Long-Term Outcomes After Pencil Beam Scanning Carbon Ion Radiotherapy for Unresectable Retroperitoneal Sarcomas

Background: To evaluate the results of pencil beam scanning (PBS) carbon ion radiotherapy (CIRT) for unresectable retroperitoneal sarcomas (RPSs). Methods and Materials: From July 2015 to March 2024, twenty-five unresectable RPS patients received CIRT. Median prescription dose was 72 Gy (RBE) in 18 fractions. Primary endpoints included overall survival (OS), local control (LC) rates and toxicity. Dose-averaged linear energy transfer (LETd) and RBE-weighted dose parameters within gross tumor volume (GTV) were analyzed. Results: At a median follow-up of 53 (range, 12~110) months, the 3- and 5-year rates of OS and LC for an advanced (FNCLCC G 2~3, 92%; multifocal lesions, 60%) and heavily pre-treated population (prior surgery, 76%; re-irradiation, 28%) were 72.4%/53.6% and 61.4%/51.2%, respectively. GTV ≥ 116.86 cc predicted worse OS (p = 0.039). BED of 129.6 Gy (RBE) correlated with the most favorable OS (p = 0.011). No grade ≥ 3 toxicity occurred. LC correlated with higher mean LETd (54.8 ± 6.2 vs. 39.5 ± 6.4 keV/μm, p = 0.032) and greater V40keV/μm (89.1 ± 18.4% vs. 47.1 ± 22.8%, p = 0.048). Conclusions: CIRT with PBS is safe and effective for unresectable RPS. Larger GTV predicts worse OS, whereas higher LETd within GTV was associated with improved LC in this exploratory analysis.