Author: Motohiro Kawashima, Adrian Kugara, Masao Nakao, Shuichi Ozawa, Mutsumi Tashiro, Maria Varnava 👨🔬
Affiliation: Gunma University Heavy Ion Medical Center, Hiroshima High-Precision Radiotherapy Cancer Center 🌍
Purpose: This study aimed to evaluate the time-dependent dose rate characteristics of a scanning carbon ion beam using a fast-response plastic scintillator detector (PSD), with a focus on its potential applicability to advanced radiotherapy techniques such as FLASH.
Methods: A Blue Physics Model 10 PSD, featuring a 1 mm diameter and length, was employed to measure dose rates in a 290 MeV carbon ion beam. The beam was configured as a 60 mm spread-out Bragg peak (SOBP) in a spot-scanning mode. Irradiation covered a 10 × 10 cm field consisting of 51 × 51 spots. A ridge filter of 3 mm thickness was applied per layer during 30-layer irradiation using range shifters. Dose rate measurements focused on the 18th layer to minimize LET-related corrections. The PSD was calibrated against absolute dose values obtained from a PTW31014 ionization chamber (0.015 cc) under identical conditions.
Results: Each row of 51 spots was scanned in approximately 3 ms, generating distinct signal peaks per row. Dose rate peaks were observed for 19 central rows, reflecting irradiation uniformity. The maximum dose rate measured was 42.0 Gy/s, approaching the FLASH therapy threshold (>40 Gy/s). This result underscores the importance of dose rate monitoring, even during standard scanning beam operations, for potential application to FLASH radiotherapy.
Conclusion: This study successfully demonstrated the capability of the PSD to measure dose rates with sub-millisecond time resolution in scanning beams. The method’s high temporal resolution makes it suitable not only for precise dose assessments in conventional scanning beams but also for characterizing dose rates in advanced applications such as FLASH therapy, highlighting its versatility in the evolving field of carbon ion radiotherapy.