(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: proton therapy studies involve ultra-high dose rates exceeding ~50 Gy/s and have been of great interest due to reduced normal tissue toxicity and equivalent tumor killing effect compared to conventional dose rates. However, the ionization chamber response to proton FLASH has not been evaluated and verified for commonly used ionization chamber designs. In this study, we aim to evaluate the ion-recombination correction factors k_s for several types of ionization chambers by means of the two-voltage method as recommended in TRS 398.
Methods: measurements were performed in a dedicated research room with a fixed proton beamline. Protons at 230 MeV energy were extracted from the cyclotron with requested beam current of 360 nA. A passive scattering system was used to create a 2.6-cm diameter uniform field at the measurement location. The dose rate was measured at 114 Gy/s with an ADCL-calibrated parallel plate chamber at the flat region of the proton beam. Subsequently, collected charge was recorded for seven other chambers (IBA-dosimetry PPC05, PPC40, CC03, CC13, Standard Imaging Extradin A12, A16 and A26) under the same irradiation conditions at the same location with two bias voltages (+400V and +200V).
Results: parallel plate chambers (PPC05, PPC40, PTW Advanced Markus) and the cylindrical chambers which have sensitive volume no greater than 0.04cc (CC04, A16 and A26) demonstrated adequate ion collection efficiency at 114 Gy/s dose rate with measured k_s less than 1% from unity. The Farmer chamber and CC13 chamber are not suitable for absolute dosimetry due to large ion recombination effects with k_s greater than 1.05 at the same dose rate.
Conclusion: addition to parallel plate chambers, small volume cylindrical chambers are surprisingly suitable for absolute dosimetry in proton FLASH irradiation. These “micro” chambers have excellent spatial resolution, which may find additional uses for small-field FLASH applications.
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