In the new study, more than 8,500 women took estrogen plus progestin, and more than 8,100 took a placebo. Participants were given a mammogram and breast exam at the start of the trial and annually thereafter.
At the one-year mark, women on the combination therapy were three times more likely to report experiencing breast tenderness than women on the placebo. About 36 percent of women on the hormone therapy reported new breast tenderness, compared with nearly 12 percent of women on a placebo.
During 5.6 years of follow-up, 0.6 percent of women who reported new breast tenderness were diagnosed with breast cancer each year, compared with 0.36 percent of women taking hormones who did not experience new breast tenderness.
Researchers aren't certain why some women on hormone therapy develop breast cancer while others don't, though the breast tenderness may provide a clue, Crandall said. Tenderness could be a sign that the cells of the breast are multiplying rapidly. Cell proliferation is a risk factor for cancer.
Mary B. Daly, director of the Personalized Cancer Risk Assessment Program at Fox Chase Cancer Center in Philadelphia, said the findings of the study, which she described as well done, should prompt women and their doctors to reassess the need for hormone replacement if such tenderness develops.
Yet individual differences -- why this happens to some women and not others -- are still not well understood, Daly said.
"Some women who start taking hormone therapy may have cells that are much more receptive to it, or more sensitive to it, while other women must be more resistant," Daly said. "Something must be different about the breast tissue to make it more or less responsive to the hormones. If we could figure that out, that would be great."
Women in the study were taking daily doses of oral conjugated equine es
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