Breast cancer and rheumatoid arthritis show inverse associations in older US women

Researchers used SEER-Medicare data from 2008–2021 to examine the relationship between rheumatoid arthritis (RA) and breast cancer among US women aged 66–99. The case-control analysis included 204,186 breast cancer cases and 2,897,862 controls, with RA identified through Medicare claims. A cohort analysis of 184,524 women with and without breast cancer assessed subsequent RA incidence. Both analyses used multivariable-adjusted models.

RA was associated with lower odds of breast cancer overall (OR 0.80, 95% CI 0.77–0.83), specifically hormone receptor-positive disease (OR 0.78, 95% CI 0.76–0.81). In the cohort analysis, breast cancer was associated with lower subsequent RA incidence (HR 0.93, 95% CI 0.88–0.98), without differences by hormone receptor status. Among women with breast cancer, radiotherapy was associated with lower RA incidence (HR 0.82, 95% CI 0.76–0.89). Among those with hormone receptor-positive disease, exemestane treatment was associated with higher RA incidence (HR 1.33, 95% CI 1.17–1.50).

The authors suggest hormone-related pathways and radiotherapy-related immune modulation as possible explanations, but these mechanisms remain unproven. The observational findings do not establish causality or justify treatment changes. Claims-based RA ascertainment and restriction to older women should inform interpretation; full-text methodological details were unavailable for this update.