Coupled ionizing-radiation/optical-photon transport Monte Carlo simulations for characterisation of light signal in an optical fiber radioluminescence dosimetry system

Optical fiber radioluminescence (RL) dosimetry has gained prominence in modern radiation therapy, offering real-time measurement and high spatial resolution. Our research group has developed a system utilizing a polymethyl methacrylate (PMMA) transmission fiber coupled with a photodetector and various scintillators, including doped silica fibers. A critical challenge in RL dosimetry lies in distinguishing the stem signal, generated by the transmission optical fiber, from the primary light signal produced by the RL sensor. To address this issue, we employed the Geant4 simulation tool, allowing for the simultaneous tracking of ionizing radiation and optical photons. In this study, the Geant4-based code, TOPAS, was utilized to conduct Monte Carlo simulations, aiming to gain insights into the radioluminescence signal in an optical fiber RL dosimeter and specifically characterize the stem signal for enhanced measurement accuracy. The simulations encompassed interactions of a medical photon beam from an Elekta linac within a solid water phantom, subsequent energy deposition within the RL sensor, and the generation and transmission of light signals within the optical fiber. Our emphasis was placed on detailed characterization of the light signals originating from both the Ge-doped silica fiber and PMMA transmission fiber. The primary focus was not only to discern the stem signal from the main signal but also to differentiate between the fluorescence and Cerenkov signals. Importantly, our study showcases how Monte Carlo simulations can be used to spectrally distinguish the stem signal from the scintillation signal of the sensor. This provides valuable information, especially in scenarios where spectrometry is unavailable, contributing to the understanding and refinement of optical fiber RL dosimetry systems.