Abstract Background: Recently, anal squamous cell carcinoma (SCC) has increased rapidly over time, particularly among older women. As immunosuppression is associated with increased anal cancer risk, a parallel increase in autoimmune conditions may be contributing to this rising trend. Methods: We conducted a population-based case-control study using the Surveillance, Epidemiology, and End Results (SEER)-Medicare linked database (years 2000-2017). Cases were individuals with first anal SCC diagnosed in SEER registries (N=4,505) and 199,566 cancer-free controls from a 5% random sample of Medicare beneficiaries. Using logistic regression, we calculated odds ratios (OR) and 95% confidence intervals (95%CI) for associations with 47 autoimmune conditions identified via Medicare claims. Models were adjusted for age (continuous), sex, race (white, others), year of selection (quartiles), average number of physician claims per year (quartiles), medicare coverage months (quartiles) and smoking status. The false discovery rate was used to correct for multiple comparisons. Results: 18% of cases and 15% of controls had any autoimmune condition diagnosed. Having any autoimmune condition was associated with an increased risk of anal SCC (OR 1.12, 95%CI 1.03-1.22). The strongest risk estimates were for sarcoidosis (OR 1.97, 95%CI 1.22-3.19), followed by systemic lupus erythematosus (1.86, 1.38-2.52) and psoriasis (1.31, 1.08-1.58). Stratified by sex, only women showed significant associations for systemic lupus erythematosus (OR 2.05, 95%CI 1.51-2.78) and men with psoriasis (1.72, 1.25-2.35) and polymyalgia rheumatica (0.33, 0.12-0.89). Conclusion: To date, this is the largest study of anal SCC exploring its association with autoimmunity, especially in this understudied old population. We report a weak to moderately increased risk of certain autoimmune diseases with anal SCC. Some associations differed by sex. Impact/Significance: Understanding the mechanism by which autoimmune diseases increase cancer anal cancer risk may guide better prevention and treatment strategies. Given the rarity of these conditions and their moderate associations with anal cancer, clinical diagnosis of autoimmune diseases cannot explain the rising trend in anal cancer. Future studies are warranted in addressing the underdiagnosis of these autoimmune conditions to better quantify the burden. Citation Format: Minkyo Song, Eric A. Engels, Megan A. Clarke, Aimée R. Kreimer, Meredith S. Shiels. Autoimmune disease and the risk of anal cancer in the U.S. elderly population [abstract]. In: Proceedings of the AACR Special Conference: Aging and Cancer; 2022 Nov 17-20; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2022;83(2 Suppl_1):Abstract nr B008.
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