Evaluation of scintillation detectors for ultrahigh dose-rate x-ray beam dosimetry

FLASH-Radiotherapy (FLASH-RT) is an emerging radiotherapy technique delivering ionizing radiation beam at ultrahigh dose rates (UHDR), typically =40 Gy/s. Animal studies have demonstrated the safety and efficacy of the technique in killing tumor cells while significantly reducing radiation toxicity in normal tissues, compared to conventional radiotherapy (dose-rate exposure <0.03 Gy/s). A reliable real-time dosimeter system is crucial for the characterization of the so-called 'FLASH-effect' and an accurate beam delivery. Standard dosimeters for conventional radiotherapy saturate at this high-intensity field or cannot provide real-time measurements. In previous work, optical fiber inorganic scintillating detectors (ISDs) showed excellent linearity with shutter exposure time and tube current, indicating scintillating signals independent of the dose and dose rate, respectively. This study aims to benchmark the performance of the ISD with plastic scintillating detectors (PSDs) for an ultrahigh dose-rate x-ray beam irradiation. Relative scintillator output, signal linearity with dose and dose rate, signal-to-noise ratio (SNR), signal stability and reliability were evaluated for all detectors. In a UHDR x-ray beam irradiation, the ISDs produced a larger SNR than the PSDs. All detectors showed good linearity with tube current (R2 > 0.975) and shutter exposure (R2 > 0.999). Gd2O2S:Tb showed excellent repeatability (coefficient of variation (CV) <0.1%) compared to other detectors, while the PSDs resulted in the highest reliability for a UHDR beam measurement with a CV of <0.1%. A further investigation regarding the positioning uncertainty of the ISDs during irradiation due to the detector's angular dependency and the optimal design of the scintillator detectors for UHDR applications are required.