A major study has found that women with the greatest risk of breast cancer are those who started hormone therapy the earliest - before or soon after menopause started. The finding means that women thought to be at lower risk for breast cancer from hormone therapy may actually be at the highest risk.
The finding comes from the Million Women Study (MWS)—a large observational study in the United Kingdom—and adds to a growing body of evidence that the use of combined hormone therapy (estrogen plus progestin) to treat menopausal symptoms increases the risk of breast cancer and deaths from the disease.
The study included more than a million postmenopausal women, one in four British women who were aged 50 to 64 during the enrollment period, from May 1996 to December 2001.The results appeared in the Journal of the National Cancer Institute on January 28, 2011.
The pattern of increased breast cancer risk “was seen across different types of hormonal therapy, among women [in the MWS] who used hormonal therapy for either short or long durations, and also in lean and in overweight and obese women,” wrote Dr. Valerie Beral of Oxford University and her colleagues. The findings support results from the Women’s Health Initiative (WHI), a randomized clinical trial that, in 2002, first reported evidence linking combined hormone use to breast cancer.
“The new findings underscore the idea that there’s really no safe window of time for women to take combined hormone therapy,” said Dr. Leslie Ford of the National Cancer Institute's Division of Cancer Prevention and the Institute’s WHI liaison. After the initial WHI results were announced, she noted, some people had argued that hormones may be safer when started at the time of menopause. “The new findings refute that argument,” she added.
WHI and MWS investigators have both reported that breast cancer incidence rates declined rapidly once women stopped taking combined hormone therapy. “It is important for women to know that if they stop using hormones, the risk of breast cancer very quickly returns to where it was before hormone therapy began,” Ford said.