The Women's Health Initiative (WHI) Estrogen-Alone trial, which randomized women with prior hysterectomy to conjugated equine estrogens (CEE) or placebo, was stopped earlier than planned because of increased stroke incidence and no reduction in risk of coronary heart disease. In contrast to substantial epidemiological evidence associating exogenous (originating externally) estrogens with increased breast cancer incidence, preliminary analyses found fewer breast cancers in women in the CEE group, prompting a detailed updated analysis of breast cancer incidence and mammographic reports.
Marcia L. Stefanick, Ph.D., of Stanford University, Stanford, Calif., and colleagues with the WHI study, analyzed the data from the CEE-alone group of the WHI study to determine the effects of CEE on breast cancers and mammographic findings. The study included 10,739 postmenopausal women aged 50 to 79 years with prior hysterectomy who were randomized to CEE or placebo at 40 U.S. clinical centers from 1993 through 1998. Mammography screenings and clinical breast examinations were performed at baseline and annually. All breast cancers diagnosed through February 29, 2004, are included. Participants received a dose of 0.625 mg/d of CEE or an identical-appearing placebo.
In an analyses of all events (n = 237 cases) occurring prior to intervention termination, after an average follow-up of 7.1 years, nonsignificant reductions were observed for invasive breast cancer (20 percent lower rate) and for total breast cancer (18 percent lower rate) in women randomized to CEE, when compared to the placebo group. The annualized rates were 0.28 (104 cases in the CEE group) and 0.34 (133 cases in the placebo group). In further analyses, fewer breast cancers with localized disease were diagnosed
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