Autoimmune diseases and breast cancer recurrence: a Danish nationwide cohort study.

Autoimmune diseases (ADs) comprise a large group of heterogeneous diseases in which the immune system attacks healthy organs. Both intrinsic changes in the body and AD treatment can compromise immune function. Impaired immune function could increase the risk of recurrent cancer. We aimed to investigate this hypothesis in a population-based epidemiological study. We examined the risk of breast cancer (BC) recurrence associated with an AD diagnosis among patients with incident stages I-III BC diagnosed during 1980-2007. Data were obtained from Danish population-based medical registries. ADs were categorized dichotomously and according to organ system of origin. Follow-up was up to 10 years or until 31 December 2009. Multivariate Cox proportional hazard regression was used to compute hazard ratios (HRs) and associated 95 % confidence intervals (95 % CIs) to evaluate the association between AD diagnosis and BC recurrence. 78,095 women with stages I-III BC were identified. Median age-at-diagnosis was 61 years (19-102 years), median follow-up was 5.7 years, and 13,545 women had a recurrence during follow-up. 6,716 women had at least one AD. In adjusted models, the association between ADs and BC recurrence was near null: HRadjusted 0.96 (95 % CI 0.89, 1.04). These results held in all AD subcategories, except for central nervous/neuromuscular system ADs, with HRadjusted 0.56 (95 % CI 0.40, 0.78). Among women with BC, a history of at least one AD diagnosis was not associated with BC recurrence, with the possible exception of ADs of the central nervous/neuromuscular system.