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American Journal of Epidemiology Advance Access originally published online on April 29, 2008
American Journal of Epidemiology 2008 167(12):1407-1415; doi:10.1093/aje/kwn090
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American Journal of Epidemiology © The Author 2008. Published by the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

ORIGINAL CONTRIBUTIONS

Conjugated Equine Estrogens and Breast Cancer Risk in the Women's Health Initiative Clinical Trial and Observational Study

Ross L. Prentice1, Rowan T. Chlebowski2, Marcia L. Stefanick3, JoAnn E. Manson4, Robert D. Langer5, Mary Pettinger1, Susan L. Hendrix6, F. Allan Hubbell7, Charles Kooperberg1, Lewis H. Kuller8, Dorothy S. Lane9, Anne McTiernan1, Mary Jo O'Sullivan10, Jacques E. Rossouw11 and Garnet L. Anderson1

1 Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA
2 Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, Los Angeles, CA
3 Stanford Prevention Research Center, School of Medicine, Stanford University, Stanford, CA
4 Division of Preventive Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, MA
5 Outcomes Research Institute, Geisinger Health System, Danville, PA
6 Department of Obstetrics and Gynecology, Wayne State University, Detroit, MI
7 Department of Medicine, University of California, Irvine, CA
8 Department of Epidemiology, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, PA
9 Department of Preventive Medicine, State University of New York, Stony Brook, NY
10 Department of Obstetrics and Gynecology, University of Miami, Miami, FL
11 National Heart, Lung, and Blood Institute, Bethesda, MD

Correspondence to Dr. Ross L. Prentice, Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, P.O. Box 19024, Seattle, WA 98109-1024 (e-mail: rprentic{at}fhcrc.org).

Received for publication February 23, 2007. Accepted for publication October 2, 2007.

The Women's Health Initiative randomized controlled trial found a trend (p = 0.09) toward a lower breast cancer risk among women assigned to daily 0.625-mg conjugated equine estrogens (CEEs) compared with placebo, in contrast to an observational literature that mostly reports a moderate increase in risk with estrogen-alone preparations. In 1993–2004 at 40 US clinical centers, breast cancer hazard ratio estimates for this CEE regimen were compared between the Women's Health Initiative clinical trial and observational study toward understanding this apparent discrepancy and refining hazard ratio estimates. After control for prior use of postmenopausal hormone therapy and for confounding factors, CEE hazard ratio estimates were higher from the observational study compared with the clinical trial by 43% (p = 0.12). However, after additional control for time from menopause to first use of postmenopausal hormone therapy, the hazard ratios agreed closely between the two cohorts (p = 0.82). For women who begin use soon after menopause, combined analyses of clinical trial and observational study data do not provide clear evidence of either an overall reduction or an increase in breast cancer risk with CEEs, although hazard ratios appeared to be relatively higher among women having certain breast cancer risk factors or a low body mass index.

breast neoplasms; clinical trial; cohort studies; estrogens; hormone replacement therapy; postmenopause

Abbreviations: CEE, conjugated equine estrogen; CI, confidence interval; HT, postmenopausal hormone therapy; WHI, Women's Health Initiative

Editor's note: An invited commentary on this article is published on page 1416.


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