Hormone Found to Raise Risk of Breast Cancer

A second major study of breast cancer risk has found that including progestin in hormone replacement therapy over a period of years increases the risk of developing the disease.

The study, by researchers at USC, found that taking a combination of estrogen and progestin increases breast cancer risk by 24% for each five years of use.

For women taking estrogen alone, the risk of breast cancer rises 6% for every five years of use.

The study will be published later this month in the Journal of the National Cancer Institute.

A study published last month in the Journal of the American Medical Assn. also found that adding progestin to hormone therapy significantly increased breast cancer risk over time.

The USC study distinguished between women who take progestin part of the month and those who follow a regimen of daily progestin use. It found a higher breast cancer risk among women who took progestin only part of the month.

“Continuous combined therapy may be better overall, since you tend to see lower doses of progestin in that therapy,” said Dr. Ronald K. Ross, a professor of preventive medicine and the lead author of the study.

Hormone therapy is prescribed to help women with the side effects of menopause, such as hot flashes. Giving estrogen after menopause, however, is also thought to greatly reduce the risks of heart disease and osteoporosis.

Progestin is often included in the therapy because giving estrogen alone--called unopposed estrogen--increases the risk of uterine cancer.

The research should not discourage the use of long-term estrogen therapy, the authors noted.

“Even with a slight increased risk of breast cancer and a more substantial increased risk of endometrial cancer, the overall risk-benefit equation for ERT [estrogen replacement therapy] balances strongly on the side of benefit,” the paper said.

However, Ross and his colleagues now suggest that the added uterine cancer risk from taking unopposed estrogen may be more acceptable than the added breast cancer risk linked to estrogen and progestin.

Breast cancer is five times more common than uterine cancer.

The study followed 1,897 post-menopausal women who had been diagnosed with breast cancer and 1,637 similar, healthy women. The women were asked about their hormone therapy use.