A Study on a wide-field dose evaluation method in heavy-ion radiotherapy using whole-body CT and Monte Carlo simulation

Hirai, Yuta*; Koba, Yusuke*; Furuta, Takuya   ; Matsumura, Akihiko*; Shinsho, Kiyomitsu*

In radiation therapy, irradiation of normal tissues is unavoidable, posing a long-term risk of radiation-induced secondary cancers. Therefore, assessment of doses to normal tissues in both vicinity of treatment site and out-of-field is important. However, conventional treatment planning systems (TPS) for carbon radiotherapy (CIRT) limit dose calculation to the treatment region, leaving out-of-field doses to normal tissues largely unexplored. In this study, we applied RT-PHITS for CIRT, a Monte Carlo-based system that reconstructs treatment beam geometry from TPS data and enables detailed, whole-body dose evaluation. Five prostate cancer treatment plans were created using the TPS with whole-body CT data from The Cancer Imaging Archive (TCIA). RT-PHITS for CIRT reproduced TPS dose distributions near the target and further quantified distant doses down to 0.01% of the isocenter dose. These results demonstrate the capability of RT-PHITS for CIRT to perform particle-resolved, wide-field dose evaluations, providing a foundation for future studies on secondary cancer risk assessment in CIRT patients.