Outcomes After Fast Neutron Therapy for Locally Advanced Base of Tongue Adenoid Cystic Carcinoma.

Purpose: Base of tongue (BOT) adenoid cystic carcinoma (ACC) is a rare, locally aggressive malignancy that is typically associated with a high rate of disease recurrence. Surgery alone is rarely curative and highly morbid. Outcomes of systemic therapies and low linear energy transfer radiation therapy (RT) are generally poor. Fast neutron therapy (NRT), a form of high linear energy transfer radiation, is advantageous in treating radioresistant salivary gland malignancies. Long-term cancer control and swallowing outcomes in patients with BOT ACC treated with NRT were retrospectively reviewed. Methods: Between 1994 and 2023, 63 patients ≥18 years (median age 61) with biopsy-confirmed BOT ACC, not treated previously with surgery or RT, underwent 3D-conformal NRT or intensity-modulated neutron therapy (IMNT) for localized M0 (77.8%) or M1 (22.2%) disease. Most patients had (T3-4) primary tumors. Kaplan-Meier method was used to estimate locoregional recurrence-free (LRFS), distant metastasis-free, and overall survival. Late toxicities (≥90 days after NRT) were reported per Common Terminology Criteria for Adverse Events v5.0. Treatment-related and post-treatment percutaneous endoscopic gastrotomy (PEG) tube utilization was evaluated. Results: For the entire cohort, 5-year LRFS was 53.4%, and 5-year overall survival was 75.1%. Among M0 patients, 3-year LRFS was 72.4%, 5-year LRFS was 59.4%, and 5-year distant metastasis-free was 44.1%. Prophylactic PEG tubes were placed in 58.1% of patients; 14.5% remained PEG-dependent ≥90 days post-treatment. Common toxicities included xerostomia and dysphagia. Among evaluable patients, 33.3% required PEG placement after 90 days due to disease or salvage therapy. Neither patient treated with IMNT required long-term PEG use. Conclusions: This is the first study to report clinical outcomes of NRT for BOT ACC. Swallowing outcomes were acceptable, with a low rate of persistent PEG dependence. NRT achieved significant locoregional control in many locally advanced primary tumors. Additional improvements in the therapeutic ratio and functional outcomes may be achievable with IMNT.