Click here to

Session: FLASH RT - Instrumentation and Dosimetry [Return to Session]

Systematic Evaluation of Ion Recombination Effects of Clinically Available Ion Chambers in FLASH Pencil Beam Scanning Proton Beam

E Lee1*, J Speth2, Y Zhang3, Z Xiao4, A Mascia5, (1) Cincinnati Children's/UC Health, Liberty Campus, OH, (2) ,,,(3) University of Cincinnati Medical Center, Liberty Township,, OH, (4) University of Cincinanti Medical Center Proton Therapy Center, ,,(5) University of Cincinnati Medical Center, Cincinnati, OH


SU-E-BRC-6 (Sunday, 7/10/2022) 1:00 PM - 2:00 PM [Eastern Time (GMT-4)]

Ballroom C

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)

Contact Email