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. What's new? The female preponderance of many autoimmune diseases suggests a possible hormonal etiology. Therefore, it is of interest to examine associations of autoimmune diseases with breast cancer by the hormone‐receptor status of the tumors. This is the first study to examine associations of systemic and organ‐specific autoimmune diseases with breast cancer according to tumor estrogen receptor (ER)‐ and progesterone receptor (PR)‐status. The findings that statistically significant alterations in risk with rheumatoid arthritis, systemic lupus erythematosus, pernicious anemia and psoriasis varied according to tumor ER‐ and PR‐status offer important clues to the complex relationship between certain autoimmune diseases and