Three-year clinical experience and data analysis of treatment plans using a rotating gantry for carbon ion radiotherapy

Purpose This study aimed to summarize clinical treatment-planning data for carbon-ion radiotherapy (CIRT) using a rotating gantry. Materials and methods A total of 730 CIRT rotating gantry treatment plans between May 2022 and May 2025 were analyzed. Planning-related parameters stored in the treatment planning system and treatment delivery system were evaluated. Results Among the 730 plans, liver tumors were the most common indication, followed by bone & soft-tissue tumors, head and neck tumors, and pancreatic cancers. In 94.5% of all plans, 2–4 fields were used. Of all beam angles applied, 78.8% were located in the upper gantry range, while beam directions potentially intersecting the couch edge were rarely selected. The maximum field dimensions used were most frequently around 10 cm and 20 cm, with 21.8% of plans requiring fields larger than 20 cm. The maximum and minimum particle energies used were approximately 418.5 MeV/u and 55.6 MeV/u, respectively. Conclusion These findings provide fundamental insights for future improvements, including expanding the range of beam angle applications constrained by physical limitations, establishing robust and site-specific beam angle selection strategies, and increasing the maximum field size to avoid uncertainties associated with patch field irradiation.