Space-making Particle Therapy Using Bioabsorbable Spacer for Bone and Soft Tissue Sarcomas: Two-institution Experience.

Background/aim: Particle therapy for bone and soft tissue sarcomas (BSTSs) is limited if the tumor is close to the intestine. Space-making particle therapy (SMPT) using a bioabsorbable spacer (BAS) may treat this condition. We aimed to evaluate SMPT outcomes using a BAS for BSTSs.

Patients and methods: We retrospectively identified 28 patients who underwent SMPT with BAS between November 2019 and August 2023. Outcomes and associations with each factor were analyzed using the Kaplan-Meier method and log-rank test. Adverse events were determined using CTCAE v5.0.

Results: The median age was 48.5 years (1-78), and the most common histological type was chordoma (25%). Among all patients, 71% were treated with proton therapy and 29% with carbon ion therapy. The most frequent dose fractionations were 70.4 Gy (relative biological effectiveness) in 16 fractions (36%) and 32 fractions (36%). The median follow-up period was 29.5 months (7-48). The 3-year overall survival, progression-free survival, and local control rates were 83%, 63%, and 80%, respectively. Tumor location (pelvis) and low aggressive sarcoma (chordoma and Ewing sarcoma) were significantly associated with better progression-free survival. Adverse events related to the BAS included grade 2 constipation in 7% of patients. Radiation toxicities included acute grade 2 and 3 dermatitis in 36% and 4% of patients, respectively. Late grade 2 toxicities included fractures (7%), peripheral motor neuropathy (4%), dermatitis (4%), superficial soft tissue fibrosis (4%), and skin pain (4%). Late grade 3 toxicities involved fractures (4%).

Conclusion: SMPT using a BAS appears effective and safe for treating BSTSs near critical organs, enabling curative dosing with minimal toxicity. Longer follow-up is needed to assess long-term outcomes.