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.