A Scintillator Beam Monitor for Real-Time FLASH Radiotherapy

BACKGROUND: FLASH Radiotherapy (RT) is a potentially new cancer radiotherapy technique where an entire therapeutic dose is delivered in about 0.1 s and at ~1000 times higher dose rate than in conventional RT. For clinical trials to be conducted safely, precise and fast beam monitoring that can generate an out-of-tolerance beam interrupt is required. PURPOSE: A FLASH Beam Scintillator Monitor (FBSM) is being developed based in part on two novel proprietary scintillator materials with capabilities that conventional RT detector technologies are unable to simultaneously provide: 1) large area coverage; 2) a low mass profile; 3) a linear response over a broad dynamic range; 4) radiation tolerance; 5) real-time analysis IEC-compliant fast beam-interrupt signal; 6) true two-dimension beam imaging with excellent spatial resolution. This paper describes the design concept and reports results from prototype devices. METHODS: The FBSM uses two types of proprietary low mass (< 1 mm WE), non-hygroscopic, radiation tolerant scintillator materials (designated PM and HM: polymeric and hybrid material, respectively) that are viewed by high frame rate machine-vision cameras. Folded optics using mirrors enable a thin monitor profile of ~10 cm. The scintillator selection is determined by the specific beam type and delivery configuration. An FPGA-based data acquisition system currently under development generates real-time analysis and a beam interrupt signal on a time scale appropriate to the FLASH RT beam modality: 100–1000 Hz for pulsed electrons and 10–20 kHz for quasi-continuous scanning proton pencil beams. Two prototype monitor devices were fabricated and tested in various radiation beams that include heavy ions, low energy protons at nA currents, FLASH level dose per pulse electron beams, and in a hospital radiotherapy clinic with electron beams. RESULTS: Results presented in this report include image quality, response linearity, radiation hardness, spatial resolution, and real-time data processing. Both scintillator materials were found to be highly radiation damage resistant. PM and HM scintillator exhibited no measurable drop in signal after a cumulative dose of 9 kGy and 20 kGy respectively. HM showed a small −0.02%/kGy signal decrease after a 212 kGy cumulative dose resulting from continuous exposure for 15 minutes at a high FLASH dose rate of 234 Gy/s. These tests established the linear response of the FBSM with respect to beam currents, dose per pulse, and material thickness. Comparison with commercial Gafchromic film indicates that the FBSM produces a high resolution 2D beam image and can reproduce a nearly identical beam profile, including primary beam tails. Double Gaussian fits to the beam profile show that the FBSM and Gafchromic film yield the same fit parameters to within 1.4% of their average values. At 20 kfps or 50 μs/frame, the real-time computation and analysis of beam position, beam shape, and beam dose takes < 1 μs. CONCLUSIONS: The FBSM is designed to provide real-time beam profile monitoring over a large active area without significantly degrading the beam quality. Prototype devices have been staged in particle beams at currents of single particles up to FLASH level dose rates, using both continuous ion beams and pulsed electron beams. Using our novel scintillators, beam profiling has been demonstrated for currents extending from single particles to 10 nA currents. Radiation damage is minimial and even under FLASH conditions would require ≥ 50 kGy of accumulated exposure in a single spot to result in a 1% decrease in signal output. Spatial resolution was comparable to radiochromic films. Real-time data processing, taking < 1 μs, is being implemented in firmware for 10–20 kHz frame rates for continuous proton beams and for pulsed electron beams from 100–1000 Hz.