Purpose: Beam monitoring for FLASH-RT is challenging due to ultra-high dose rate conditions. In this study, dose-rate independent radioluminescent based techniques are presented for pulse-resolved beam characterization and real-time feedback to a FLASH enabled Linac.
Methods: A modified clinical linac delivered a radially symmetric 10 MeV electron FLASH beam (~300 Gy/s, 1 Gy/Pulse) to isocenter. Lateral projected 2D dose distributions for each linac pulse were imaged in a fluorophore-doped water tank using a time-gated camera. An inverse Abel transform reconstruction provided 3D images for on-axis depth dose values. The central axis depth dose values were compared against film data. For pulse-by-pulse feedback, a scintillating point dosimeter was coupled to a gated integrator and a field programmable gate array (FPGA)-based control system.
Results: The camera images were reconstructed at a spatial resolution of 0.5 mm. The Dmax, R₅₀, and Rp measured with film and camera based method agreed to within 1 mm for a 1.5 cm circular beam and the beam with jaws wide open (40 x 40 cm²). Central axis cross beam profiles for both beams agreed with film data with > 95% passing rate (2%/2mm gamma criteria). Beam energy measurements based on the R50 criteria using optical measurements revealed a stable beam energy between adjacent pulses. However, a ramp-up period was observed where the dose per pulse increased for the first few pulses and then stabilized. The dose-read out scheme for individual pulses using the gated integrator and the FPGA based system was characterized.
Conclusion: Radioluminescence based techniques were presented as a high spatio-temporal resolution technique for online monitoring of FLASH beams. The presence of a ramp-up requires that feedback to the Linac should be based on dose accumulation. Therefore, the gated integrator and the FPGA based control system can enable accurate feedback for FLASH.