Carbon Ion Radiotherapy for Unresectable Primary or Recurrent Soft Tissue Sarcomas: Long-Term Outcomes based on the Local Effect Model System

Background The application of carbon ion radiotherapy (CIRT) for unresectable primary or recurrent soft tissue sarcoma (STS) remains controversial and existing data are limited. We herein present our institutional experience with CIRT for unresectable primary or recurrent STS. Methods We retrospectively evaluated the outcomes of CIRT in unresectable primary or recurrent STS patients at our center. We assessed the 4 - and 5 - year local control (LC), overall survival (OS), distant metastasis free survival (DMFS), progression free survival (PFS), as well as acute and late toxicities. Additionally, we analyzed the prognostic factors associated with the treatment outcomes. Results Between June 2015 and August 2022, 48 consecutive patients with unresectable primary or recurrent STS were treated with CIRT at our center. The median follow-up duration was 48.6 months. The 4-year LC, OS, DMFS, and PFS rates were 65.8%, 59.7%, 50.3%, and 36.7%, respectively. The 5-year LC, OS, DMFS, and PFS rates were 60.3%, 54.7%, 44.0%, and 28.0%, respectively. The median survival time was 67.3 months. Late toxicities included grade 3 dermatitis (n = 2, 4.2%), grade 3 arthrofibrosis (n = 1, 2.1%), and grade 4 neuropathy (n = 1, 2.1%). Patients who received a higher prescribed dose (BED ≥ 129 Gy) exhibited significantly better LC ( p  = 0.010) and PFS ( p  = 0.004) compared to those who received a lower prescribed dose. A pre-CIRT neutrophil-to-lymphocyte ratio (NLR) ≥ 3 was associated was significantly associated with inferior DMFS ( p  = 0.020). Conclusions In patients with unresectable primary or recurrent STS, CIRT may serve as a potentially safe and effective treatment option.