(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose:
Recent pre-clinical studies showed that ultra-high dose rate, called FLASH, radiotherapy significantly reduce normal tissue toxicity while maintaining tumor control. Cincinnati Children’s Hospital and University of Cincinnati Medical Center Proton Therapy Center (CPCT) in collaboration with Varian, Inc. has upgraded the existing research-dedicated gantry room for the clinical FLASH proton radiotherapy. The purpose of this work is to compare the absorbed dose to water in ultra-high dose rate (~65 Gy/s) pencil beam scanning proton beams measured with ion chambers with a calorimeter from National Physical Laboratory (NPL) in UK as a reference.
Methods:
A total of 8 rectangular 250 MeV single layer fields (5×6, 5×8, 5×10, 5×12 cm² and transposed fields for each) were developed for treating bone metastasis of extremities using plateau region of the field. The absolute dose measurements at 5 cm water equivalence depth (WED) for each field were performed to compare dose determination by the Advance Markus (AMC) and PPC05 ion chambers with a graphite calorimeter. Gafchromic film was used as a secondary validation tool as well as for the verification of 2D dose uniformity and field size.
Results:
The measured absolute dose average of all 8 fields with calorimeter is 8.47±0.04 Gy as our reference RBE dose. The ratio of AMC dose to the calorimeter reference dose is 1.00. PPC05 chamber showed 3% higher reading (the ratio of 1.03). The dose measured with Gafchromic film shows 2% over-response (the ratio of 1.02).
Conclusion:
AMC absolute dose measurement agreed with a calorimeter reference dose within 0.02%. PPC05 chamber showed 3% over-response compared to the reference dose measured with the calorimeter. For FLASH PBS proton radiotherapy both AMC and PPC05 chambers can be a reference dosimeter due to accurate and stable measurements while Gafchromic film may be an excellent secondary dosimeter for 2D dose distribution.
Keywords
Protons, Ionization Chamber, Dosimetry Protocols
Taxonomy
TH- External Beam- Particle/high LET therapy: Proton therapy – experimental dosimetry
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