ELogs/JamesGolbourn

Electronic Log for James Golbourn

Project Direction

The project will be split into two sections, the first will be similar to the research essay and will be a survey on current quality assurance procedures clinically used. The second will be on focused on the features of an ideal integrated QA device, including what beam characteristics it will measure, how long it will take and the precision of the measurements.

Scintillator Columns

Raffy and I briefly discussed a method for QA measurements which involves stacking scintillator screens and then splitting the screens into columns such that depth and transverse profile measurements can be taken. The beam would be centred on the column and the spot position will be a measure of the deviation of the beam away from the predicted position.

To Do

Short Term

• Update links in wiki.
• Start to think about project direction.
• Summarise Research Essay with links.

Long Term

• Get in contact with Laurent contact at Med Austrom about QA procedures.
• Prepare practice presentation for project (8/2/19)
• Write up report on meeting with UCLH.
• Look at what functioning centres are using as QA.
• Look at advantages and disadvantages of QA systems.

Research Essay Device Summary

Sun Nuclear QA 3

Property measured	Component	Description	Precision
Output	CAX	The CAX is located at the centre of the SOBP and output is determined from charge collected in CAX. This is compared to a reference value.	2-3  %
Range	Electron Ionization Chambers	Top left and bottom right chambers are positioned in distal fall off region, top right and bottom left are positioned in SOBP centre using range compensators of different thickness to modify beam range. Ratio of charge measured in chambers is used to infer beam range fluctuations.	1 mm
Symmetry	Electron Ionization Chambers	Charge ratio between top right and bottom left chambers.
Spot position	4 diode triplet sets	Central diode of each diode was exposed to a dose at 50 % lateral fall off and the position determined by signal measured in neighbouring two diodes.	1.5 mm

Links to papers

1. Xiaoning Ding et al. “A novel daily QA system for proton therapy”. In:Journal of applied clinical medical physics 14.2 (2013), pp. 115–126. https://aapm.onlinelibrary.wiley.com/doi/full/10.1120/jacmp.v14i2.4058
2. James E. Younkin et al. “Technical Note: An efficient daily QA procedure forproton pencil beam scanning”. In: Medical Physics 45.3 (2018), pp. 1040–1049. doi: 10.1002/mp.12787. https://www.ncbi.nlm.nih.gov/pubmed/29394447

IBA Matrixx

IBA Matrixx formed of 1020 parallel plate ionization chambers arranged in a 2D array. There are 2 versions used in PBT, the Matrixx PT and Matrixx Evolution. The PT is more precise than the Evolution with the chamber spacing improving from 5 mm in the Evolution to 2 mm in the PT this improves the resolution. PT is more sensitive (1.3 nC/ Gy) than the Evolution (2.4 nC/Gy).

There are large dose uncertainties in the Evolution, particularly in the penumbra regions of upto 4 %, this is improved with the PT device to 1 % primarily due to the improved resolution. The Matrixx is a useful device as it provides results in real time and has been demonstrated to have a similar precision to film dosimetry techniques.

Evolution

Property measured	Component	Description	Tolerance
Output	3 x 3 array of chambers	Area exposed by beam and dose measured from charge in ionization chambers. Uncertainty is reduced by averaging over several chambers. Plastic water was placed in front of chambers to make sure chambers were at centre of modulations. Value compared to farmer ionization chamber reference.	1 %
Flatness and Symmetry	Ionization Chambers	Flatness determined by how the mean dose varied across the central 80% of the full width half maximum of the beam whilst the symmetry was determined from the difference in the largest points.	1 %

Links to Papers

1. Bijan Arjomandy et al. “Use of a two-dimensional ionization chamber array for proton therapy beam quality assurance”. In: Medical Physics 35.9 (2008), pp. 3889–3894. https://www.ncbi.nlm.nih.gov/pubmed/18841839
2. Liyong Lin et al. “Use of a novel two-dimensional ionization chamber array for pencil beam scanning proton therapy beam quality assurance”. In: Journal of Applied Clinical Medical Physics 16.3 (2015), pp. 270–276. doi:10.1120/jacmp.v16i3.5323. https://europepmc.org/articles/pmc5690130
3. M. Varasteh Anvar et al. “Quality assurance of carbon ion and proton beams: A feasibility study for using the 2D MatriXX detector”. In: Physica Medica 32.6 (2016), pp. 831–837. issn: 1120-1797. doi: https://doi.org/10.1016/j.ejmp.2016.05.058. https://www.ncbi.nlm.nih.gov/pubmed/27246359

IBA Lynx and Sphinx

Scintillator based device. High spatial resolution of 0.5 mm and a large active surface area of 300 mm x 300 mm. The CCD pixels have a physical size of 4.65 µm x 4.65 µm. The Sphinx is designed to compliment the Lynx and is made up of blocks of RW3 of varying thickness and shape (wedged and non wedged). By scanning a beam across the block range measurements can be performed. The Sphinx is mechanically bolted to the Lynx and a parallel plate ionization chamber is contained in one of the RW3 blocks for output measurements.

The Lynx was irradiated with a uniform spot pattern consisting of 9 identical spots in a 3 x 3 array with each spot separated by 100 mm. The centre of the spot was calculated as the coordinate of the midpoint between the 50% levels on the beam profiles. The detector was irradiated for an exposure time of 300 s.

Property measured	Component	Description	Precision
Spot Size	Diode	The spot size was determined from the full width half maximum of the spot	2 %
Spot Position	Diode	spot position was measured as the deviation between the measured and expected position of the spot.	0.5 mm
Range	Sphinx blocks and diodes.	The beam is directed along the wedge to obtain the depth-dose curve and from this, range characteristics such as the proximal and distal range were obtained.	2 mm
Output	Ionization chamber.	No method described but could focus beam on block containing chamber and compare to reference chamber.

Links to Papers

1. S. Russo et al. “Characterization of a commercial scintillation detector for 2-D dosimetry in scanned proton and carbon ion beams”. In: Physica Medica 34 (2017), pp. 48–54. issn: 1120-1797. https://www.ncbi.nlm.nih.gov/pubmed/28118950
2. Lorenzo Placidi et al. “Range resolution and reproducibility of a dedicated phantom for proton PBS daily quality assurance”. In: Zeitschrift für Medizinische Physik 28.4 (2018), pp. 310–317. issn: 0939-3889. https://www.ncbi.nlm.nih.gov/pubmed/29548595

IBA Zebra and Giraffe

The Zebra is a multilayer ionization chamber (MLIC) consisting of 180 parallel plate ionization chambers separated by 2 mm and the chambers have a diameter of 2.5 cm. Similarly the Giraffe is an MLIC consisting of 180 parallel plate chambers. The chambers in the Giraffe have a diameter of 12 cm so the Giraffe is a lot bigger than the Zebra.

Both these devices can be utilised for QA measurements of the depth-dose profile. The accuracy of the measurements is limited by the resolution of the detector. The range and SOBP width are measured with tolerances of 2 mm and 5 mm respectively.

Links to Papers

1. Sandeep Dhanesar et al. “Quality assurance of proton beams using a multilayer ionization chamber system”. In: Medical Physics 40.9 (2013), p. 092102. doi: 10.1118/1.4817481. https://www.ncbi.nlm.nih.gov/pubmed/24007171
2. Christian Baumer et al. “Evaluation of detectors for acquisition of pristine depth-dose curves in pencil beam scanning”. In: Journal of Applied Clinical Medical Physics 16.6 (2015). https://www.ncbi.nlm.nih.gov/pubmed/26699567

Dosimetry

Very insightful paper on different dosimetric techniques that can be utilised in ion beam therapy. Christian P Karger et al. “Dosimetry for ion beam radiotherapy”. In:Physics in Medicine & Biology 55.21 (2010), R193. https://www.ncbi.nlm.nih.gov/pubmed/20952816

Tasks Completed

• Presentation to PBT group.
• Project progress report.
• Project outline.
• First plan for research essay.
• Prepare practise presentation to Agapi.
• Email Alison for details of Haakan Nystrom.
• Met with Alison and Andrew form UCLH
• Research Essay

Key Dates

Task	Deadline
Project Outline	03/12/2018
Project Progress Report	31/01/2019
Short Presentation	08/02/2019
Research Essay	31/03/2019
Thesis Deadline	22/08/2019
Final Presentation	02/09/2019

Second Meeting With Alison and Andrew

Firstly I updated Alison and Andrew on what progress I had made so far in my project. We briefly discussed some of the devices that I had come across as part of my research and the common beam characteristics that had been measured as part of daily QA.

From my Research

As before the characteristics measured as part of daily QA were:

• The beam energy.
• Output.
• Spot position (not so much uniformity as difficult to do).
• Spot size (and to an extent shape).

Summary of detectors:

• Sun Nuclear DQA3: The DQA3 is a diode and ion chamber device used to measure the beam output, range, symmetry, spot position and size. Very useful device.
• IBA Matrixx: Parallel plate chamber based device used to measure output, beam flatness and symmetry. Not as useful.
• IBA Zebra: Multilayer ionization chamber device. Not as useful.
• IBA Lynx: Scintillator based device used to measure spot size and position. Very useful device however limited due to cost.
• IBA Sphinx and Lynx: Combination of Lynx and Sphinx used to measure spot position, size and range. Very useful but again limited by cost.
• Gaffchromic film: Film based detector used to obtain dose profile.

Clinical Centres:

• PSI use a Lynx and Sphinx combination for daily QA.
• Med Austrom use a Giraffe, Lynx and chamber in solid water on a couch for daily QA.
• Both of these are very expensive approaches.

Discussion

Andrew and Alison confirmed that they planned to initially use the Sun Nuclear DQA3 device for their daily QA measurements. This decision was based on a compromise between cost and which measurements could be carried out. The IBA Lynx and Sphinx is the "gold standard" in daily QA systems however would cost roughly £500,000 which is not feasible. Conversely, the DQA3 device is about £8,000 and has the capability to perform the required measurements.

The output and range measurements are carried out using the central 5 ionization chambers and the spot position and size with the diodes. The range measured is a measure of range fluctuations rather, this is sufficient for daily QA. Different buildups are placed in front of the ionization chambers to allow for different range measurements. The diodes used are at a set depth and position and so get limited information from them but could be sufficient for daily QA. To measure the position the beam is directed at the innermost diode of the triplet and a half Gaussian is constructed by the detection from the triplet of diodes. This is combined with the half Gaussian obtained from the triplet on the opposite side and if the combined shape is roughly Gaussian then that acts as a confirmation of spot position. There are difficulties measuring the spot size as the beam will not be circular when measured, and, for example, could be elliptical in shape and there are therefore difficulties in obtaining the size directly (a scintillator would be better to get size).

The spots delivered are ideally Gaussian, and the spots should be separated such that the intensity is constant to roughly 2/3 %. The intensity cannot be measured with the DQA3. The intensity distribution is very sensitive to spot position deviations. Small changes in the spot position can greatly effect the uniformity of the intensity distribution. This increases the precision at which the position must be measured.

In order to activate the DQA3, the central axis chamber, used to measure the output, must be irradiated to trigger measurements. Can either irradiate the whole surface and perform measurements. Or quickly perform measurements on diodes after irradiation. One option is to obtain multiple data sets/ fields and then put together to reconstruct data.

Monthly QA was also discussed but they did not have an ideal set of measurements to take for this and is dependent on what they get out the daily QA. One option is to use the Zebra (multilayer ionization chamber).

From daily QA, the goal was to look for big deviations for the energy, beam profile and output, apart from with the spot spacing (position).

For me to potentially work on in the future:

• Look at putting different build up in front of the chambers to generate different depths. The buildups are made up of plastic WET material. In the patient the dose distribution is dominated by the scattering interactions rather than the beam energy.
• Look at doing a DQA3 prototype and maybe test it in Denmark.

Emails with Hakan

I emailed Hakan regarding the daily QA procedures at the PBT facilities at Skandionkliniken and Aarhus. We discussed essential daily QA checks and he confirmed that the:

• Beam energy (range)
• Spot positon
• Spot size
• Output
• Imaging systems

Something that I had left out from my research so far was how the daily QA of the imaging systems. Modern PBT is very image guided and so if the imaging geometry is out then the treatment will also be out. Furthermore, a geometrical offset of the imaging system is likely and happens all the time, and so needs to be incorporated into daily QA.

Hakan also confirmed what Alison and Andrew had alluded to about the beam energy (range). Although it is an important parameter, it is very unlikely to go wrong due to technical reasons (true for cyclotron systems). Therefore he suggested that the energy could even be left to be checked on a weekly or monthly basis.

A viable QA system should take no more than 30 mins per gantry. The QA at Aarhus takes about 15 mins and is dependent on the parameters and the number of irradiations.

Comments made about presentation

• Understand what range modulators do.
• Look into whether Zebra was initially designed for X-Rays.
• Zebra not often used for daily QA due to the very long initial set up time.
• Investigate film dosimetry e.g Gafchromic film.
• Research Lynx scintillator.
• Range calorimeter - need a moving absorber, e.g water tank, to measure energy and range.
• Know difference between scintillator screens and fibres - fibres use fibre optics and are not used due to needing something to read output from each fibre which is expensive.
• UCLH will have 4 treatment rooms.
• Put slide number on presentation.
• Expand on each daily QA:
  • Table of devices
  • Tolerances
  • How long it takes
• Collect standards and tolerances on energy, spot size and output.

Useful Links for Research Essay

Devices and Detectors

• Sun Nuclear QA 3 Device:
  • A novel daily QA system for proton therapy
    • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5714372/
  • Daily QA in proton therapy using a single commercially available detector
    • https://aapm.onlinelibrary.wiley.com/doi/pdf/10.1120/jacmp.v15i6.5005
  • Sun Nuclear QA 3 Fact Sheet
    • https://www.sunnuclear.com/documents/datasheets/dailyqa3-rfdaily.pdf
  • Technical Note: An efficient daily QA procedure for proton pencil beam scanning
    • https://aapm.onlinelibrary.wiley.com/doi/10.1002/mp.12787

Clinical Procedures

• PSI
  • O Actis et al, "A comprehensive and efficient daily quality assurance for PBS proton therapy", Physics in Medicine & Biology, 6 February 2017.
  • https://iopscience.iop.org/article/10.1088/1361-6560/aa5131/meta
  • PSI use their own device for daily QA. The device is formed of a scintillating screen, dosimetry phantom and multilayer ionization chambers.