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

## TL;DR

Carbon ion radiotherapy shows promising survival and control rates for patients with unresectable retroperitoneal sarcoma.

## Contribution

This study provides clinical evidence supporting carbon ion radiotherapy as a definitive treatment for unresectable retroperitoneal sarcoma.

## Key findings

- 3- and 5-year overall survival rates were 68.3% and 49.4%.
- Local control rates were 79.0% and 72.0% at 3 and 5 years.
- Late grade ≥3 adverse events occurred in 5.2% of patients.

## Abstract

Retroperitoneal sarcoma (RPS) is a rare and challenging malignancy, particularly when surgical resection is not feasible due to anatomical complexity or patient-related factors. Carbon ion radiotherapy (CIRT), with its superior dose distribution and high linear energy transfer, offers a promising alternative for unresectable RPS. This single-institution retrospective study evaluated the clinical outcomes of 76 patients treated with CIRT between 2000 and 2022. The 3- and 5-year overall survival rates were 68.3% and 49.4%, respectively. The 3- and 5-year local control rates were 79.0% and 72.0%, respectively. Among 47 treatment-naïve patients, the 3- and 5-year abdominal recurrence-free survival rates were 51.1% and 29.1%, respectively. Late grade ≥3 adverse events occurred in 5.2% of patients. Compared to the STRASS trial (EORTC-62092), our cohort presented a more medically complex population, excluded from standard surgical approaches. These findings support CIRT as a definitive treatment option for unresectable RPS, warranting further multicenter studies to validate its efficacy and explore histology-specific therapeutic strategies.

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.