Autoimmune diseases and breast cancer risk by tumor hormone-receptor status among elderly women.

The female preponderance of many autoimmune diseases suggests a possible hormonal etiology. Little research exists on systemic and organ-specific autoimmune diseases and risk of breast cancer by tumor estrogen receptor (ER)- and progesterone receptor (PR)- status. Here, we evaluate associations between selected systemic and organ-specific autoimmune diseases and breast cancer risk overall and by tumor ER- and PR-status. We used linked Surveillance, Epidemiology and End Results (SEER)-Medicare data, with first female breast cancer cases ages ≥66 years identified by SEER registries (years 1992-2011; N = 209,929). We selected female controls (N = 200,000) from a stratified 5% random sample of Medicare recipients who were alive and breast cancer-free. We assessed exposures until 12 months before breast cancer diagnosis/selection using Medicare claims data. We estimated odds ratios (OR) and 99.9% confidence intervals (CI) using unconditional and multinomial logistic regression. We found reduced breast cancer risk among those with rheumatoid arthritis (OR = 0.84; 99.9% CI 0.79-0.89), systemic lupus erythematosus (OR = 0.82; 99.9% CI 0.70-0.97) and pernicious anemia (OR = 0.90; 99.9% CI 0.83-0.97) and increased risk among those with psoriasis (OR = 1.16; 99.9% CI 1.06-1.27). Statistically significant alterations in risk for rheumatoid arthritis were limited to ER-positive (+) breast cancer, whereas those for the other three conditions were further limited to ER+/PR+ breast cancer. However, only differences for rheumatoid arthritis by ER-status were statistically significant (p-heterogeneity = 0.0001). The reasons for these associations need to be investigated in future studies accounting for host characteristics and autoimmune disease treatment.