Definitive proton beam therapy for adenoid cystic carcinoma of the nasopharynx involving the base of skull.

Objectives: Management of unresectable adenocystic carcinoma (ACC) of the nasopharynx is challenging given the high dose required for tumor control while respecting dose constraints. We evaluated long-term outcomes and toxicity in patients with unresectable ACC of the nasopharynx treated with definitive proton beam therapy.

Methods: Between 2000 and 2013, 14 patients with ACC of the nasopharynx were treated. Ninety-three percent had T4 disease. All had involvement of the skull base. Seventy-nine percent and 21% of patients underwent biopsy and endoscopic debulking surgery, respectively. Median dose was 73.8Gy (RBE). Fifty percent of patients received concurrent chemotherapy. Locoregional control and overall survival probabilities were estimated by the Kaplan-Meier method. Treatment toxicity was scored by the Common Terminology Criteria for Adverse Events version 4.0.

Results: Median follow-up of surviving patients was 69months. There were 3 local, 1 regional, and 4 distant failures. Median time of local failures was 69months (range: 63-161). All local recurrences were within previous high-dose regions. Four patients developed metastatic disease at a median of 30months (range: 4-64). Five-year overall survival was 59%. The most common cause of death was due to metastatic disease. There was one acute grade 3 toxicity. No patient required gastrostomy tube or hospitalization. Three patients developed grade 3 or higher late toxicity. Two of these patients received combined modality treatment. With 176months follow-up, no second cancer was observed.

Conclusion: Proton beam therapy results in promising local control with acceptable toxicity in patients with unresectable ACC of the nasopharynx. As late recurrence is common, longer follow-up is necessary to confirm our findings.