Carbon-Ion Radiation Therapy for Unresectable Solitary Fibrous Tumor of the Intermediate Category: A Nationwide Multi-institutional Observational Study by the Japan Carbon-Ion Radiation Oncology Study Group

Purpose To evaluate the efficacy and safety of carbon-ion radiation therapy(C-ion RT) for patients with unresectable intermediate-category solitary fibrous tumor (SFT) in a nationwide multi-institutional observational study. Methods and Materials This study included patients with unresectable intermediate-category SFT treated with C-ion RT at institutions affiliated with the Japan Carbon-Ion Radiation Oncology Study Group between April 2016 and December 2023. Primary endpoints were local control (LC) and overall survival. Secondary endpoints included progression-free survival, toxicity, and quality of life (QOL). Survival outcomes were analyzed using the Kaplan–Meier method. Toxicities were evaluated using the Common Terminology Criteria for Adverse Events version 5.0. QOL was assessed using the Functional Mobility Scale and EuroQol 5-Dimension 5-Level. Results Ten patients from 3 institutions were enrolled in this study. The prescribed dose was 64.0 to 70.4 Gy in 16 fractions. The median follow-up was 49 months. Four-year overall survival, LC, and progression-free survival rates were 100% (95% CI, not available [NA]-NA), 100% (95% CI, NA-NA), and 67% (95% CI, 20%-90%), respectively. No local recurrences occurred, and 2 patients developed distant metastases. One patient experienced grade 2 late toxicity (peripheral neuropathy), and no grade ≥3 toxicities were observed. QOL scores were preserved throughout treatment. The median baseline EuroQol 5-Dimension 5-Level utility score was 0.85 (IQR, 0.76-0.97), and the median score at the final observation was 0.94 (IQR, 0.88-1.00), with no significant difference observed ( P = .225). Conclusions C-ion RT demonstrated favorable LC without severe toxicity, while maintaining QOL in patients with unresectable intermediate-category SFT.