低線量放射線に対するエネルギー付着および応答のマイクロスケール解析のための新規システム

Micrometer-scale evaluation of energy deposition is important for radiation protection and therapy as well as for advancing knowledge of responses to radiation in materials and biological systems. Due to the stochastic nature of radiation interactions, there is significant variation in energy deposition in micrometer-sized targets, especially at low doses. This variability underscores the need for a framework for microdosimetry, particularly in low-dose scenarios.

The goal of this work is to develop a novel system for micron-scale characterization of energy deposition and response to radiation that is applicable at low doses, using a combination of Monte Carlo (MC) simulations and experimental techniques.

EBT3 radiochromic film samples are irradiated to absorbed doses of 0.003-0.5 Gy using the 6-MV beam from a clinical linear accelerator. To quantify energy deposition, MC simulations of the experimental irradiations are conducted to evaluate specific energy deposited within micron-scale voxels in the active layer of the film. To investigate the dose response of the film, the following two methods are employed: (i) flatbed scanner measurement of changes in optical density (OD) of the film, and (ii) Raman spectroscopy (RS) to measure response intensity across doses with micron-scale resolution. Experimental film responses are compared to predictions from the microdosimetric one-hit model.

Specific energy distributions obtained from MC simulations show large variation in energy deposition at low doses and within small targets; the "microdosimetric spread" (relative standard deviation) is significantly higher ( $>$ 10 times) at 0.003 Gy than at 0.5 Gy, and is observed to decrease with increases in dose and target size. Both RS and OD measurements exhibit a near linear dose-response relationship, reflecting the film's sensitivity across micro- and macroscopic spatial scales. Overall, the OD and RS values determined using the one-hit model with MC-obtained specific energy distributions fit well to experimental measurements, with percentage differences up to 15 and 9.8%, respectively. An initial comparison of the relative standard deviation of RS and OD measurements (corrected for offset signal) shows qualitative agreement with the trends observed for MC-determined microdosimetric spread.

This study provides first results of a system that combines simulations with experimental techniques to investigate radiation response in micron-scale targets, with a focus on low-dose radiation exposure. The system shows promise in enabling future investigations of energy deposition within small volumes at low doses, where biological responses may be heterogeneous as some cells may receive high energy deposits and incur damage, while others may experience minimal or no deposition.