Close Margins and Adjuvant Radiotherapy in Acinic Cell Carcinoma of the Parotid Gland.

Importance: The precise indications and oncologic effects of adjuvant radiotherapy in acinic cell carcinoma of the parotid gland are not well known, particularly in patients with negative, but close (≤1 mm), margins without other high-risk histopathologic factors. Objective: To evaluate the oncologic outcomes of patients with acinic cell carcinoma of the parotid gland and the results of adjuvant therapy for those with close (≤1-mm) margins. Design, Setting, and Participants: In a retrospective case series with medical record review at a single academic tertiary referral center, patients treated surgically from January 2000 to December 2014 for acinic cell carcinoma of the parotid gland were identified from an institutional database. All data analysis was performed in September 2017. Exposures: All patients underwent parotidectomy with or without adjuvant radiotherapy or chemoradiotherapy. Main Outcomes and Measures: The primary end point was locoregional control. Secondary end points included recurrence patterns and survival. Results: Forty-five patients were identified in this case series (23 [51%] female), with a mean (SD) age of 47.1 (19.5) years. The median follow-up in surviving patients was 56.7 months (range, 18.5-204 months). Four patients (9%) experienced recurrence (1 local and 3 distant) at a median of 67.3 months (range, 12.7-136 months) after surgery. Thirteen patients (29%) had at least one high-risk histopathologic factor (advanced T category, nodal disease, lymphovascular or perineural invasion, high-grade, or positive margins). The remaining 32 patients (71%) without these high-risk factors had significantly improved disease-free survival (hazard ratio, 0.08; 95% CI, 0.01-0.71). Of patients without high-risk factors, those with close (≤1-mm) margins were significantly more likely to receive adjuvant radiotherapy (10 [56%] vs 1 [7%]; difference, 49%; 95% CI, 16%-82%), although this was not associated with disease control. At a median follow-up of 64.3 months (range, 33-204 months) in the 18 patients with close (≤1-mm) margins without other high-risk factors (10 with adjuvant radiotherapy and 8 without adjuvant therapy), only 1 patient (who had received adjuvant radiotherapy) experienced a recurrence, at 136 months after surgery. Conclusions and Relevance: Patients with acinic cell carcinoma of the parotid gland whose only histopathologic risk factor is a close (≤1 mm) but negative margin do not appear to benefit from adjuvant radiotherapy. Recurrent disease is rare but may occur many years after initial treatment, and patients with acinic cell carcinoma could benefit from lifelong clinical surveillance.