Carbon Ion Radiotherapy for Retroperitoneal Sarcoma: A Single-Institution Study

Background/Objectives: Surgery remains the mainstay of treatment for retroperitoneal sarcoma (RPS); however, definitive therapeutic strategies for patients with insufficient surgical margins and unresectable disease owing to locally advanced RPS remain unclear. Carbon ion radiotherapy (CIRT) has been employed in patients with unresectable RPS. This study aimed to evaluate the effectiveness of CIRT in this patient population. Methods: A retrospective analysis was conducted in 76 patients with unresectable RPS treated with CIRT. Of these, 95% had a confirmed prognosis until 2022. In 74 patients, the prescribed relative biological effectiveness dose was 70.4 Gy, delivered in 16 fractions over 4 weeks. Respiratory gating was used, and spot scanning irradiation has been performed in all patients since 2016. Results: The 3- and 5-year overall survival rates for the entire cohort were 68.3% and 49.4%, respectively, with a median overall survival time of 58.1 months. The 3- and 5-year local control rates were 79.0% and 72.0%, respectively. Among 47 naïve patients with treatment-naïve tumors, the 3- and 5-year abdominal recurrence-free survival rates were 51.1% and 29.1%, respectively. Late adverse events of grade 3 or higher occurred in 4 (5.2%) patients. Conclusions: CIRT represents a definitive treatment option for patients with unresectable RPS. In the future, multicenter studies should be conducted to evaluate the effectiveness of CIRT for RPS in larger patient cohorts.