Part of the Women’s Health Initiative study of hormonal replacement therapy in the late 1990s included a group of hysterectomised women who received oestrogen alone. Over 10 000 post-menopausal women were randomised to 0.625mg of conjugated equine oestrogen (CEE) daily or placebo over a period of seven years before the trial was stopped because of an increase in stroke incidence without cardiovascular benefit. Unlike the main study of combined progesterone plus CEE in women with an intact uterus, the incidence of breast cancer in those receiving CEE alone decreased modestly with a hazard ratio of 0.80. However, this reduction has to be balanced against an increase of abnormal mammograms in the CEE alone group, especially in the first year of follow-up. The cumulative percentages requiring follow-up for mammogram abnormalities was 36% for CEE alone recipients and 28% for those on the placebo.
This study had the same flaws as the larger combined progesterone plus CEE research, with most of the women being over 60 years old on recruitment and being overweight, but it does raise interesting reflections that oestrogen alone does not increase breast cancer risk. Would women in their fifties with a progesterone-releasing intrauterine system be able to use oestrogen alone with the same protection from breast cancer? (Stefanick et al JAMA 2006;295:1647-57)
