Persistent Long-Term Risk After Primary Surgery for Head and Neck Adenoid Cystic Carcinoma: Competing-Risk and Conditional Estimates

<b>Background/Objectives</b>: Head and neck adenoid cystic carcinoma (HNAdCC) is characterized by indolent growth but sustained long-term risk of late recurrence and disease-related mortality. Data describing very long-term outcomes using analytic approaches that explicitly account for competing mortality remain limited. We aimed to characterize late failures, competing causes of death, and clinically interpretable long-horizon risk estimates after primary surgery for HNAdCC. <b>Methods:</b> We performed a retrospective single-center cohort study of patients with HNAdCC treated with curative-intent surgery between 1984 and 2020. Overall survival (OS) and cancer-specific survival (CSS) were estimated using Kaplan-Meier method. Competing risks of disease-related and other-cause death, as well as first-failure patterns, were analyzed using cumulative incidence functions, including a 5-year landmark analysis. Conditional mortality and restricted mean survival time (RMST; τ = 25 years) were additionally assessed. <b>Results:</b> Fifty-seven patients were included (median age 54 years). Median follow-up was 133 months overall and 212 months among survivors. A first failure occurred in 19/57 (33.3%) of patients, with distant metastasis as the most common pattern; 7/19 (36.8%) of failures occurred beyond 5 years. OS at 5, 10, and 25 years was 68.4%, 64.9%, and 37.5%, respectively; corresponding CSS was 78.9%, 74.8%, and 51.7%. At 25 years, cumulative incidence of disease-related death was 41.7%, compared with 20.9% for other-cause death. Older age and advanced T category were independently associated with worse OS, while older age and perineural invasion predicted worse CSS. Among 5-year survivors, conditional risk of disease-related death by 25 years remained 32.7%. RMST analyses demonstrated substantial long-term life-years lost associated with perineural invasion and T3-4 disease. <b>Conclusions:</b> HNAdCC exhibits persistent long-term risk with clinically meaningful late failures and substantial competing mortality over decades. Conditional and RMST-based estimates provide patient-centered measures that support lifelong, risk-adapted surveillance, particularly focused on detection of distant metastases.