Electron and proton FLASH beam dosimetry using unified alanine, EBT-XD, and HD-V2 Gafchromic film dosimeters

Background

Ultra-high dose rate (UHDR) radiotherapy, or FLASH RT, has shown potential to spare normal tissues while maintaining tumor control. However, accurate dosimetry at UHDR remains challenging, as conventional ionization chambers suffer from recombination effects. Although radiochromic films and alanine dosimeters have both been investigated independently for FLASH dosimetry, their separate use hinders robust validation and direct comparison of their measurements.

Purpose

This study aims to develop and evaluate a unified dosimeter containing both alanine and radiochromic film for electron and proton FLASH beam dosimetry. The design allows for simultaneous, co-located irradiation of both dosimeter types, enabling a direct comparison between them. This configuration eliminates confounding factors such as positional offsets, alignment errors, and beam fluctuations, thereby facilitating the validation of measurements and enhancing confidence in FLASH dosimetry.

Methods

The unified alanine and EBT-XD/HD-V2 film dosimeter was designed with the same outer dimensions as the Advanced Markus chamber (PTW-Freiburg), allowing compatibility with commercial QA phantoms. Alanine and film dosimeters were calibrated under conventional electron and proton beams, traceable to absorbed dose to water from Co-60 gamma rays. The unified dosimeter was used to measure dose from a 9 MeV electron FLASH beam (Varian Clinac iX) and a 230 MeV proton FLASH beam (IBA machine), with alanine and film irradiated simultaneously at the same location.

Results

The alanine dosimeter measured the dose per pulse, instantaneous dose rate, and mean dose rate at a source-to-surface distance of 100 cm for the electron FLASH beam as 0.99 $\pm$ 0.02 Gy/pulse, 2.48 $\times$ 10⁵ Gy/s, and 357 Gy/s, respectively. The EBT-XD film showed good agreement (within a 2.0% relative difference) in the 10–30-Gy range, whereas the HD-V2 indicated a larger difference (up to 5.9%) compared to the alanine dosimeter. The mean dose rate for the proton FLASH beam, measured by the alanine dosimeter, was 115.4$\pm$ 1.1 Gy/s. The EBT-XD showed a 4.3% relative difference with the alanine dosimeter in the 10–30-Gy range.

Conclusions

The unified alanine and film dosimeters enabled simultaneous irradiation of the alanine and the films, with combined relative standard uncertainties of 2.4% (k = 1) for the alanine dosimeter and 3.5% (k = 1) for the EBT-XD films at the electron FLASH beam. For the proton FLASH beam, these uncertainties were 3.2% (k = 1) for both the alanine dosimeter and the EBT-XD films. Until dosimetry guidelines for the FLASH RT community are established by a working group such as AAPM TG-359, the dosimetry protocol proposed in this study can serve as a promising approach for FLASH RT facilities worldwide.