Abstract Background FLASH radiotherapy requires further preclinical and clinical investigation to establish its biological effectiveness and define optimal beam parameters. In conventional (CONV) radiotherapy, redundant beam termination systems are a cornerstone ensuring patient safety, yet analogous safeguards for FLASH delivery are not well established, creating a critical barrier to safely enabling such studies. Purpose To develop and evaluate a real‐time, in vivo, point‐dose monitoring system capable of terminating electron FLASH beam delivery as an additional monitoring system on a modified medical linear accelerator (LINAC). Methods A decommissioned LINAC was modified to deliver electron FLASH beams with stable dose per pulse (DPP) at 300 Hz pulse frequency. A commercial plastic scintillation detector system was adapted through hardware and firmware modifications to enable pulse‐based and dose‐based beam termination via the LINAC MLC interface. The detector was cross‐calibrated against radiochromic film under FLASH conditions. System performance was evaluated through measurements of control accuracy, and detector response as a function of DPP. Results Stable electron FLASH delivery was achieved with an average dose rate of 127.514.91 Gy/s and an approximate beam energy of 5.2 MeV. Pulse‐based control terminated delivery within +3 pulses of the requested value (requested 1‐20 pulses), with overshoot attributable to downstream circuitry latency. Dose‐based control agreed with film measurements within 1.110.81 Gy for surface‐based control (in vivo setup) and −1.450.38 Gy at depth (stable dosimetry) (tested dose deliveries between 2‐15 Gy). The detector response versus DPP in the FLASH range (0.11–0.78 Gy/p) could be roughly approximated as linear before detector saturation, with only marginal improvement seen when using quadratic fitting. Conclusion A modified scintillation‐based system was implemented as a real‐time in vivo beam termination mechanism for electron FLASH radiotherapy under stable DPP and specific experimental conditions. While not intended for primary beam control, the system may provide a practical redundant safety layer for mitigating gross delivery errors in experimental and translational FLASH applications.
DeFrancisco et al. (Tue,) studied this question.