Comprehensive characterization and validation of a fast-resolving (1000 Hz) plastic scintillator for ultra-high dose rate electron dosimetry.

BACKGROUND

The normal tissue sparing effect of ultra-high dose rate irradiation (≥40 Gy/s, UHDR), as compared to conventional dose rate (CONV), has attracted significant research interest for FLASH radiotherapy (RT). Accurate, dose rate independent, fast-responding dosimeters capable of resolving the spatiotemporal characteristics of UHDR beams are urgently needed to facilitate FLASH research and support its clinical translation. Tissue-equivalent scintillators, with millimeter-level spatial resolution and millisecond-level temporal resolution, possess these required characteristics and show strong potential for use in UHDR dosimetry.

PURPOSE

We investigated the performance of the HYPERSCINT RP-FLASH scintillator system at up to 1000 Hz sampling frequency (f) for UHDR electron beam dosimetry.

METHODS

The scintillator was exposed to CONV and UHDR electron irradiation using a LINAC-based FLASH platform. Its spectral characteristics were delineated with a four-component calibration, followed by a signal-to-dose calibration using 18 MeV CONV electron beam. The dose linearity and dosimetric accuracy in response to CONV and UHDR irradiation at 1 and 1000 Hz f were quantified against ion chamber and EBT-XD film measurements. The response of the scintillator system was investigated as a function of beam energy (6 and 18 MeV), field size (2 × 2 to 25 × 25 cm), dose per pulse (DPP, 0.8-2.3 Gy/pulse), and pulse repetition frequency (PRF, 30-180 Hz). Relative signal sensitivity was quantified against accumulated dose to account for the scintillator's radiation degradation. Pulse-resolved dose measurements at 18 MeV UHDR, obtained using the scintillator with 1000 Hz f for a train of 10 pulses at 180 Hz PRF, were validated with a PMT-fiber optic scattered radiation detector.

RESULTS

The scintillator system at 1 Hz f demonstrated high accuracy in dose measurements, remaining within 0.5% of ion chamber measurements over the dose range of 0.1-35 Gy under CONV irradiation. For the UHDR irradiation, the scintillator showed <3% dose error compared to film measurements up to 40 Gy at 1000 Hz f. Its response was found to be minimally dependent on energy, field size, and PRF. The scintillator under-responded by ∼4% over the 0.8-2.3 Gy/pulse range, although the dose difference relative to film remained within 2%. The radiation degradation of the scintillation detector followed a 2-order polynomial fit between 0 and 10 kGy, and a linear fit with a slope of -2.6%/kGy in the range of 0-2 kGy. The pulse-resolved dose measured by the scintillator was verified to be within 3% accuracy when compared to the measurements obtained using the PMT-fiber optic detector.

CONCLUSIONS

With routine dose calibration to account for radiation induced degradation, the fast-responding scintillator system can accurately provide millisecond-resolved inter-pulse measurements for electron beams at conventional and ultra-high dose rates, with minimal dependence on beam parameters. This suggests that the HYPERSCINT RP-FLASH scintillator system could serve as a detector of choice for electron FLASH research.