Timing of HRT may affect breast cancer risk

Women who undergo hormone replacement therapy (HRT) at the beginning of menopause may have a greater risk of breast cancer than women who wait until later to start treatment, a new study from Oxford University suggests.

The so-called Million Women Study evaluated prospective information on HRT use and other factors related to breast cancer risk provided by 1,129,025 postmenopausal women in the United Kingdom. Women who started combination HRT (estrogen-progestin) as they entered menopause or less than 5 years after it began developed breast cancer at a rate of 0.61% per year, compared with 0.46% per year in those who started HRT 5 or more years after onset of menopause and 0.30% per year in women who didn’t use hormone therapy. Comparable incidence rates for women taking estrogen-only HRT were .43% (less than 5 years after menopause) and .32% (5 years or more).

“In this large study, we found greater risks of breast cancer if hormonal therapy use began either before or soon after menopause than after a longer gap,” the authors write. “And this pattern of risk was seen across different types of hormonal therapy, among women who used hormonal therapy for either short or long durations, and also in lean and in overweight and obese women.”

Epidemiologist Dame Valerie Beral, the lead author, notes that 90% of the women taking HRT in the Million Women Study began treatment before or within 5 years of the start of menopause, a far larger number than in previous studies that found evidence of breast cancer risk among early starters of HRT, including the 2002 Women’s Health Initiative randomized trial.

The large study population is somewhat offset, however, by the observational nature of the study, which makes the findings less conclusive than results of a randomized trial.

The study was published in The Journal of the National Cancer Institute (2011;103:296-305).