Re-irradiation using boron neutron capture therapy for recurrent oral squamous cell carcinoma: the impact of tumor volume and early response on survival.

PURPOSE: Re-irradiation for recurrent oral cancer remains clinically challenging due to limited treatment efficacy and severe toxicity. In this study, we evaluated the clinical outcomes of boron neutron capture therapy (BNCT) and explored prognostic factors associated with survival in oral squamous cell carcinoma (OSCC). METHODS: We conducted a retrospective single-institution cohort study of 45 patients with recurrent OSCC treated with accelerator-based BNCT between June 2020 and December 2022. Overall survival (OS), local control (LC), and progression-free survival (PFS) were estimated using the Kaplan-Meier method. Prognostic factors were assessed using Cox proportional hazards models. RESULTS: The objective response rate was 80%. At a median follow-up of 25 months, the 2‑year OS, LC, and PFS rates were 51.1%, 31.8%, and 22.2%, respectively. In the univariate analysis, tumor volume was associated with OS (p = 0.036). Exploratory analyses suggested that tumor volumes ≥ 20 cm3 may be associated with inferior OS, and ≥ 25 cm3 with inferior OS and LC (p < 0.05). Patients achieving complete response (CR) within 90 days had better OS than those without CR (HR, 3.73; 95% CI, 1.50-9.30; p = 0.005). Grade ≥ 3 acute mucositis occurred in 49% of patients, and late grade ≥ 3 osteoradionecrosis occurred in 10%. CONCLUSION: BNCT may be an option for selected patients with recurrent OSCC after prior irradiation. Favorable responses and OS were observed, although LC remained limited and toxicities required careful management. Baseline tumor volume warrants further study for survival risk stratification, while early response may aid prognostic assessment.