Pulse-resolved spatial dosimetry of ultra-high dose rate electron FLASH beams using optical imaging.

PURPOSE: Ultra-high dose rate FLASH radiotherapy has shown the potential to reduce normal tissue toxicity while maintaining tumor control. However, investigating the underlying radiobiological mechanisms requires dosimetry systems capable of resolving spatial and temporal characteristics of FLASH beams. In this work, an optical imaging system for pulse-resolved two-dimensional dosimetry of a 10 MeV electron FLASH beam with a PRF of 400 Hz was developed and characterized. METHODS: The system is based on a plastic scintillation plate imaged by a high-speed CMOS camera operating at 800 frames per second. Custom Python software was used for image acquisition and processing, including geometric correction, noise filtering, and automatic frame selection. The luminescence signal was corrected for Cerenkov light contributions and calibrated to absolute dose. The performance of the system was evaluated by comparing absolute dose, depth-dose distributions, and lateral dose profiles with simultaneously irradiated Gafchromic EBT-XD films. RESULTS: The absolute dose measured with the scintillation plate agreed with film within 4%. Comparisons of depth-dose curves and lateral profiles showed mean deviations ranging from 0.3%±2.1% to -5.3%±16.3%, with larger variations mainly attributed to film uncertainties at doses below 3 Gy. The system demonstrated high reproducibility and enabled visualization of individual pulses and pulse gaps during beam delivery. CONCLUSION: The presented optical imaging approach enables real-time, pulse-resolved measurement of both absolute dose and spatial dose distributions in electron FLASH beams. These results demonstrate the potential of scintillation-based optical imaging as a practical tool for beam characterization and as a candidate for a standard in FLASH dosimetry.