Estrogen plus progestin (E+P) and breast cancer incidence and mortality.

Abstract

1501 Background: In the WHI clinical trial, E+P increased both breast cancer incidence and breast cancer mortality (JAMA 2010;304:1684). In contrast, breast cancers associated with E+P use in most observational studies have a more favorable prognosis. To address differences, a cohort of WHI Observational Study participants with characteristics similar to the WHI clinical trial was identified to examine E+P association with invasive breast cancer incidence and outcome. Methods: 41,449 postmenopausal women with no prior hysterectomy and mammogram negative for breast cancer < 2 years before who either were not hormone users (25,328) or were using E+P (16,121) were identified. Breast cancers were verified by centralized medical record review. Adjusted Cox proportional hazard regression was used to calculate hazard ratios (HRs) with 95% confidence intervals (CI). Additional analyses adjusted for breast cancer screening, censoring participants for incidence analyses who had a > 2 year interval without a mammogram. Results: After a mean (SD) follow-up 11.3 (3.1) years, 2,236 breast cancers were diagnosed. Breast cancer incidence was higher in E+P users (0.60% vs 0.42%, annualized rate, respectively: HR 1.55, 95% CI 1.41-1.70, P<0.001). Screening adjusted analyses had stronger breast cancer association (0.63% vs 0.39%, HR 1.72, 95% CI 1.54-1.93; P<0.001). Survival following breast cancer, measured from diagnosis date, was similar in E+P users and non-users (HR 0.95, 95% CI 0.74-1.23). Breast cancer mortality, analyzed from cohort entry date, are shown in the table. Conclusions: E+P use is associated with increased breast cancer incidence. As breast cancer prognosis following diagnosis on E+P is similar to that of nonusers, the higher incidence with E+P leads to increased breast cancer mortality. [Table: see text]