No Link Between Estrogen Alone and Breast Cancer

No Link Between Estrogen Alone and Breast Cancer

According to updated results from the Womens Health Initiative, postmenopausal hormone therapy with estrogen alone does not increase the risk of breast cancer. These results were published in the Journal of the American Medical Association.

The Womens Health Initiative (WHI) included two clinical trials of postmenopausal hormone therapy-one included estrogen alone in 10,739 women without a uterus; the other included estrogen plus progestin in 16,608 women with a uterus. (Only women without a uterus were given estrogen alone because estrogen alone increases the risk of uterine cancer.)

The studies enrolled women between the ages of 50 and 79. In each study, women were randomly assigned to receive either the hormone or a placebo (a sugar pill). The primary purpose of the hormone studies was to determine the effect of hormone therapy on a womans risk of cardiovascular disease, fractures, and breast cancer.

The study of estrogen plus progestin was stopped early because women taking the hormones had a higher risk of heart disease, breast cancer, stroke, and blood clots.

Women taking estrogen plus progestin had fewer fractures and were less likely to develop colorectal cancer, but these benefits were thought to be outweighed by the risks.

The study of estrogen alone continued, but eventually also stopped earlier than planned. Compared to women taking the placebo, women taking estrogen alone had an increased risk of strokes and a decreased risk of fractures.

Women taking estrogen alone did not have an increased risk of heart disease or breast cancer. In fact, there was some suggestion of a decreased risk of breast cancer among women taking estrogen alone. The researchers noted that the breast cancer results would need to be further evaluated when more complete information from the study was available.

The current study updates the breast cancer results from the WHI study of estrogen alone. After a median of 7.l years of follow-up, there were 237 invasive breast cancers and 55 in situ breast cancers.

  • The rate of breast cancer in women taking estrogen alone was not significantly different than the rate of breast cancer in women taking the placebo.
  • Women taking estrogen alone were more likely than women taking the placebo to have an abnormal mammogram that required a short interval follow-up (a repeat mammogram in six months).

In spite of a higher frequency of abnormal mammograms, risk of breast cancer was not increased among women taking estrogen alone. The researchers conclude, Initiation of [estrogen] alone in women after hysterectomy should continue to be based on careful consideration of potential risks and benefits for a given individual.

Reference: Stefanick ML, Anderson GL, Margolis KL et al. Effects of Conjugated Equine Estrogens on Breast Cancer and Mammography Screening in Postmenopausal Women with Hysterectomy. JAMA. 2006;295:1647-1657.

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