Scintillating Micro-Probe Detectors for Conventional and FLASH Proton Dosimetry: Optical Transport and Geometry...

Optical fiber-based inorganic scintillating detectors (ISDs) are increasingly investigated for radiation therapy applications, including microbeam and ultra-high-dose-rate dosimetry. Accurate characterization of these compact detectors is particularly important for proton beams delivered under high-flux and FLASH conditions. Simplified analytical and ray-tracing models can estimate detector saturation limits but may inadequately account for internal optical scattering and the fiber insertion geometry within the scintillating volume, leading to inaccurate estimates of the effective sensitive volume and dose-rate response. In this study, we developed a detailed computational framework that explicitly models the fiber-scintillator geometry and volumetric optical photon transport. Five detector geometries with different sensitive volumes were evaluated under 25 and 230 MeV proton irradiation using a fully volumetric GATE Monte Carlo optical transport model incorporating intra-dosimeter optical scattering. The results revealed a clear inverse relationship between detector sensitive volume and maximum measurable dose rate before saturation. At 25 MeV, the maximum measurable dose rate (D˙max) ranged from 364.5 ± 7.6 Gy/s for the smallest detector to 13.5 ± 0.2 Gy/s for the largest detector, while at 230 MeV, it ranged from 392.6 ± 8.1 Gy/s down to 15.6 ± 0.3 Gy/s at 230 MeV, respectively. These findings demonstrate that fiber insertion geometry and volumetric optical scattering significantly influence the dose-rate response and saturation threshold of scintillating micro-probes. Accurate modeling of these effects is therefore essential for optimizing detector geometry, establishing reliable quantitative dosimetry across conventional and proton-FLASH dose-rate regimes.