calorimeter (QuARC), at the Clatterbridge cancer centre (CCC) in Wirral, UK for the measurement of pristine and spread-out Bragg peaks (SOBPs). The QuARC uses a series of 14 optically-isolated 100×100×2.85 mm polystyrene scintillator sheets, read out by a series of photodiodes. The detector system is housed in a custom 3D-printed enclosure mounted directly to the nozzle and a numerical model was used to fit measured depth-light curves and correct for scintillator light quenching. Main results. Measurements of the pristine 60 MeV proton Bragg curve found the QuARC able to measure proton ranges accurate to 0.2 mm and reduced QA measurement times from several minutes down to a few seconds. A new framework of the quenching model was deployed to successfully fit depth-light curves of SOBPs with similar range accuracy. Significance. The speed, range accuracy and simplicity of the QuARC make the device a promising candidate for ocular proton range QA. Further work to investigate the performance of SOBP fitting at higher energies/greater depths is warranted.
Fenwick, John D. and Mayhew, Christopher and Jolly, Simon and Amos, Richard A. and Hawkins, Maria A., “Navigating the straits: realizing the potential of proton FLASH through physics advances and further pre-clinical characterization”, Frontiers in Oncology 14 p.1420337 (2024)
DOI: 10.3389/fonc.2024.1420337
Keywords & Abstract

Abstract:
Ultra-high dose-rate 'FLASH' radiotherapy may be a pivotal step forward for cancer treatment, widening the therapeutic window between radiation tumour killing and damage to neighbouring normal tissues. The extent of normal tissue sparing reported in pre-clinical FLASH studies typically corresponds to an increase in isotoxic doselevels of 5–20%, though gains are larger at higher doses. Conditions currently thought necessary for FLASH normal tissue sparing are a dose-rate ≥40 Gy s^-1, dose-per-fraction ≥5–10 Gy and irradiation duration ≤0.2–0.5 s. Cyclotron proton accelerators are the first clinical systems to be adapted to irradiate deep-seated tumours at FLASH dose-rates, but even using these machines it is challenging to meet the FLASH conditions. In this review we describe the challenges for delivering FLASH proton beam therapy, the compromises that ensue if these challenges are not addressed, and resulting dosimetric losses. Some of these losses are on the same scale as the gains from FLASH found pre-clinically. We therefore conclude that for FLASH to succeed clinically the challenges must be systematically overcome rather than accommodated, and we survey physical and pre-clinical routes for achieving this.
Ocampo, Jeremy et al., “Determination of output factor for CyberKnife using scintillation dosimetry and deep learning”, Physics in Medicine & Biology 69 (2) p.025024 (2024)
DOI: 10.1088/1361-6560/ad1b69
Keywords & Abstract

Keywords: cyberknife, deep learning, output factor, photography, small field dosimetry, solid plastic scintillator

Abstract:
Objective. Small-field dosimetry is an ongoing challenge in radiotherapy quality assurance (QA) especially for radiosurgery systems such as CyberKnifeTM. The objective of this work is to demonstrate the use of a plastic scintillator imaged with a commercial camera to measure the output factor of a CyberKnife system. The output factor describes the dose on the central axis as a function of collimator size, and is a fundamental part of CyberKnife QA and integral to the data used in the treatment planning system. Approach. A self-contained device consisting of a solid plastic scintillator and a camera was build in a portable Pelicase. Photographs were analysed using classical methods and with convolutional neural networks (CNN) to predict beam parameters which were then compared to measurements. Main results. Initial results using classical image processing to determine standard QA parameters such as percentage depth dose (PDD) were unsuccessful, with 34% of points failing to meet the Gamma criterion (which measures the distance between corresponding points and the relative difference in dose) of 2 mm/2%. However, when images were processed using a CNN trained on simulated data and a green scintillator sheet, 92% of PDD curves agreed with measurements with a microdiamond detector to within 2 mm/2% and 78% to 1%/1 mm. The mean difference between the output factors measured using this system and a microdiamond detector was 1.1%. Confidence in the results was enhanced by using the algorithm to predict the known collimator sizes from the photographs which it was able to do with an accuracy of less than 1 mm. Significance. With refinement, a full output factor curve could be measured in less than an hour, offering a new approach for rapid, convenient small-field dosimetry.
Heyes, G et al., “Measurement of output factor for Cyberknife using scintillation dosimetry and deep learning”, In proceedings of “Annual Meeting of the European-Society-for-Radiotherapy-and-Oncology (ESTRO)” Radiotherapy And Oncology 194 (Supplemental 1) p.S3181–S3182 (2024)
DOI: 10.1016/S0167-8140(24)00877-6
Keywords & Abstract

Keywords: Science & Technology, Life Sciences & Biomedicine, Oncology, Radiology, Nuclear Medicine & Medical Imaging, scintillation, deep-learning

PBT - 2023

Saad Shaikh, “QuARC — A Quality Assurance Range Calorimeter for Proton Therapy”, PhD Thesis, University College London (2023)
URL: https://discovery.ucl.ac.uk/id/eprint/10178868/
Keywords & Abstract

Abstract:
Proton beam therapy (PBT) is a type of radiotherapy used to treat cancerous tumours. Compared to conventional X-ray radiotherapy, PBT offers significant advantages due to its superior dose conformity, allowing better sparing of healthy tissue. This dose conformity is due to the well-defined range of protons in matter. The proton range in water is a key parameter in optimising patient safety and is measured everyday during daily quality assurance (QA). However, this is a lengthy process that takes valuable time away from patient treatment. This project covers the design and construction of a scintillator-based detector for range measurements in PBT to reduce the time taken for daily QA from an hour down to a few minutes. This work focuses on the implementation of a new photodiode-based front-end data acquisition (DAQ) system and mechanical setup for the detector. Overhauling the DAQ system increased the detector data-rate from 25 Hz to 6 kHz, allowing real-time tracking of scanned treatment delivery, and increased the dynamic range of the detector by several orders of magnitude, allowing for proton range measurements in the FLASH (ultra-high dose rate) regime. Optimisation of the back-end data analysis process introduced real-time function fitting to facilitate live proton range reconstruction to sub-mm precision at up to 5 Hz. A total of seven experiments were conducted across several different PBT facilities to optimise the detector setup and scintillator composition, with the range accuracy of the detector found to be within 0.5 mm. FLASH PBT experiments found the detector able to measure beam currents up to 50 nA, though some non-linearity was observed. A specialised, compact version of the detector was also developed for the successful range measurements of spread-out Bragg peaks. Overall, this new device presents an attractive alternative to current commercial offerings for fast, accurate proton range QA measurements.
Jeremy Ocampo, “Designing Convolutional Neural Networks for Scintillation Photography and General Applications”, PhD Thesis, University College London (2023)
URL: https://discovery.ucl.ac.uk/id/eprint/10173798/
Keywords & Abstract

Abstract:
Modern radiotherapy treatments provide complex dose distributions which are difficult to measure and verify. This is due to beams having high-dose gradients, timevarying intensity and sizes reducing to millimetre scales. A dosimeter was designed to provide improvements over standard detectors that are unsuitable for measuring complex small fields. The proposed detector system consists of an irradiated scintillator sheet that is photographed, from which the dose is reconstructed. This provides a cheap, fast, and high-resolution solution. But scintillation images come with a variety of visual artefacts that need correcting. Convolutional neural networks (CNN) have been shown to have excellent accuracy in extracting useful information from noisy images. This requires thousands to millions of training images. In scintillation photography, there is not enough data to achieve an acceptable performance for CNNs. A novel method using domain randomisation was used to solve this issue, where thousands of images were simulated with varying physical parameters. This data can be used to train the CNN to be robust to visual artefacts. These CNNs are designed to assist in our image processing by predicting relative dose distributions and (un)known physical parameters, which gives confidence that the measured images are correct. Results showed that CNNs performed better than classical methods and could provide dose distributions that are suitable for routine QA. This work was extended by designing a novel CNN layer which can be generalised to non-Euclidean domains while maintaining scalability, e.g. the sphere which has many applications. This is done by developing modern methods in group convolutions and helping them scale to high resolution. This method leverages symmetries in the data, which improves the CNNs ability to generalize to "unseen" data with only a few thousand training examples. The models were tested on spherical data benchmarks for which state-of-the-art performance was achieved.
Shaikh, S et al., “O 146 - QuARC: A Quality Assurance Range Calorimeter for proton therapy”, In proceedings of “60th Annual Conference of the Particle Therapy Cooperative Group” (2023)
DOI: 10.14338/ijpt-23-ptcog60-9.4
URL: https://www.sciencedirect.com/science/article/pii/S2331518023002482?via%3Dihub#cesec4400
Keywords & Abstract

Abstract:
Range uncertainties remain the largest source of uncertainty in proton therapy and prevent taking full advantage of its superior dose conformity. To optimise patient safety, daily quality assurance (QA) procedures are carried out each day before treatment begins, which are often time-consuming. In FLASH proton therapy, short treatment delivery times and high dose rates mean that typical ionisation chamber-based dosimetry methods become unusable. These dose rates are currently estimated to be approximately 40 Gy/s or 600 nA to the patient. The Quality Assurance Range Calorimeter (QuARC) is currently under development at UCL to provide fast, accurate, water-equivalent proton range measurements to speed up daily QA, with the capability to operate at FLASH dose rates. The detector is a series of optically isolated plastic scintillator sheets that sample the proton dose deposition along its path length. Each scintillator sheet is coupled to a photodiode that measures its light output directly. An analytical depth-light model is used to fit the data and measure the proton range. Two preliminary beam tests at UCLH with pencil beams between 70–110 MeV found the QuARC able to consistently recover proton ranges with good accuracy, even at low dose rates. Due to its large dynamic range — able to measure at least a 100-fold increase in dose — this will scale up to FLASH dose rates. Live fitting of the captured data enables stable real-time range reconstruction at 40 Hz. Further measurements are required to fully characterise the detector performance and light output with FLASH.

PBT - 2021

Laurent Kelleter, “A Scintillator-Based Range Telescope for Particle Beam Radiotherapy”, PhD Thesis, University College London (2021)
This research was funded by the European Union's Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement No 675265.
URL: https://discovery.ucl.ac.uk/id/eprint/10119396
Keywords & Abstract

Keywords: Scintillator, Proton therapy, Particle therapy, Range telescope, Quality assurance, Mixed beam, Theranostics, Bragg curve, Depth-Light curve

Abstract:
Particle beam therapy (PBT) is a form of radiation therapy that is used for cancer treatment. Recently, interest in scintillator-based detectors for the measurement of depth-dose curves of therapeutic particle beams has been growing. In this work, a novel range telescope based on plastic scintillator and read out by a large-scale CMOS image sensor is presented. The detector is made of a stack of 49 plastic scintillator sheets with a thickness of 2–3 mm and a transverse area of 100×100mm^2. A novel Bragg curve model that incorporates scintillator quenching effects was developed for the beam range reconstruction from depth-light curves with low depth resolution. Measurements to characterise the performance of the detector were carried out at three different PBT centres across Europe. The maximum difference between the measured proton range and the reference range was found to be 0.46 mm. An evaluation of the radiation hardness proved that the range reconstruction algorithm is robust following the deposition of 6,300 Gy peak dose into the detector. Variations in the beam spot size, the transverse beam position and the beam intensity were shown to have a negligible effect on the range reconstruction accuracy. Range measurements of helium, carbon and oxygen ion beams were also performed. A novel technique for online range verification based on a mixed helium/carbon ion beam and the range telescope was investigated. The helium beam range modulation by a narrow air gap of 1 mm thickness in a plastic phantom that affected less than a quarter of the beam particles was detected, demonstrating the outstanding sensitivity of the mixed-beam technique. Using two anthropomorphic pelvis phantoms it was shown that small rotations of the phantom as well as simulated bowel gas movements cause detectable range changes. The future prospects and limitations of the helium-carbon mixing as well as its technical feasibility are discussed.

PBT - 2020

Simon Jolly and Hywel Owen and Marco Schippers and Carsten Welsch, “Technical challenges for FLASH proton therapy”, Physica Medica: European Journal of Medical Physics 78 p.71–82 (2020)
DOI: 10.1016/j.ejmp.2020.08.005
Keywords & Abstract

Keywords: FLASH, Proton therapy, PBT, Proton FLASH therapy, Proton radiotherapy, Ultra-rapid irradiation, Accelerator, Cyclotron, Synchrotron, Synchro-cyclotron, Linac, Medical accelerator

Abstract:
There is growing interest in the radiotherapy community in the application of FLASH radiotherapy, wherein the dose is delivered to the entire treatment volume in less than a second. Early pre-clinical evidence suggests that these extremely high dose rates provide significant sparing of healthy tissue compared to conventional radiotherapy without reducing the damage to cancerous cells. This interest has been reflected in the proton therapy community, with early tests indicating that the FLASH effect is also present with high dose rate proton irradiation. In order to deliver clinically relevant doses at FLASH dose rates significant technical hurdles must be overcome in the accelerator technology before FLASH proton therapy can be realised. Of these challenges, increasing the average current from the present clinical range of 1–10 nA to in excess of 100 nA is at least feasible with existing technology, while the necessity for rapid energy adjustment on the order of a few milliseconds is much more challenging, particularly for synchrotron-based systems. However, the greatest challenge is to implement full pencil beam scanning, where scanning speeds 2 orders of magnitude faster than the existing state-of-the-art will be necessary, along with similar improvements in the speed and accuracy of associated dosimetry. Hybrid systems utilising 3D-printed patient specific range modulators present the most likely route to clinical delivery. However, to correctly adapt and develop existing technology to meet the challenges of FLASH, more pre-clinical studies are needed to properly establish the beam parameters that are necessary to produce the FLASH effect.
Jacinta Yap et al., “Beam characterisation studies of the 62 MeV proton therapy beamline at the Clatterbridge Cancer Centre”, Physica Medica: European Journal of Medical Physics 77 p.108-120 (2020)
DOI: 10.1016/j.ejmp.2020.08.002
Keywords & Abstract

Abstract:
The Clatterbridge Cancer Centre (CCC) in the United Kingdom is the world's first hospital proton beam therapy facility, providing treatment for ocular cancers since 1989. A 62 MeV beam of protons is produced by a Scanditronix cyclotron and transported through a passive delivery system. In addition to the long history of clinical use, the facility supports a wide programme of experimental work and as such, an accurate and reliable simulation model of the treatment beamline is highly valuable. However, as the facility has seen several changes to the accelerator and beamline over the years, a comprehensive study of the CCC beam dynamics is needed to firstly examine the beam optics. An extensive analysis was required to overcome facility related constraints to determine fundamental beamline parameters and define an optical lattice written with the Methodical Accelerator Design (MAD-X) and the particle tracking Beam Delivery Simulation (BDSIM) code. An optimised case is presented and simulated results of the optical functions, beam distribution, losses and the transverse rms beam sizes along the beamline are discussed. Corresponding optical and beam information was used in TOPAS to simulate transverse beam profiles and compared to EBT3 film measurements. We provide an overview of the magnetic components, beam transport, cyclotron, beam and treatment related parameters necessary for the development of a present day optical model of the facility. This work represents the first comprehensive study of the CCC facility to date, as a basis to determine input beam parameters to accurately simulate and completely characterise the beamline.
Laurent Kelleter et al., “A scintillator-based range telescope for particle therapy”, Physics in Medicine and Biology 65 (16) p.165001 (2020)
DOI: 10.1088/1361-6560/ab9415
Inspire entry: Kelleter:2020qen
Keywords & Abstract

Keywords: Bragg curve, CMOS sensor, light quenching, particle therapy, pencil beam range, plastic scintillator, quality assurance

Abstract:
The commissioning and operation of a particle therapy centre requires an extensive set of detectors for measuring various parameters of the treatment beam. Among the key devices are detectors for beam range quality assurance. In this work, a novel range telescope based on a plastic scintillator and read out by a large-scale CMOS sensor is presented. The detector is made of a stack of 49 plastic scintillator sheets with a thickness of 2-3 mm and an active area of 100×100 mm^2, resulting in a total physical stack thickness of 124.2 mm. This compact design avoids optical artefacts that are common in other scintillation detectors. The range of a proton beam is reconstructed using a novel Bragg curve model that incorporates scintillator quenching effects. Measurements to characterise the performance of the detector were carried out at the Heidelberger Ionenstrahl-Therapiezentrum (HIT, Heidelberg, GER) and the Clatterbridge Cancer Centre (CCC, Bebington, UK). The maximum difference between the measured range and the reference range was found to be 0.41 mm at a proton beam range of 310 mm and was dominated by detector alignment uncertainties. With the new detector prototype, the water-equivalent thickness of PMMA degrader blocks has been reconstructed within 0.1 mm. An evaluation of the radiation hardness proves that the range reconstruction algorithm is robust following the deposition of 6,300 Gy peak dose into the detector. Furthermore, small variations in the beam spot size and transverse beam position are shown to have a negligible effect on the range reconstruction accuracy. The potential for range measurements of ion beams is also investigated.
Laurent Kelleter and Simon Jolly, “A mathematical expression for depth-light curves of therapeutic proton beams in a quenching scintillator”, Medical Physics 47 (5) p.2300-2308 (2020)
DOI: 10.1002/mp.14099
Keywords & Abstract

Abstract:
Purpose: Recently, there has been increasing interest in the development of scintillator-based detectors for the measurement of depth��dose curves of therapeutic proton beams (Beaulieu and Beddar [2016], Phys Med Biol., 61:R305��R343). These detectors allow the measurement of single beam parameters such as the proton range or the reconstruction of the full three-dimensional dose distribution. Thus, scintillation detectors could play an important role in beam quality assurance, online beam monitoring, and proton imaging. However, the light output of the scintillator as a function of dose deposition is subject to quenching effects due to the high-specific energy loss of incident protons, particularly in the Bragg peak. The aim of this work is to develop a model that describes the percent depth-light curve in a quenching scintillator and allow the extraction of information about the beam range and the strength of the quenching. Methods: A mathematical expression of a depth-light curve, derived from a combination of Birks�� law (Birks [1951], Proc Phys Soc A., 64:874) and Bortfeld��s Bragg curve (Bortfeld [1997], Med Phys., 24:2024��2033) that is termed a ��quenched Bragg�� curve, is presented. The model is validated against simulation and measurement. Results: A fit of the quenched Bragg model to simulated depth-light curves in a polystyrene-based scintillator shows good agreement between the two, with a maximum deviation of 2.5% at the Bragg peak. The differences are larger behind the Bragg peak and in the dose build-up region. In the same simulation, the difference between the reconstructed range and the reference proton range is found to be always smaller than 0.16��mm. The comparison with measured data shows that the fitted beam range agrees with the reference range within their respective uncertainties. Conclusions: The quenched Bragg model is, therefore, an accurate tool for the range measurement from quenched depth��dose curves. Moreover, it allows the reconstruction of the beam energy spread, the particle fluence, and the magnitude of the quenching effect from a measured depth-light curve.
L. Volz et al., “Experimental exploration of a mixed helium/carbon beam for online treatment monitoring in carbon ion beam therapy”, Physics in Medicine and Biology 65 (5) p.055002 (2020)
DOI: 10.1088/1361-6560/ab6e52
Keywords & Abstract

Abstract:
Recently, it has been proposed that a mixed helium/carbon beam could be used for online monitoring in carbon ion beam therapy. Fully stripped, the two ion species exhibit approximately the same mass/charge ratio and hence could potentially be accelerated simultaneously in a synchrotron to the same energy per nucleon. At the same energy per nucleon, helium ions have about three times the range of carbon ions, which could allow for simultaneous use of the carbon ion beam for treatment and the helium ion beam for imaging. In this work, measurements and simulations of PMMA phantoms as well as anthropomorphic phantoms irradiated sequentially with a helium ion and a carbon ion beam at equal energy per nucleon are presented. The range of the primary helium ion beam and the fragment tail of the carbon ion beam exiting the phantoms were detected using a novel range telescope made of thin plastic scintillator sheets read out by a flat-panel CMOS sensor. A 10:1 carbon to helium mixing ratio is used, generating a helium signal well above the carbon fragment background while adding little to the dose delivered to the patient. The range modulation of a narrow air gap of 1 mm thickness in the PMMA phantom that affects less than a quarter of the particles in a pencil beam were detected, demonstrating the achievable relative sensitivity of the presented method. Using two anthropomorphic pelvis phantoms it is shown that small rotations of the phantom as well as simulated bowel gas movements cause detectable changes in the helium/carbon beam exiting the phantom. The future prospects and limitations of the helium/carbon mixing as well as its technical feasibility are discussed.

PBT - 2019

Kelleter, Laurent et al., “Technical Note: Simulation of dose buildup in proton pencil beams”, Medical Physics 46 (8) p.3734-3738 (2019)
DOI: 10.1002/mp.13660
E-print: https://aapm.onlinelibrary.wiley.com/doi/pdf/10.1002/mp.13660
URL: https://aapm.onlinelibrary.wiley.com/doi/abs/10.1002/mp.13660
Keywords & Abstract

Keywords: dose buildup, proton therapy, secondary particles, skin sparing

Abstract:
Purpose: The purpose of this study is to characterize the magnitude and depth of dose buildup in pencil beam scanning proton therapy. Methods: We simulate the integrated depth-dose curve of realistic proton pencil beams in a water phantom using the Geant4 Monte Carlo toolkit. We independently characterize the electronic and protonic components of dose buildup as a function of proton beam energy from 40 to 400 MeV, both with and without an air gap. Results: At clinical energies, electronic buildup over a distance of about 1 mm leads to a dose reduction at depth of the basal layer (0.07 mm) by up to 6% compared to if no buildup effect were present. Protonic buildup reduces the dose to the basal layer by up to 16% and has effects at depths of up to 150 mm. Secondary particles with a mass number A>1 do not contribute to dose buildup. An air gap of 1 m has no significant effect on protonic buildup but reduces electronic buildup below 1%. Conclusions: Protonic and electronic dose buildup are relevant for accurate dosimetry in proton therapy although a realistic air gap reduces the electronic buildup to levels where it can be safely neglected. We recommend including electrons and secondary protons in Monte Carlo-based treatment planning systems down to a predicted range of 10–20 μm in order to accurately model the dose at depths of the basal layer, no matter the size of the air gap between nozzle and patient.

PBT - 2016

Hywel Owen and Antony Lomax and Simon Jolly, “Current and future accelerator technologies for charged particle therapy”, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 809 p.96-104 (2016)
Advances in detectors and applications for medicine.
DOI: 10.1016/j.nima.2015.08.038
URL: https://www.sciencedirect.com/science/article/pii/S0168900215009729
Keywords & Abstract

Keywords: Accelerator, Radiotherapy, Proton, Cyclotron, Synchrotron, Linac, Proton Therapy, Conceptual Design, Beam-Transport, Radiotherapy

Abstract:
The past few years have seen significant developments both of the technologies available for proton and other charged particle therapies, and of the number and spread of therapy centres. In this review we give an overview of these technology developments, and outline the principal challenges and opportunities we see as important in the next decade. Notable amongst these is the ever-increasing use of superconductivity both in particle sources and for treatment delivery, which is likely to greatly increase the accessibility of charged particle therapy treatments to hospital centres worldwide.
Chirvase, C and Barlow, R and Basharina-Freshville, A and Jolly, S and Saakyan, R, “Faster QA through improved proton calorimetry. Another spin-off from particle physics”, In proceedings of “International Conference on Translational Research in Radiation Oncology/ Physics for Health in Europe (ICTR-PHE 2016)” Radiotherapy And Oncology 118 (Supplemental 1) p.S25 (2016)
DOI: 10.1016/s0167-8140(16)30050-0
URL: https://www.thegreenjournal.com/issue/S0167-8140(15)X0016-8
Keywords & Abstract

Abstract:
We have used the calorimeter module originally designed for the SuperNEMO experiment to measure the energies of protons at the Clatterbridge proton therapy cyclotron. Such measurements are necessary for time consuming QA checks, and by improving the rates and energy resolution this time can be considerably reduced. Preliminary results show that an energy resolution of σ = 0.7% can be achieved for low rates which will later be compared to the high rate data. We hope to extend this technique to proton radiography as well.

Useful PBT Publications

Wayne D. Newhauser and Rui Zhang, “The physics of proton therapy”, Physics in Medicine and Biology 60 (8) (2015)
DOI: 10.1088/0031-9155/60/8/R155
Keywords & Abstract

Abstract:
The physics of proton therapy has advanced considerably since it was proposed in 1946. Today analytical equations and numerical simulation methods are available to predict and characterize many aspects of proton therapy. This article reviews the basic aspects of the physics of proton therapy, including proton interaction mechanisms, proton transport calculations, the determination of dose from therapeutic and stray radiations, and shielding design. The article discusses underlying processes as well as selected practical experimental and theoretical methods. We conclude by briefly speculating on possible future areas of research of relevance to the physics of proton therapy.
Christian P. Karger and Oliver J��kel and Hugo Palmans and Tatsuaki Kanai, “Dosimetry for ion beam radiotherapy”, Physics in Medicine and Biology 55 (21) (2010)
DOI: 10.1088/0031-9155/55/21/R01
Keywords & Abstract

Abstract:
Recently, ion beam radiotherapy (including protons as well as heavier ions) gained considerable interest. Although ion beam radiotherapy requires dose prescription in terms of iso-effective dose (referring to an iso-effective photon dose), absorbed dose is still required as an operative quantity to control beam delivery, to characterize the beam dosimetrically and to verify dose delivery. This paper reviews current methods and standards to determine absorbed dose to water in ion beam radiotherapy, including (i) the detectors used to measure absorbed dose, (ii) dosimetry under reference conditions and (iii) dosimetry under non-reference conditions. Due to the LET dependence of the response of films and solid-state detectors, dosimetric measurements are mostly based on ion chambers. While a primary standard for ion beam radiotherapy still remains to be established, ion chamber dosimetry under reference conditions is based on similar protocols as for photons and electrons although the involved uncertainty is larger than for photon beams. For non-reference conditions, dose measurements in tissue-equivalent materials may also be necessary. Regarding the atomic numbers of the composites of tissue-equivalent phantoms, special requirements have to be fulfilled for ion beams. Methods for calibrating the beam monitor depend on whether passive or active beam delivery techniques are used. QA measurements are comparable to conventional radiotherapy; however, dose verification is usually single field rather than treatment plan based. Dose verification for active beam delivery techniques requires the use of multi-channel dosimetry systems to check the compliance of measured and calculated dose for a representative sample of measurement points. Although methods for ion beam dosimetry have been established, there is still room for developments. This includes improvement of the dosimetric accuracy as well as development of more efficient measurement techniques. �� 2010 Institute of Physics and Engineering in Medicine Printed in the UK.
J. Perl and J. Shin and J. Sch��mann and B. Faddegon and H. Paganetti, “TOPAS: An innovative proton Monte Carlo platform for research and clinical applications”, Medical Physics 39 (11) (2012)
DOI: 10.1118/1.4758060
Keywords & Abstract

Abstract:
Purpose: While Monte Carlo particle transport has proven useful in many areas (treatment head design, dose calculation, shielding design, and imaging studies) and has been particularly important for proton therapy (due to the conformal dose distributions and a finite beam range in the patient), the available general purpose Monte Carlo codes in proton therapy have been overly complex for most clinical medical physicists. The learning process has large costs not only in time but also in reliability. To address this issue, we developed an innovative proton Monte Carlo platform and tested the tool in a variety of proton therapy applications. Methods: Our approach was to take one of the already-established general purpose Monte Carlo codes and wrap and extend it to create a specialized user-friendly tool for proton therapy. The resulting tool, TOol for PArticle Simulation (TOPAS), should make Monte Carlo simulation more readily available for research and clinical physicists. TOPAS can model a passive scattering or scanning beam treatment head, model a patient geometry based on computed tomography (CT) images, score dose, fluence, etc., save and restart a phase space, provides advanced graphics, and is fully four-dimensional (4D) to handle variations in beam delivery and patient geometry during treatment. A custom-designed TOPAS parameter control system was placed at the heart of the code to meet requirements for ease of use, reliability, and repeatability without sacrificing flexibility. Results: We built and tested the TOPAS code. We have shown that the TOPAS parameter system provides easy yet flexible control over all key simulation areas such as geometry setup, particle source setup, scoring setup, etc. Through design consistency, we have insured that user experience gained in configuring one component, scorer or filter applies equally well to configuring any other component, scorer or filter. We have incorporated key lessons from safety management, proactively removing possible sources of user error such as line-ordering mistakes. We have modeled proton therapy treatment examples including the UCSF eye treatment head, the MGH stereotactic alignment in radiosurgery treatment head and the MGH gantry treatment heads in passive scattering and scanning modes, and we have demonstrated dose calculation based on patient-specific CT data. Initial validation results show agreement with measured data and demonstrate the capabilities of TOPAS in simulating beam delivery in 3D and 4D. Conclusions: We have demonstrated TOPAS accuracy and usability in a variety of proton therapy setups. As we are preparing to make this tool freely available for researchers in medical physics, we anticipate widespread use of this tool in the growing proton therapy community. �� 2012 American Association of Physicists in Medicine.
Harald Paganetti, “Range uncertainties in proton therapy and the role of Monte Carlo simulations”, Physics in Medicine and Biology 57 (11) (2012)
DOI: 10.1088/0031-9155/57/11/R99
Keywords & Abstract

Abstract:
The main advantages of proton therapy are the reduced total energy deposited in the patient as compared to photon techniques and the finite range of the proton beam. The latter adds an additional degree of freedom to treatment planning. The range in tissue is associated with considerable uncertainties caused by imaging, patient setup, beam delivery and dose calculation. Reducing the uncertainties would allow a reduction of the treatment volume and thus allow a better utilization of the advantages of protons. This paper summarizes the role of Monte Carlo simulations when aiming at a reduction of range uncertainties in proton therapy. Differences in dose calculation when comparing Monte Carlo with analytical algorithms are analyzed as well as range uncertainties due to material constants and CT conversion. Range uncertainties due to biological effects and the role of Monte Carlo for in vivo range verification are discussed. Furthermore, the current range uncertainty recipes used at several proton therapy facilities are revisited. We conclude that a significant impact of Monte Carlo dose calculation can be expected in complex geometries where local range uncertainties due to multiple Coulomb scattering will reduce the accuracy of analytical algorithms. In these cases Monte Carlo techniques might reduce the range uncertainty by several mm. �� 2012 Institute of Physics and Engineering in Medicine.
Harald Paganetti, “Proton Therapy Physics”, (2016)
DOI: 10.1201/9781032616858
A. J. Lomax, “Intensity modulated proton therapy and its sensitivity to treatment uncertainties 1: The potential effects of calculational uncertainties”, Physics in Medicine and Biology 53 (4) (2008)
DOI: 10.1088/0031-9155/53/4/014
Keywords & Abstract

Abstract:
The effects of calculational uncertainties on 3D and distal edge tracking (DET) intensity modulated proton therapy (IMPT) treatment plans have been investigated. Dose calculation uncertainties have been assessed by comparing analytical and Monte Carlo dose calculations, and potential range uncertainties by recalculating plans with all CT values modified by ��3%. Analysis of the volume of PTV agreeing to within ��3% between the two calculations shows that the 3D approach provides significantly improved agreement (87.1 versus 80.3% of points for the 3D and DET approaches, respectively). For the DET approach, doses in the CTV have also been found to globally change by 5% as a result of 3% changes in CT value. When varying the intra-field gradients of the plans a similar trend is seen, but with the more complex plans also being found to be more sensitive to both uncertainties. In conclusion, the DET approach has been found to be relatively sensitive to the calculational errors investigated here. In contrast, the 3D approach appears to be quite robust, unless strong internal gradients are present. Nevertheless, the routine use of uncertainty analysis is advised when assessing all forms of IMPT plans. �� 2008 Institute of Physics and Engineering in Medicine.
A. J. Lomax, “Intensity modulated proton therapy and its sensitivity to treatment uncertainties 2: The potential effects of inter-fraction and inter-field motions”, Physics in Medicine and Biology 53 (4) (2008)
DOI: 10.1088/0031-9155/53/4/015
Keywords & Abstract

Abstract:
Simple tools for studying the effects of inter-fraction and inter-field motions on intensity modulated proton therapy (IMPT) plans have been developed, and have been applied to both 3D and distal edge tracking (DET) IMPT plans. For the inter-fraction motion, we have investigated the effects of misaligned density heterogeneities, whereas for the inter-field motion analysis, the effects of field misalignment on the plans have been assessed. Inter-fraction motion problems have been analysed using density differentiated error (DDE) distributions, which specifically show the additional problems resulting from misaligned density heterogeneities for proton plans. Likewise, for inter-field motion, we present methods for calculating motion differentiated error (MDE) distributions. DDE and MDE analysis of all plans demonstrate that the 3D approach is generally more robust to both inter-fraction and inter-field motions than the DET approach, but that strong in-field dose gradients can also adversely affect a plan's robustness. An important additional conclusion is that, for certain IMPT plans, even inter-fraction errors cannot necessarily be compensated for by the use of a simple PTV margins, implying that more sophisticated tools need to be developed for uncertainty management and assessment for IMPT treatments at the treatment planning level. �� 2008 Institute of Physics and Engineering in Medicine.
Xuemin Bai and Gino Lim and David Grosshans and Radhe Mohan and Wenhua Cao, “Robust optimization to reduce the impact of biological effect variation from physical uncertainties in intensity-modulated proton therapy”, Physics in Medicine and Biology 64 (2) (2019)
DOI: 10.1088/1361-6560/aaf5e9
Keywords & Abstract

Abstract:
Robust optimization (RO) methods are applied to intensity-modulated proton therapy (IMPT) treatment plans to ensure their robustness in the face of treatment delivery uncertainties, such as proton range and patient setup errors. However, the impact of those uncertainties on the biological effect of protons has not been specifically considered. In this study, we added biological effect-based objectives into a conventional RO cost function for IMPT optimization to minimize the variation in biological effect. One brain tumor case, one prostate tumor case and one head & neck tumor case were selected for this study. Three plans were generated for each case using three different optimization approaches: planning target volume (PTV)-based optimization, conventional RO, and RO incorporating biological effect (BioRO). In BioRO, the variation in biological effect caused by IMPT delivery uncertainties was minimized for voxels in both target volumes and critical structures, in addition to a conventional voxel-based worst-case RO objective function. The biological effect was approximated by the product of dose-averaged linear energy transfer (LET) and physical dose. All plans were normalized to give the same target dose coverage, assuming a constant relative biological effectiveness (RBE) of 1.1. Dose, biological effect, and their uncertainties were evaluated and compared among the three optimization approaches for each patient case. Compared with PTV-based plans, RO plans achieved more robust target dose coverage and reduced biological effect hot spots in critical structures near the target. Moreover, with their sustained robust dose distributions, BioRO plans not only reduced variations in biological effect in target and normal tissues but also further reduced biological effect hot spots in critical structures compared with RO plans. Our findings indicate that IMPT could benefit from the use of conventional RO, which would reduce the biological effect in normal tissues and produce more robust dose distributions than those of PTV-based optimization. More importantly, this study provides a proof of concept that incorporating biological effect uncertainty gap into conventional RO would not only control the IMPT plan robustness in terms of physical dose and biological effect but also achieve further reduction of biological effect in normal tissues.
Antony John Lomax, “Myths and realities of range uncertainty”, The British Journal of Radiology 93 (1107) (2020)
DOI: 10.1259/bjr.20190582
Keywords & Abstract

Abstract:
Range uncertainty is a much discussed topic in proton therapy. Although a very real aspect of proton therapy, its magnitude and consequences are sometimes misunderstood or overestimated. In this article, the sources and consequences of range uncertainty are reviewed, a number of myths associated with the effect discussed with the aim of putting range uncertainty into clinical context and attempting to de-bunk some of the more exaggerated claims made as to its consequences.
Albin Fredriksson and Rasmus Bokrantz, “A critical evaluation of worst case optimization methods for robust intensity-modulated proton therapy planning”, Medical Physics 41 (8) (2014)
DOI: 10.1118/1.4883837
Keywords & Abstract

Abstract:
Purpose: To critically evaluate and compare three worst case optimization methods that have been previously employed to generate intensity-modulated proton therapy treatment plans that are robust against systematic errors. The goal of the evaluation is to identify circumstances when the methods behave differently and to describe the mechanism behind the differences when they occur. Methods: The worst case methods optimize plans to perform as well as possible under the worst case scenario that can physically occur (composite worst case), the combination of the worst case scenarios for each objective constituent considered independently (objectivewise worst case), and the combination of the worst case scenarios for each voxel considered independently (voxelwise worst case). These three methods were assessed with respect to treatment planning for prostate under systematic setup uncertainty. An equivalence with probabilistic optimization was used to identify the scenarios that determine the outcome of the optimization. Results: If the conflict between target coverage and normal tissue sparing is small and no dose-volume histogram (DVH) constraints are present, then all three methods yield robust plans. Otherwise, they all have their shortcomings: Composite worst case led to unnecessarily low plan quality in boundary scenarios that were less difficult than the worst case ones. Objectivewise worst case generally led to nonrobust plans. Voxelwise worst case led to overly conservative plans with respect to DVH constraints, which resulted in excessive dose to normal tissue, and less sharp dose fall-off than the other two methods. Conclusions: The three worst case methods have clearly different behaviors. These behaviors can be understood from which scenarios that are active in the optimization. No particular method is superior to the others under all circumstances: composite worst case is suitable if the conflicts are not very severe or there are DVH constraints whereas voxelwise worst case is advantageous if there are severe conflicts but no DVH constraints. The advantages of composite and voxelwise worst case outweigh those of objectivewise worst case. �� 2014 American Association of Physicists in Medicine.
D. Pflugfelder and J. J. Wilkens and U. Oelfke, “Worst case optimization: A method to account for uncertainties in the optimization of intensity modulated proton therapy”, Physics in Medicine and Biology 53 (6) (2008)
DOI: 10.1088/0031-9155/53/6/013
Keywords & Abstract

Abstract:
The sharp dose gradients which are possible in intensity modulated proton therapy (IMPT) not only offer the possibility of generating excellent target coverage while sparing neighbouring organs at risk, but can also lead to treatment plans which are very sensitive to uncertainties in treatment variables such as the range of individual Bragg peaks. We developed a method to account for uncertainties of treatment variables in the optimization based on a worst case dose distribution. The worst case dose distribution is calculated using several possible realizations of the uncertainties. This information is used by the objective function of the inverse treatment planning system to generate treatment plans which are acceptable under all considered realizations of the uncertainties. The worst case optimization method was implemented in our in-house treatment planning software KonRad in order to demonstrate the usefulness of this approach for clinical cases. In this paper, we investigated range uncertainties, setup uncertainties and a combination of both uncertainties. Using our method the sensitivity of the resulting treatment plans to these uncertainties is considerably reduced. �� 2008 Institute of Physics and Engineering in Medicine.
Sebastian Tattenberg et al., “Proton range uncertainty reduction benefits for skull base tumors in terms of normal tissue complication probability (NTCP) and healthy tissue doses”, Medical Physics 48 (9) (2021)
DOI: 10.1002/mp.15097
Keywords & Abstract

Abstract:
Purpose: Proton therapy allows for more conformal dose distributions and lower organ at risk and healthy tissue doses than conventional photon-based radiotherapy, but uncertainties in the proton range currently prevent proton therapy from making full use of these advantages. Numerous developments therefore aim to reduce such range uncertainties. In this work, we quantify the benefits of reductions in range uncertainty for treatments of skull base tumors. Methods: The study encompassed 10��skull base patients with clival tumors. For every patient, six treatment plans robust to setup errors of 2��mm and range errors from 0% to 5% were created. The determined metrics included the brainstem and optic chiasm normal tissue complication probability (NTCP) with the endpoints of necrosis and blindness, respectively, as well as the healthy tissue volume receiving at least 70% of the prescription dose. Results: A range uncertainty reduction from the current level of 4% to a potentially achievable level of 1% reduced the probability of brainstem necrosis by up to 1.3 percentage points in the nominal scenario in which neither setup nor range errors occur and by up to 2.9 percentage points in the worst-case scenario. Such a range uncertainty reduction also reduced the optic chiasm NTCP with the endpoint of blindness by up to 0.9 percentage points in the nominal scenario and by up to 2.2 percentage points in the worst-case scenario. The decrease in the healthy tissue volume receiving at least 70% of the prescription dose ranged from ��7.8 to 24.1��cc in the nominal scenario and from ��3.4 to 38.4��cc in the worst-case scenario. Conclusion: The benefits quantified as part of this study serve as a guideline of the OAR and healthy tissue dose benefits that range monitoring techniques may be able to achieve. Benefits were observed between all levels of range uncertainty. Even smaller range uncertainty reductions may therefore be beneficial.
Bijan Arjom et al., “AAPM task group 224: Comprehensive proton therapy machine quality assurance”, Medical Physics 46 (8) (2019)
DOI: 10.1002/mp.13622
Keywords & Abstract

Abstract:
Purpose:��Task Group (TG) 224 was established by the American Association of Physicists in Medicine's Science Council under the Radiation Therapy Committee and Work Group on Particle Beams. The group was charged with developing comprehensive quality assurance (QA) guidelines and recommendations for the three commonly employed proton therapy techniques for beam delivery: scattering, uniform scanning, and pencil beam scanning. This report supplements established QA guidelines for therapy machine performance for other widely used modalities, such as photons and electrons (TG 142, TG 40, TG 24, TG 22, TG 179, and Medical Physics Practice Guideline 2a) and shares their aims of ensuring the safe, accurate, and consistent delivery of radiation therapy dose distributions to patients. Methods:��To provide a basis from which machine-specific QA procedures can be developed, the report first describes the different delivery techniques and highlights the salient components of the related machine hardware. Depending on the particular machine hardware, certain procedures may be more or less important, and each institution should investigate its own situation. Results:��In lieu of such investigations, this report identifies common beam parameters that are typically checked, along with the typical frequencies of those checks (daily, weekly, monthly, or annually). The rationale for choosing these checks and their frequencies is briefly described. Short descriptions of suggested tools and procedures for completing some of the periodic QA checks are also presented. Conclusion:��Recommended tolerance limits for each of the recommended QA checks are tabulated, and are based on the literature and on consensus data from the clinical proton experience of the task group members. We hope that this and other reports will serve as a reference for clinical physicists wishing either to establish a proton therapy QA program or to evaluate an existing one.
X. Ronald Zhu et al., “Towards effective and efficient patient-specific quality assurance for spot scanning proton therapy”, Cancers 7 (2) (2015)
DOI: 10.3390/cancers7020631
Keywords & Abstract

Abstract:
An intensity-modulated proton therapy (IMPT) patient-specific quality assurance (PSQA) program based on measurement alone can be very time consuming due to the highly modulated dose distributions of IMPT fields. Incorporating independent dose calculation and treatment log file analysis could reduce the time required for measurements. In this article, we summarize our effort to develop an efficient and effective PSQA program that consists of three components: measurements, independent dose calculation, and analysis of patient-specific treatment delivery log files. Measurements included two-dimensional (2D) measurements using an ionization chamber array detector for each field delivered at the planned gantry angles with the electronic medical record (EMR) system in the QA mode and the accelerator control system (ACS) in the treatment mode, and additional measurements at depths for each field with the ACS in physics mode and without the EMR system. Dose distributions for each field in a water phantom were calculated independently using a recently developed in-house pencil beam algorithm and compared with those obtained using the treatment planning system (TPS). The treatment log file for each field was analyzed in terms of deviations in delivered spot positions from their planned positions using various statistical methods. Using this improved PSQA program, we were able to verify the integrity of the data transfer from the TPS to the EMR to the ACS, the dose calculation of the TPS, and the treatment delivery, including the dose delivered and spot positions. On the basis of this experience, we estimate that the in-room measurement time required for each complex IMPT case (e.g., a patient receiving bilateral IMPT for head and neck cancer) is less than 1 h using the improved PSQA program. Our experience demonstrates that it is possible to develop an efficient and effective PSQA program for IMPT with the equipment and resources available in the clinic.
Suresh Rana and Jaafar Bennouna and E. James Jebaseelan Samuel and Alonso N. Gutierrez, “Development and long-term stability of a comprehensive daily QA program for a modern pencil beam scanning (PBS) proton therapy delivery system”, Journal of Applied Clinical Medical Physics 20 (4) (2019)
DOI: 10.1002/acm2.12556
Keywords & Abstract

Abstract:
Purpose: The main purpose of this study is to demonstrate the clinical implementation of a comprehensive pencil beam scanning (PBS) daily quality assurance (QA) program involving a number of novel QA devices including the Sphinx/Lynx/parallel-plate (PPC05) ion chamber and HexaCheck/multiple imaging modality isocentricity (MIMI) imaging phantoms. Additionally, the study highlights the importance of testing the connectivity among oncology information system (OIS), beam delivery/imaging systems, and patient position system at a proton center with multi-vendor equipment and software. Methods: For dosimetry, a daily QA plan with spot map of four different energies (106, 145, 172, and 221��MeV) is delivered on the delivery system through the OIS. The delivery assesses the dose output, field homogeneity, beam coincidence, beam energy, width, distal-fall-off (DFO), and spot characteristics �� for example, position, size, and skewness. As a part of mechanical and imaging QA, a treatment plan with the MIMI phantom serving as the patient is transferred from OIS to imaging system. The HexaCheck/MIMI phantoms are used to assess daily laser accuracy, imaging isocenter accuracy, image registration accuracy, and six-dimensional (6D) positional correction accuracy for the kV imaging system and robotic couch. Results: The daily QA results presented herein are based on 202 daily sets of measurements over a period of 10��months. Total time to perform daily QA tasks at our center is under 30��min. The relative difference (�� rel ) of daily measurements with respect to baseline was within��1% for field homogeneity, ��0.5��mm for range, width and DFO, ��1��mm for spots positions, ��10% for in-air spot sigma, ��0.5 spot skewness, and ��1��mm for beam coincidence (except 1 case: �� rel ��=��1.3��mm). The average �� rel in dose output was ��0.2% (range: ��1.1% to 1.5%). For 6D IGRT QA, the average absolute difference (�� abs ) was ��0.6��0.4��mm for translational and ��0.5�� for rotational shifts. Conclusion: The use of novel QA devices such as the Sphinx in conjunction with the Lynx, PPC05 ion chamber, HexaCheck/MIMI phantoms, and myQA software was shown to provide a comprehensive and efficient method for performing daily QA of a number of system parameters for a modern proton PBS-dedicated treatment delivery unit.
P. Trnkov�� and A. Bolsi and F. Albertini and D. C. Weber and A. J. Lomax, “Factors influencing the performance of patient specific quality assurance for pencil beam scanning IMPT fields”, Medical Physics 43 (11) (2016)
DOI: 10.1118/1.4964449
Keywords & Abstract

Abstract:
Purpose: A detailed analysis of 2728 intensity modulated proton therapy (IMPT) fields that were clinically delivered to patients between 2007 and 2013 at Paul Scherrer Institute (PSI) was performed. The aim of this study was to analyze the results of patient specific dosimetric verifications and to assess possible correlation between the quality assurance (QA) results and specific field metrics. Methods: Dosimetric verifications were performed for every IMPT field prior to patient treatment. For every field, a steering file was generated containing all the treatment unit information necessary for treatment delivery: beam energy, beam angle, dose, size of air gap, nuclear interaction (NI) correction factor, number of range shifter plates, number of Bragg peaks (BPs) with their position and weight. This information was extracted and correlated to the results of dosimetric verification of each field which was a measurement of two orthogonal profiles using an orthogonal ionization chamber array in a movable water column. Results: The data analysis has shown more than 94% of all verified plans were within defined clinical tolerances. The differences between measured and calculated dose depend critically on the number of BPs, total thickness of all range shifter plates inserted in the beam path, and maximal range. An increase of the dose difference was observed with smaller number of BPs (i.e., smaller tumor) and smaller ranges (i.e., superficial tumors). The results of the verification do not depend, however, on the prescribed dose, NI correction, or the size of the air gap. There is no dependency of the transversal and longitudinal spot position precision on the beam angle. The value of NI correction depends on the number of spots and number of range shifter plates. Conclusions: The presented study has shown that the verification method used at Centre for Proton Therapy at Paul Scherrer Institute is accurate and reproducible for performing patient specific QA. The results confirmed that the dose discrepancy is dependent on the size and location of the tumor.
Moyed Miften et al., “Tolerance limits and methodologies for IMRT measurement-based verification QA: Recommendations of AAPM Task Group No. 218”, Medical Physics 45 (4) (2018)
DOI: 10.1002/mp.12810
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Purpose: Patient-specific IMRT QA measurements are important components of processes designed to identify discrepancies between calculated and delivered radiation doses. Discrepancy tolerance limits are neither well defined nor consistently applied across centers. The AAPM TG-218 report provides a comprehensive review aimed at improving the understanding and consistency of these processes as well as recommendations for methodologies and tolerance limits in patient-specific IMRT QA. Methods: The performance of the dose difference/distance-to-agreement (DTA) and �� dose distribution comparison metrics are investigated. Measurement methods are reviewed and followed by a discussion of the pros and cons of each. Methodologies for absolute dose verification are discussed and new IMRT QA verification tools are presented. Literature on the expected or achievable agreement between measurements and calculations for different types of planning and delivery systems are reviewed and analyzed. Tests of vendor implementations of the �� verification algorithm employing benchmark cases are presented. Results: Operational shortcomings that can reduce the �� tool accuracy and subsequent effectiveness for IMRT QA are described. Practical considerations including spatial resolution, normalization, dose threshold, and data interpretation are discussed. Published data on IMRT QA and the clinical experience of the group members are used to develop guidelines and recommendations on tolerance and action limits for IMRT QA. Steps to check failed IMRT QA plans are outlined. Conclusion: Recommendations on delivery methods, data interpretation, dose normalization, the use of �� analysis routines and choice of tolerance limits for IMRT QA are made with focus on detecting differences between calculated and measured doses via the use of robust analysis methods and an in-depth understanding of IMRT verification metrics. The recommendations are intended to improve the IMRT QA process and establish consistent, and comparable IMRT QA criteria among institutions.
Nicola Bizzocchi and Francesco Fracchiolla and Marco Schwarz and Carlo Algranati, “A fast and reliable method for daily quality assurance in spot scanning proton therapy with a compact and inexpensive phantom”, Medical Dosimetry 42 (3) (2017)
DOI: 10.1016/j.meddos.2017.05.001
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In a radiotherapy center, daily quality assurance (QA) measurements are performed to ensure that the equipment can be safely used for patient treatment on that day. In a pencil beam scanning (PBS) proton therapy center, spot positioning, spot size, range, and dose output are usually verified every day before treatments. We designed, built, and tested a new, reliable, sensitive, and inexpensive phantom, coupled with an array of ionization chambers, for daily QA that reduces the execution times while preserving the reliability of the test. The phantom is provided with 2 pairs of wedges to sample the Bragg peak at different depths to have a transposition on the transverse plane of the depth dose. Three ��boxes�� are used to check spot positioning and delivered dose. The box thickness helps spread the single spot and to fit a Gaussian profile on a low resolution detector. We tested whether our new QA solution could detect errors larger than our action levels: 1 mm in spot positioning, 2 mm in range, and 10% in spot size. Execution time was also investigated. Our method is able to correctly detect 98% of spots that are actually in tolerance for spot positioning and 99% of spots out of 1 mm tolerance. All range variations greater than the threshold (2 mm) were correctly detected. The analysis performed over 1 month showed a very good repeatability of spot characteristics. The time taken to perform the daily quality assurance is 20 minutes, a half of the execution time of the former multidevice procedure. This ��in-house build�� phantom substitutes 2 very expensive detectors (a multilayer ionization chamber [MLIC] and a strip chamber, reducing by 5 times the cost of the equipment. We designed, built, and validated a phantom that allows for accurate, sensitive, fast, and inexpensive daily QA procedures in proton therapy with PBS.
Dennis Mackin et al., “Spot-Scanning Proton Therapy Patient-Specific Quality Assurance: Results from 309 Treatment Plans”, International Journal of Particle Therapy 1 (3) (2014)
DOI: 10.14338/ijpt-14-00017.1
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Purpose:We report the outcomes of patient-specific quality assurance (PSQA) for spot- scanning proton therapy (SSPT) treatment plans by disease site. Patients and Methods: We analyzed quality assurance outcomes for 309 SSPT plans. The PSQA measurements consisted of 2 parts: (1) an end-to-end test in which the beam was delivered at the prescribed gantry angle and (2) dose plane measurements made from gantry angle 2708. The HPlusQ software was used for gamma analysis of the dose planes using dose-tolerance and distance-to-agreement levels of 2%, 2 mm and 3%, 3 mm, respectively. Passingwas defined as a gamma score,1 in at least 90%of the pixels. Results: The overall quality assurance measurement passing rate was 96.2% for the gamma index criteria of 3%, 3mmbut fell to 85.3%when the criteria were tightened to 2%, 2 mm. The passing rate was dependent on the treatment site.With the 3%, 3 mm criteria, the passing rate was 95%for head-and-neck treatment plans and 100% for prostate plans. No significant difference was found between passing rates for multi-field and single-field optimized plans. The passing rate was 94.8% 6 0.6% for fields with range shifters and 99.0% 6 0.6% for those without (P�� .002). Most low gamma index scores were due to steep dose gradients transverse to the measured plane. A less frequent cause of failures was an apparent systematic overestimation of the calculated dose at depths proximal to the spread-out Bragg peak. Conclusion: A comprehensive PSQA program serves to ensure the safety of a specific treatment plan and acts as a check on the entire treatment system. We propose that the 3%, 3 mm with 90% pixel passing rate is a reasonable action level for 2-dimensional comparisons of dose planes in SSPT, although more restrictive tolerance levels would be appropriate for prostate treatment plans
Maria F. Chan et al., “Patient-Specific QA of Spot-Scanning Proton Beams Using Radiochromic Film”, International Journal of Medical Physics, Clinical Engineering and Radiation Oncology 06 (02) (2017)
DOI: 10.4236/ijmpcero.2017.62011
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Background and Aims��Cardiovascular disease (CVD) is among the leading causes of morbidity and mortality worldwide. Traditional risk factors predict 75-80% of an individual's risk of incident CVD. However, the role of early life experiences in future disease risk is gaining attention. The Barker hypothesis proposes fetal origins of adult disease, with consistent evidence demonstrating the deleterious consequences of birth weight outside the normal range. In this study, we investigate the role of birth weight in CVD risk prediction. Methods and Results��The Women's Health Initiative (WHI) represents a large national cohort of post-menopausal women with 63 815 participants included in this analysis. Univariable proportional hazards regression analyses evaluated the association of 4 self-reported birth weight categories against 3 CVD outcome definitions, which included indicators of coronary heart disease, ischemic stroke, coronary revascularization, carotid artery disease and peripheral arterial disease. The role of birth weight was also evaluated for prediction of CVD events in the presence of traditional risk factors using 3 existing CVD risk prediction equations: one body mass index (BMI)-based and two laboratory-based models. Low birth weight (LBW) (< 6 lbs.) was significantly associated with all CVD outcome definitions in univariable analyses (HR=1.086, p=0.009). LBW was a significant covariate in the BMI-based model (HR=1.128, p<0.0001) but not in the lipid-based models. Conclusion��LBW (<6 lbs.) is independently associated with CVD outcomes in the WHI cohort. This finding supports the role of the prenatal and postnatal environment in contributing to the development of adult chronic disease.
Lukas Cornelius Wolter and Fabian Hennings and Jozef Bokor and Christian Richter and Kristin Stuetzer, “Validity of one-time phantomless patient-specific quality assurance in proton therapy with regard to the reproducibility of beam delivery”, Medical Physics 52 (5) p.3173-3182 (2025)
DOI: 10.1002/mp.17637
Heng Li et al., “Use of treatment log files in spot scanning proton therapy as part of patient-specific quality assurance”, Medical Physics 40 (2) (2013)
DOI: 10.1118/1.4773312
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Purpose: The purpose of this work was to assess the monitor unit (MU) values and position accuracy of spot scanning proton beams as recorded by the daily treatment logs of the treatment control system, and furthermore establish the feasibility of using the delivered spot positions and MU values to calculate and evaluate delivered doses to patients. Methods: To validate the accuracy of the recorded spot positions, the authors generated and executed a test treatment plan containing nine spot positions, to which the authors delivered ten MU each. The spot positions were measured with radiographic films and Matrixx 2D ion-chambers array placed at the isocenter plane and compared for displacements from the planned and recorded positions. Treatment logs for 14 patients were then used to determine the spot MU values and position accuracy of the scanning proton beam delivery system. Univariate analysis was used to detect any systematic error or large variation between patients, treatment dates, proton energies, gantry angles, and planned spot positions. The recorded patient spot positions and MU values were then used to replace the spot positions and MU values in the plan, and the treatment planning system was used to calculate the delivered doses to patients. The results were compared with the treatment plan. Results: Within a treatment session, spot positions were reproducible within ��0.2 mm. The spot positions measured by film agreed with the planned positions within ��1 mm and with the recorded positions within ��0.5 mm. The maximum day-to-day variation for any given spot position was within ��1 mm. For all 14 patients, with ��1 500 000 spots recorded, the total MU accuracy was within 0.1% of the planned MU values, the mean (x, y) spot displacement from the planned value was (-0.03 mm, -0.01 mm), the maximum (x, y) displacement was (1.68 mm, 2.27 mm), and the (x, y) standard deviation was (0.26 mm, 0.42 mm). The maximum dose difference between calculated dose to the patient based on the plan and recorded data was within 2%. Conclusions: The authors have shown that the treatment log file in a spot scanning proton beam delivery system is precise enough to serve as a quality assurance tool to monitor variation in spot position and MU value, as well as the delivered dose uncertainty from the treatment delivery system. The analysis tool developed here could be useful for assessing spot position uncertainty and thus dose uncertainty for any patient receiving spot scanning proton beam therapy. �� 2013 American Association of Physicists in Medicine.
Maria Francesca Belosi et al., “Treatment log files as a tool to identify treatment plan sensitivity to inaccuracies in scanned proton beam delivery”, Radiotherapy and Oncology 125 (3) (2017)
DOI: 10.1016/j.radonc.2017.09.037
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Dose distributions delivered at Gantry 2 at the Paul Scherrer Institut (PSI) can be reconstructed on the patient anatomy based on machine log files. With the present work, the dependency of the log file calculation on the planning optimization technique and on other planning parameters, such as field direction and tumour size, has been investigated. Interestingly, and despite the typically higher modulation of Intensity Modulated Proton Therapy (IMPT) plans, the results for both Single Field Uniform Distribution and IMPT approaches have been found to be similar. In addition, complex fields with steep in-field dose gradients, such as Simultaneous Integrated Boost, and with couch movements in between the delivery, also resulted in good agreement between planned and reconstructed doses. Nevertheless, highly modulated plans can have regions of larger local dose deviations and attention should therefore be paid during the planning stage to the location of isolated, highly weighted pencil beams. We propose also, that further effort should be invested in order to predict field robustness to delivery fluctuations before the clinical delivery of the plan as part of the plan specific Quality Assurance.
M. Matter et al., “Update on yesterday's dose-Use of delivery log-files for daily adaptive proton therapy (DAPT)”, Physics in Medicine and Biology 65 (19) (2020)
DOI: 10.1088/1361-6560/ab9f5e
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In daily adaptive proton therapy (DAPT), the treatment plan is re-optimized on a daily basis. It is a straightforward idea to incorporate information from the previous deliveries during the optimization to refine this daily proton delivery. A feedback signal was used to correct for delivery errors and errors from an inaccurate dose calculation used for plan optimization. This feedback signal consisted of a dose distribution calculated with a Monte Carlo algorithm and was based on the spot delivery information from the previous deliveries in the form of log-files. We therefore called the method Update On Yesterday's Dose (UYD). The UYD method was first tested with a simulated DAPT treatment and second with dose measurements using an anthropomorphic phantom. For both, the simulations and the measurements, a better agreement between the delivered and the intended dose distribution could be observed using UYD. Gamma pass rates (1%/1 mm) increased from around 75% to above 90%, when applying the closed-loop correction for the simulations, as well as the measurements. For a DAPT treatment, positioning errors or anatomical changes are incorporated during the optimization and therefore are less dominant in the overall dose uncertainty. Hence, the relevance of algorithm or delivery machine errors even increases compared to standard therapy. The closed-loop process described here is a method to correct for these errors, and potentially further improve DAPT.
Carla Winterhalter et al., “Log file based Monte Carlo calculations for proton pencil beam scanning therapy”, Physics in Medicine and Biology 64 (3) (2019)
DOI: 10.1088/1361-6560/aaf82d
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Patient specific quality assurance is crucial to guarantee safety in proton pencil beam scanning. In current clinical practice, this requires extensive, time consuming measurements. Additionally, these measurements do not consider the influence of density heterogeneities in the patient and are insensitive to delivery errors. In this work, we investigate the use of log file based Monte Carlo calculations for dose reconstructions in the patient CT, which takes the combined influence of calculational and delivery errors into account. For one example field, 87%/90% of the voxels agree within ��3% when taking either calculational or delivery uncertainties into account (analytical versus Monte Carlo calculation/Monte Carlo from planned versus Monte Carlo from log file). 78% agree when considering both uncertainties simultaneously (nominal field versus Monte Carlo from log files). We then show the application of the log file based Monte Carlo calculations as a patient specific quality assurance tool for a set of five patients (16 fields) treated for different indications. For all fields, absolute dose scaling factors based on the log file Monte Carlo agree within ��3% to the measurement based absolute dose scaling. Relative comparison shows that more than 90% of the voxels agree within �� 5% between the analytical calculated plan and the Monte Carlo based on log files. The log file based Monte Carlo approach is an end-to-end test incorporating all requirements of patient specific quality assurance. It has the potential to reduce the workload and therefore to increase the patient throughput, while simultaneously enabling more accurate dose verification directly in the patient geometry.
Arturs Meijers et al., “Feasibility of patient specific quality assurance for proton therapy based on independent dose calculation and predicted outcomes”, Radiotherapy and Oncology 150 (2020)
DOI: 10.1016/j.radonc.2020.06.027
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Purpose: Patient specific quality assurance (PSQA) is required to verify the treatment delivery and the dose calculation by the treatment planning system (TPS). The objective of this work is to demonstrate the feasibility to substitute resource consuming measurement based PSQA (PSQAM) by independent dose recalculations (PSQAIDC), and that PSQAIDC results may be interpreted in a clinically relevant manner using normal tissue complication probability (NTCP) and tumor control probability (TCP) models. Methods and materials: A platform for the automatic execution of the two following PSQAIDC workflows was implemented: (i) using the TPS generated plan and (ii) using treatment delivery log files (log-plan). 30 head and neck cancer (HNC) patients were retrospectively investigated. PSQAM results were compared with those from the two PSQAIDC workflows. TCP/NTCP variations between PSQAIDC and the initial TPS dose distributions were investigated. Additionally, for two example patients that showed low passing PSQAM results, eight error scenarios were simulated and verified via measurements and log-plan based calculations. For all error scenarios ��TCP/NTCP values between the nominal and the log-plan dose were assessed. Results: Results of PSQAM and PSQAIDC from both implemented workflows agree within 2.7% in terms of gamma pass ratios. The verification of simulated error scenarios shows comparable trends between PSQAM and PSQAIDC. Based on the 30 investigated HNC patients, PSQAIDC observed dose deviations translate into a minor variation in NTCP values. As expected, TCP is critically related to observed dose deviations. Conclusions: We demonstrated a feasibility to substitute PSQAM with PSQAIDC. In addition, we showed that PSQAIDC results can be interpreted in clinically more relevant manner, for instance using TCP/NTCP.
M. Matter et al., “Alternatives to patient specific verification measurements in proton therapy: A comparative experimental study with intentional errors”, Physics in Medicine and Biology 63 (20) (2018)
DOI: 10.1088/1361-6560/aae2f4
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Patient specific verification (PSV) measurements for pencil beam scanning (PBS) proton therapy are resource-consuming and necessitate substantial beam time outside of clinical hours. As such, efforts to safely reduce the PSV-bottleneck in the clinical work-flow are of great interest. Here, capabilities of current PSV methods to ensure the treatment integrity were investigated and compared to an alternative approach of reconstructing the dose distribution directly from the machine control- or delivery log files with the help of an independent dose calculation (IDC). Scenarios representing a wide range of delivery or work-flow failures were identified (e.g. error in spot position, air gap or pre-absorber setting) and machine files were altered accordingly. This yielded 21 corrupted treatment files, which were delivered and measured with our clinical PSV protocol. IDC machine- and log file checks were also conducted and their sensitivity at detecting the errors compared to the measurements. Although some of the failure scenarios induced clinically relevant dose deviations in the patient geometry, the PSV measurement protocol only detected one out of 21 error scenarios. However, 11 and all 21 error scenarios were detected using dose reconstructions based on the log and machine files respectively. Our data suggests that, although commonly used in particle therapy centers, PSV measurements do a poor job detecting data transfer failures and imperfect delivery machine performance. Machine- and log-file IDCs have been shown to successfully detect erroneous work-flows and to represent a reliable addition to the QA procedure, with the potential to replace PSV.
Xiaoning Ding and James E. Younkin and Jiajian Shen and Martin Bues and Wei Liu, “A critical review of the practices of proton daily quality assurance programs”, Therapeutic Radiology and Oncology 5 (2021)
DOI: 10.21037/TRO-21-11
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Demand for proton therapy (PT) continues to grow due to its dosimetric advantages over conventional radiotherapy. New PT facilities being constructed to meet this demand will need quality assurance (QA) programs to ensure that treatments are delivered safely and accurately. However, in contrast to conventional radiotherapy, proton QA practices are constantly evolving and few commercial solutions are available. As a result, QA programs at most operational proton facilities rely on a variety of in-house developed hardware and software. An important part of these QA programs is proton daily QA, which verifies clinically-acceptable proton delivery system operation each morning before starting patient treatment. In this review article, we summarize current proton daily QA practices by providing a brief introduction to PT, describing proton delivery techniques and their particular QA requirements, and then reviewing implementations of several proton daily QA programs. Although the QA instrumentation is quite heterogeneous, the literature shows that the dosimetric daily QA results among proton facilities are comparable. We also present a typical set of proton daily QA data from our institution that includes output, range, and spot position measurements. Based on the literature review and our institutional experience, we make recommendations for future proton daily QA programs.
O. Actis and D. Meer and S. K��nig and D. C. Weber and A. Mayor, “A comprehensive and efficient daily quality assurance for PBS proton therapy”, Physics in Medicine and Biology 62 (5) (2017)
DOI: 10.1088/1361-6560/aa5131
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There are several general recommendations for quality assurance (QA) measures, which have to be performed at proton therapy centres. However, almost each centre uses a different therapy system. In particular, there is no standard procedure for centres employing pencil beam scanning and each centre applies a specific QA program. Gantry 2 is an operating therapy system which was developed at PSI and relies on the most advanced technological innovations. We developed a comprehensive daily QA program in order to verify the main beam characteristics to assure the functionality of the therapy delivery system and the patient safety system. The daily QA program entails new hardware and software solutions for a highly efficient clinical operation. In this paper, we describe a dosimetric phantom used for verifying the most critical beam parameters and the software architecture developed for a fully automated QA procedure. The connection between our QA software and the database allows us to store the data collected on a daily basis and use it for trend analysis over longer periods of time. All the data presented here have been collected during a time span of over two years, since the beginning of the Gantry 2 clinical operation in 2013. Our procedure operates in a stable way and delivers the expected beam quality. The daily QA program takes only 20 min. At the same time, the comprehensive approach allows us to avoid most of the weekly and monthly QA checks and increases the clinical beam availability.

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