Adenoid Cystic Carcinoma of the Nasopharynx: Response to Radiation Therapy and Outcomes in a Retrospective Case Series

Background: Adenoid cystic carcinoma (ACC) is a malignant salivary gland tumor with high rates of local recurrence, frequent perineural invasion, and a tendency toward distant metastasis. ACC is typically considered a radioresistant tumor, with surgery being the primary treatment modality. Nasopharyngeal ACC (NACC) is rare, and a small percentage of advanced nasopharyngeal ACC tumors invade the skull base. Due to difficulty of access and the morbidity of surgical resection, these tumors are typically treated with primary radiation therapy.

Purpose: To examine a single-center cohort of patients with nasopharyngeal ACC for pathologic features, skull base invasion, overall survival, and disease free survival, with a focus on response to proton beam radiation therapy.

Methods and Materials: The institutional cancer registry was used to retrospectively identify patients treated for ACC of the nasopharynx from 2000 to 2016. Outcomes included 5-year overall survival and locoregional control. Specimen slides from NACC were also tested for HPV positivity to evaluate the role of HPV positivity in response to radiation therapy.

Results: Twelve patients with ACC of the nasopharynx were included [AL1] in the study population. All patients presented with advanced disease and were treated with primary radiation therapy, typically proton beam therapy. Only two (17%) underwent a surgical attempt at resection. A majority of cases had a cribriform growth pattern (67%). The 5-year survival was 75% and rate of locoregional control rate at 5 years was 50%, comparable to other ACC cohort studies that included earlier stage tumors in various subsites. Seven patients (58%) had invasion of tumor into skull base, three of whom (43%) were alive at 5 years. HPV testing is pending.

Conclusion: Although ACC is traditionally noted to be radioresistant, ACC of the nasopharynx was responsive to radiotherapy in our cohort, potentially owing to greater efficacy of proton beam radiation. Reasons for this improved survival, despite advanced stage and skull base invasion, are unclear and suggest the need for further pathologic and genetic characterization of nasopharyngeal ACC.