Under-response correction for EBT3 films in the presence of proton spread out Bragg peaks.
We present a study of the under-response of the new Gafchromic EBT3 films and a procedure to accurately perform 2D and 3D proton dosimetry measurements for both pristine and spread out Bragg peaks (SOBP) of any energy. These new films differ from the previous EBT2 generation by a slightly different active layer composition, which we show has not effected appreciably their response. The procedure and the beam quality correction factor curve have been benchmarked using 29 MeV modulated proton beams. In order to show the correction to apply when EBT3 films are used as treatment verification tools in anthropomorphic phantoms, two simulation studies involving clinical energies are presented: a SOBP for eye treatments and a SOBP to treat 20 cm deep and 5 cm thick tumours. We find maximum under-responses of 37%, 30% and 7.7% for the modulated 29 MeV beam, eye and deep tumour treatment, respectively, which were attained close to the end of the peak tails, due to a higher proportion of very low energy protons. The maximum deviations between corrected and uncorrected doses were for the three cases, respectively, 20.7%, 8.3% and 2.1% of the average dose across flat region of the SOBP. These values were obtained close to the distal edge of the SOBPs, where the proportion of low energy protons was not as high as on the tail, but there still was a number of protons high enough to deposit a reasonable amount of dose in the films.
Gray Institute for Radiation Oncology and Biology, Department of Oncology, University of Oxford, Roosevelt Drive, Oxford OX3 7DQ, UK. Electronic address: francesca.fiorini@oncology.ox.ac.uk.
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F
Fiorini
Hall-Edwards Radiotherapy Research Group, Department of Medical Physics, University Hospital Birmingham NHS Trust, Birmingham B15 2TH, UK.
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D
Kirby
Gray Institute for Radiation Oncology and Biology, Department of Oncology, University of Oxford, Roosevelt Drive, Oxford OX3 7DQ, UK.
J
J
Thompson
Hall-Edwards Radiotherapy Research Group, Department of Medical Physics, University Hospital Birmingham NHS Trust, Birmingham B15 2TH, UK.
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S
Green
School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
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DJ
Parker
Gray Institute for Radiation Oncology and Biology, Department of Oncology, University of Oxford, Roosevelt Drive, Oxford OX3 7DQ, UK.
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B
Jones
Gray Institute for Radiation Oncology and Biology, Department of Oncology, University of Oxford, Roosevelt Drive, Oxford OX3 7DQ, UK.
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MA
Hill
D002138:Calibration; D005373:Film Dosimetry / Q000379:methods*; D019047:Phantoms, Imaging; D011522:Protons; D014867:Water
D011522:Protons; D014867:Water
3D dosimetry; EBT3 Gafchromic films; FLUKA Monte Carlo code; Under-response correction