Autoimmune disease and the risk of anal cancer in the US population aged 66 years and over.

Background: In the United States, anal squamous cell carcinoma rates have increased rapidly, particularly among women 50 or older than 66 years of age. As immunosuppression is associated with increased risk, autoimmune conditions may be associated with greater risk of anal squamous cell carcinoma.

Methods: We conducted a population-based, case-control study using Surveillance, Epidemiology, and End Results-Medicare data (2000-2017). Anal squamous cell carcinoma cases (n = 4505) were matched to 200 000 cancer-free controls. Using multivariable logistic regression, we calculated odds ratios (ORs) and 95% confidence intervals (CIs) for associations between 47 autoimmune conditions diagnosed before selection, identified using Medicare claims, and anal squamous cell carcinoma. The Bonferroni threshold was used to correct for multiple comparisons. Population attributable fractions were calculated for conditions nominally associated with anal squamous cell carcinoma.

Results: In total, 18% of anal squamous cell carcinoma cases and 15% of cancer-free controls had a diagnosed autoimmune condition. Any autoimmune condition was associated with an increased risk of anal squamous cell carcinoma (OR = 1.11, 95% CI = 1.02 to 1.21; population attributable fraction = 1.8%). Anal squamous cell carcinoma was associated with systemic lupus erythematosus (OR = 1.79, 95% CI = 1.32 to 2.42; population attributable fraction = 0.4%) and nominally associated (P < .05) with sarcoidosis (OR = 2.09, 95% CI = 1.30 to 3.37; population-attributable fraction = 0.2%) and psoriasis (OR = 1.28, 95% CI = 1.06 to 1.56; population attributable fraction = 0.5%). Stratified by sex, only women showed statistically significant associations for systemic lupus erythematosus (OR = 1.97, 95% CI = 1.46 to 2.68). Statistically significant interaction was observed by sex for psoriasis (men vs women: OR = 1.68 [95% CI = 1.03 to 4.28] vs OR = 1.12 [95% CI = 0.88 to 1.43]) and polymyalgia rheumatica (OR = 0.33 [95% CI = 0.12 to 0.89] vs OR = 0.99 [95% CI = 0.75 to 1.30]).

Conclusion: Systemic lupus erythematosus, sarcoidosis, and psoriasis were associated with a moderately increased risk of anal squamous cell carcinoma. Given these conditions' rarity and moderate associations with anal squamous cell carcinoma, autoimmune diseases cannot explain the rising trend in this disease.