Proton Therapy for Major Salivary Gland Cancer: Clinical Outcomes

PURPOSE: To report clinical outcomes in terms of disease control and toxicity in patients with major salivary gland cancers (SGCs) treated with proton beam therapy. MATERIALS AND METHODS: Clinical and dosimetric characteristics of patients with SGCs treated from August 2011 to February 2020 on an observational, prospective, single-institution protocol were abstracted. Local control and overall survival were calculated by the Kaplan-Meier method. During radiation, weekly assessments of toxicity were obtained, and for patients with ≥ 90 days of follow-up, late toxicity was assessed. RESULTS: Seventy-two patients were identified. Median age was 54 years (range, 23-87 years). Sixty-three patients (88%) received postoperative therapy, and nine patients (12%) were treated definitively. Twenty-six patients (36%) received concurrent chemotherapy. Nine patients (12%) had received prior radiation. All (99%) but one patient received unilateral treatment with a median dose of 64 GyRBE (relative biological effectiveness) (interquartile range [IQR], 60-66), and 53 patients (74%) received intensity-modulated proton therapy with either single-field or multifield optimization. The median follow-up time was 30 months. Two-year local control and overall survival rates were 96% (95% confidence interval [CI] 85%-99%) and 89% (95% CI 76%-95%], respectively. Radiation dermatitis was the predominant grade-3 toxicity (seen in 21% [n = 15] of the patients), and grade ≥ 2 mucositis was rare (14%; n = 10 patients). No late-grade ≥ 3 toxicities were reported. CONCLUSION: Proton beam therapy for treatment of major SGCs manifests in low rates of acute mucosal toxicity. In addition, the current data suggest a high rate of local control and minimal late toxicity.