Breast cancer risk higher with hormone therapy started close to menopause, study indicates

Women who begin taking hormone therapy near menopause have a higher risk for breast cancer than those who start taking it later, a new study reports.

Researchers looked at the U.K. Million Women Study to examine whether the timing of hormone therapy initiation affected breast cancer risk. Data on 1,129,025 postmenopausal women with 4.05 million woman-years of follow-up were included. The study results were published early online Jan. 28 by the Journal of the National Cancer Institute and will appear in the Feb. 16 print issue.

A total of 15,759 cases of incident breast cancer occurred during follow-up, 7,107 (45%) in women currently using hormone therapy. The average age of participants was 56.6 years at study entry. Women who were currently using hormone therapy had a higher risk for breast cancer if they started therapy before or shortly after menopause. They were also at higher risk for breast cancer than those who had never taken hormone therapy, but the increased risk disappeared within a few years after therapy was discontinued. Those currently using estrogen alone had little if any increased risk if they had started therapy five or more years after menopause (relative risk, 1.05, 95% CI, 0.89 to 1.24) but statistically significant higher risk if they had started it before or within five years of menopause (relative risk, 1.43, 95% CI, 1.35 to 1.51). For those taking estrogen-progestin, the risk pattern relative to timing of therapy and menopause was similar (relative risk, 1.53, 95% CI, 1.38 to 1.70 and 2.04, 95% CI, 1.95 to 2.14, respectively), although the relative risk was higher in this group.

The authors noted that their results could have been affected by residual misclassification of hormone therapy use. However, they concluded that breast cancer risk varied significantly according to when hormone use began. Women who used estrogen-progestin compared with estrogen alone and those who began taking hormones close to menopause compared with at least five years afterward were at higher risk. An editorial cautioned that while the Women's Health Initiative also found a relationship between timing of hormone therapy initiation and breast cancer, there were differences regarding whether risk is altered according to the use of estrogen alone or estrogen-progestin combinations, and that further study is required.