Influence of Estrogen Plus Progestin on Breast Cancer and Mammography in Healthy Postmenopausal Women: The Women’s Health Initiative Randomized Trial

TLDR: Relatively short-term combined estrogen plus progestin use increases incident breast cancers, which are diagnosed at a more advanced stage compared with placebo use, and also substantially increases the percentage of women with abnormal mammograms, a pattern which continued for the study duration.

Abstract: The large-scale Women's Health Initiative has confirmed that, in postmenopausal women, combined estrogen/ progestin therapy entails an increased risk of invasive breast cancer. The investigators have now explored this relationship in detail, characterizing the cancers that developed and seeking to learn whether hormonal effects on the mammogram can influence diagnosis. A total of 16,608 postmenopausal women 50 to 79 years of age, all with an intact uterus, were randomly assigned to receive active treatment (0.625 mg conjugated equine estrogens plus 2.5 mg medroxyprogesterone acetate daily in a single tablet) or placebo. The participants, seen at 40 clinical centers, were to be followed from 1993 to 1998 by annual clinical breast examinations and mammograms, but the trial was ended after a mean interval of 5.2 years. Intent-to-treat analyses demonstrated a hazard ratio of 1.24 for both total cancers and invasive cancers in women given hormone therapy compared with the placebo group. There was some suggestion of an increased risk for in situ breast cancer in hormone-treated women. An increased risk of breast cancer in treated women emerged after 3 years in those not receiving hormones previously and after 2 years in previously treated women. The findings were similar when women in specific risk categories were analyzed, and race and ethnicity were not significant factors. Invasive cancers associated with combined hormone therapy were larger than those in placebo recipients, more likely to be node-positive, and diagnosed when more advanced. There was, however, no difference in tumor grade or in the distribution of histologic types of breast cancer. After the first year, hormone-treated women more often had abnormal or highly suspicious mammograms than did those given placebo. In this prospective, randomized trial, combined estrogen/progestin treatment of postmenopausal women increased both breast cancer risk and the frequency of abnormal mammograms requiring medical assessment. In addition, cancers in treated women were more advanced when diagnosed than was the case for placebo recipients.