2842 - Salivary Gland Cancer Patterns of Failure after Radiotherapy

Purpose/Objective(s): Adjuvant radiotherapy (RT) is standard for resected salivary gland cancers with high-risk features. Given the rarity & heterogeneity of this disease, limited data are available regarding factors associated with increased risk of failure, location of & interval to first recurrence. We examined factors associated with recurrence & patterns of failure post-RT.

Materials/Methods: A single-institution retrospective review of patients who received RT for salivary gland cancers from 2005-2022 was performed. Eligible patients had M0 disease at diagnosis & received curative-intent RT. Demographic & clinical data were extracted from the medical record. Associations between characteristics & recurrence were evaluated by Fisher’s exact test.

Results: Characteristics for 82 patients meeting inclusion criteria are summarized in the table (* indicates subset not listed in pathology report). Mean age at diagnosis was 59.0 yr (SD 14.4). Eighty underwent surgery, with median time from surgery to RT start of 7.0 weeks (IQR 5.9-9.0). Median RT dose to the primary site was 64.0 Gy (IQR 60.0-66.0) & to the neck was 51.6 Gy (IQR 50.0-54.0). Median follow-up was 4.8 yr. Recurrences occurred in 32.9% of patients. Median time from RT completion to recurrence was 66 weeks (IQR 20-135). Most recurrences were distant (85.2%), followed by primary site (25.9%), & neck (11.1%). Recurrence was significantly associated with histology (p=0.01): 44.4% adenoid cystic carcinoma (12/24 with recurrence), 18.5% acinic cell carcinoma (5/12), 18.5% salivary duct carcinoma (5/12), 7.4% carcinoma ex pleomorphic adenoma (2/7), 7.4% mucoepidermoid carcinoma (2/15), & 3.7% neuroendocrine large cell (1/1). Recurrence was significantly associated with positive margins (p=0.05), lymphovascular invasion (LVI; p=0.04), & high tumor grade (p=0.03), but not perineural invasion (PNI; p=0.08). All patients with regional recurrences (n=3) received neck RT, but 2 discontinued RT early (after 6 Gy & 48.6 Gy).

Conclusion: Most salivary gland cancer recurrences after RT are distant & occur within 3 years, suggesting that patients with high-risk features (e.g., adenoid cystic, acinic cell, or salivary duct histology, positive margins, LVI, and/or grade 3 disease) may benefit from systemic therapy. Early termination of RT portends poorly for regional disease control. Regular restaging imaging should include distant sites to identify early recurrence amenable to intervention such as ablative radiotherapy. Additional analyses of these data are planned, including comparison to a cohort of patients treated with surgery alone.