Researchers ponder why estrogen-only HRT may reduce breast cancer risk


Breast cancer risk may decline in postmenopausal women who take estrogen-only hormone replacement therapy, a review released Thursday concludes. That represents a substantial change from how estrogen-only hormone therapy was viewed a decade ago, the authors said.

Older observational studies suggested that both of the major forms of hormone therapy -- estrogen-only for women who have had hysterectomies and an estrogen-progestin combination for women with uteruses -- raised breast cancer risk. The landmark Women’s Health Initiative found that the estrogen-progestin combination did significantly increase the risk of breast cancer in postmenopausal women. However, the estrogen-alone arm of the study was inconclusive.

A reanalysis of data from that study published earlier this month found that estrogen-only did not raise the risk of breast cancer and even lowered it in women who do not have risk factors for the disease, such as a family history or a history of benign breast disease.


The new review, led by Dr. Rowan T. Chlebowski of the Los Angeles Biomedical Research Institute at Harbor UCLA Medical Center, again examined the Women’s Health Initiative data and concluded that estrogen-alone significantly decreased the risk of breast cancer while estrogen-progestin significantly increased the risk.

A couple of points about hormone therapy and breast cancer are coming into focus. One is that some of the observational studies of decades past indicating that estrogen-alone increased breast cancer may have been skewed by the fact that hormone users were also more likely to get mammograms and have their cancers detected, the authors said.

Second, it appears that breast cancer cells in postmenopausal women are highly sensitive to changes in levels of hormones and “can survive only a limited range of estrogen exposures,” they said. A substantial change in estrogen levels can halt tumor growth.

If you’re a postmenopausal woman and you’re confused, no wonder. As the authors state: “[C]oncepts regarding menopausal hormone therapy and breast cancer have undergone considerable change in the last decade.”

Women with miserable menopausal symptoms should talk to their doctors about treatment options. Estrogen-progestin for women with uteruses will increase breast cancer risk, but women who have had hysterectomies have more choices. For everyone, hormone replacement therapy involves weighing a number of risks and benefits.

The paper appears in the Journal of the National Cancer Institute.