Purpose: To assess in a systematic study the dose rate dependency of ion recombination effects in clinically used plane-parallel ion chambers in ultra-high dose rate pencil beam scanning proton beam irradiations.
Methods: A 5×12 cm² rectangular 250 MeV single layer transmission proton field was delivered to the Advanced Markus and PPC05 parallel plate chambers at 5 cm water equivalent depth. Bias voltages were varied from 50 to 400 Volts. For each voltage, collected charges were measured as a function of dose rates ranging from voxel-based dose rate of 5 Gy/s to 60 Gy/s. The extrapolation methods and two-voltage technique following the IAEA TRS-398 protocol were used to determine the ion recombination correction factor ks.
Results: At voltages greater than 200 V, the values of ks for the PTW Advanced Markus chamber are saturated less than 0.7% from unity and increase rapidly as the bias voltage decreases. The relative difference in the ks values determined from between the extrapolation methods and the two-voltage technique for the Advanced Markus chamber are insignificant and the trends as function of dose rate vary from 0.01% to 0.5%. The IBA PPC05 shows a flat response in terms of ion recombination effects based on the ks values calculated using the two-voltage method. The ion recombination correction factors are less than 0.2% over the range of dose rates investigated.
Conclusion: The dose rate dependency of the ion recombination effects of both ion chambers in PBS proton FLASH transmission beams is negligible in the range of dose rates between 5 – 60 Gy/s for bias voltages above 200 V. A typical clinical bias voltage of -300 V is suitable for these chambers in FLASH environment. Both chambers are reliable to be used in cyclotron-based FLASH proton pencil beam systems.
Absolute Dosimetry, Collection Efficiency, Protons
TH- External Beam- Particle/high LET therapy: Proton therapy - calibration protocols and primary standards (including detector response simulation)