Changes in breast density associated with initiation, discontinuation, and continuing use of hormone replacement therapy.

CONTEXT Initiation of hormone replacement therapy (HRT) has been shown to increase breast density. Evidence exists that increased breast density decreases mammographic sensitivity. The effects on breast density of discontinuing and continuing HRT have not been studied systematically. OBJECTIVE To examine the effects of initiation, discontinuation, and continued use of HRT on breast density in postmenopausal women. DESIGN, SETTING, AND PARTICIPANTS Observational cohort study of 5212 naturally postmenopausal women aged 40 to 96 years and enrolled in a large health maintenance organization in western Washington State who had 2 screening mammograms between 1996 and 1998. MAIN OUTCOME MEASURES Breast density, assessed using the clinical radiologists' BI-RADS 4-point scale, compared among women who did not use HRT before either mammogram (nonusers); who used HRT before the first but not before the second mammogram (discontinuers); who used HRT before the second but not before the first mammogram (initiators); and who used HRT prior to both mammograms (continuing users). RESULTS Relative to nonusers, women who initiated HRT were more likely to show increases in breast density (relative risk [RR], 2.57; 95% confidence interval [CI], 2.12-3.08), while women who discontinued HRT use were more likely to show decreases in density (RR, 1.81; 95% CI, 1.06-2.98) and women who continued to use HRT were more likely to show both increases in density (RR, 1.33; 95% CI, 1.13-1.55) and sustained high density (RR, 1.45; 95% CI, 1.33-1.58). CONCLUSIONS These results indicate that breast density changes associated with HRT are dynamic, increasing with initiation, and decreasing with discontinuation.