ESTRO 2024 - Abstract Book

S3204

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Beam monitoring for radiotherapy at Ultrahigh Dose Rates (UHDR) using pulsed beams, i.e. Very High Energy Electron (VHEE) beams, is a major challenge, as a very high dose-per-pulse is necessary to achieve the mean dose rates required for FLASH. The currently used transmission ionisation monitor chambers exhibit large recombination effects under these conditions. This work presents the Fibre Optic FLASH Monitor (FOFM) as a novel technology for real-time beam monitoring for FLASH beams. The FOFM uses a CMOS camera to measure the Cherenkov radiation emitted by the beam as it passes through the fibre optic sensors and is able to provide pulse-by-pulse measurements of the beam intensity and profile in the UHDR regime.

Material/Methods:

The Fibre Optic FLASH Monitor (FOFM) consists of two arrays (one each in the vertical and horizontal axes) made up of fused silica optical fibres 0.4 mm in diameter. Each of the arrays are read out using a 2.3 MP Basler ace CMOS camera. Experiments were conducted at the CLEAR facility at CERN using 200 MeV and 160 MeV electrons to assess the FOFM's response linearity as the dose-per-pulse (DPP) measured by radiochromic films in water, increased into the UHDR range essential for FLASH radiotherapy. The FOFM's capability to provide pulse-by-pulse beam profile measurements was also compared to beam profile measurements made using radiochromic film and scintillating YAG screens.

Results:

The FOFM demonstrated excellent response linearity from 0.9 Gy/pulse to 57.4 Gy/pulse (R² = 0.999). Furthermore, it exhibited no significant energy dependence between 160 MeV and 200 MeV or instantaneous dose rate dependence between 3.2 × 10 9 Gy/s (400 pC/bunch) and 6.8 × 10 8 Gy/s (70 pC/bunch), as shown in the figure. Gaussian fits applied to vertical beam profile measurements indicated that the FOFM could accurately provide pulse-by-pulse beam size measurements, agreeing within < 9% of YAG screen measurements and < 3% of radiochromic film measurements. Measurements with SiPM and PMT photodetectors were also conducted and demonstrated that they could be integrated into the final FOFM design to provide information about the instantaneous dose rate.