Plastic scintillator sheets for quality assurance in electron FLASH and minibeam radiation therapy.

FLASH radiotherapy has a significant potential in changing tumor treatment. However, the short duration, ultra-high dose-per-pulse (UHDP), and ultra-high dose rate (UHDR) of the beams needed to trigger the FLASH effect can easily lead to detector nonlinearities, hence posing serious challenges to dosimetry, beam monitoring, and quality control. Several technologies have been explored recently for these purposes. Among them, plastic scintillators seem particularly attractive because of their linear dose response, fast decay time, and water and tissue equivalence at a reasonable cost. Their high spatial resolution may be useful for spatially-fractionated dose distributions, which are also being investigated. The goal of this work was to assess the performance of a plastic scintillator sheet imaged by an optical imaging system as a potential tool to measure the dose distribution of low-energy electron beams delivered in UHDR/UHDP and spatially-fractionated modalities. A 0.5-mm-thick plastic scintillator was irradiated with 9-MeV electrons and imaged by a charge-coupled-device-based camera. The linearity of the dose response was verified, the capability to measure the percentage depth dose curve was evaluated in two configurations, and the Cerenkov contribution to the overall collected light was studied. The capability to image spatially-fractionated dose patterns was also investigated and compared to EGSnrc Monte Carlo simulations. The dose response was found to be linear up to almost 12 Gy/pulse (3 MGy/s average instantaneous dose rate), with a maximum deviation of 5%, while the spatial features of fractionated beams were reconstructed (measured spatial resolution of 156 μm$\mu{\rm m}$ ). Plastic scintillators are able to measure the percentage depth dose curve of electron FLASH beams, possibly in one acquisition and a single beam pulse, but a correction for the Cerenkov contribution has to be implemented for accurate dose measurements. These experiments suggest that plastic scintillator sheets seem interesting candidates for imaging dose distributions delivered with UHDR/UHDP and spatially-fractionated electron beams.