ESTRO 2024 - Abstract Book

S3200

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Development of next-generation ion chambers for reference dosimetry in FLASH

Kevin Liu 1 , Ahtesham Khan 2 , Shannon Holmes 3 , Larry DeWerd 2 , Sam Beddar 1 , Emil Schueler 1

1 MD Anderson, Radiation Physics, Houston, USA. 2 University of Winsconsin, Medical Physics, Madison, USA. 3 Standard Imaging, Physics, Middleton, USA

Purpose/Objective:

Purpose: Ionization chambers are recommended for reference dosimetry applications of medical linear accelerators, but their performance is severely compromised in ultra-high dose-rate (FLASH) beamlines. In this study, we developed novel prototype ion chambers capable of accurate dose measurements in UHDR beamlines and have pinpointed their various dependencies on UHDR beam parameters.

Material/Methods:

Materials and Methods: Three different ion chambers were evaluated: a A11 parallel plate chamber with variable electrode spacing from 0.2-1.0 mm, a A11 and A10 parallel plate chamber with fixed electrode spacing of 0.3 mm. The chamber response (charge collection efficiency (CCE) and polarity (P pol ) were investigated as a function of different beam parameters such as dose-per-pulse (DPP), pulse width (PW), mean and instantaneous dose-rate, as well as chamber design such as collector diameter, electrode spacing, and electric field strength.

Results:

The chamber response was found to be dependent on DPP and PW, whose dependencies can be mitigated with larger electric field strengths (≥1000 V/mm) and smaller electrode spacing (≤3 mm).

In comparing ion chambers with different electrode spacings (0.2-1.0 mm), we measured a higher CCE in ion chambers with smaller electrode spacing as a function of DPP (1-9 Gy) when the electric field strength (1000 V/mm) remained constant.

For ion chambers with identical electrode spacings, higher electric field strengths were found to yield higher CCE at larger DPPs.

However, at electric field strengths 1000 V/mm or higher, there was no significant advantage (<1-2%) in the measured CCE in the ion chambers as a function of DPP under the experimental conditions tested.

A PW dependence was observed at low electric field strengths (500 V/mm), but at electric field strengths of 1000 V/mm and higher, the effects become negligible.

When comparing the CCE in ion chambers with identical electrode spacing (0.3 mm), but large differences in the collector diameter, we found that the ion chamber with a smaller collector diameter yielded higher CCE as a function of DPP.