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There is new evidence to explain why fewer American women have been diagnosed with breast cancer in recent years. Some experts have pointed to changes in mammograms and screening. But a study published in the "New England Journal of Medicine" points, clearly, to a drop off in the use of hormone therapy. NPR's Allison Aubrey reports.

ALLISON AUBREY: The first year that experts noticed a decrease in the incidence of breast cancer was back in 2003. What they didn't know was: Why? Some experts wondered if women might be growing complacent and getting fewer mammograms. This might explain fewer cases of breast cancer being diagnosed. Other experts thought the decrease might be related to hormone replacement therapy. Many women stopped using hormones in 2002. For anyone who doesn't remember the golden age of HRT, as it was called, it was a period when it was thought as something akin to the fountain of youth. Women who took this combination of estrogen and progestin to treat hot flashes or night sweats, seemed to get other benefits too.

Dr. DEBORAH ARMSTRONG (Physician, Johns Hopkins Kimmel Cancer Center): People thought that hormone replacement therapy could keep your bones healthy, could keep your heart healthy, could keep your skin young. And so, people thought there really wasn't a problem.

AUBREY: That's physician Deborah Armstrong, of the Johns Hopkins Kimmel Cancer Center. What she sees in her practice is similar to what's happened nationwide: A drop-off in the number of women using hormone therapy - from a high of 60 million prescriptions, down to 20 million by 2005. The decline began after a huge randomized trial including thousands of postmenopausal women found hormones increased the risk of breast cancer and heart problems. JoAnn Manson is chief of preventive medicine at Brigham and Women's Hospital, and a co-author of the new paper. She says, this study clears up the confusion about the roles of both mammograms and hormone therapy, and the numbers of breast cancer cases.

Dr. JOANN MANSON (Chief, Preventive Medicine, Brigham and Women's Hospital): What's new here is that the role of mammographic practices was looked at, in detail, to understand whether or not it contributed to this reduced risk of breast cancer.

AUBREY: And as it turned out, Manson says, it's clear that any change in the rate of mammograms had little or no effect on breast cancer. Marcia Stefanick, of the Stanford Prevention Research Center, is co-author of the study. She says, some experts had argued that women taking hormones were more likely to get mammograms, which could lead to more frequent detection of tumors.

Dr. MARCIA STEFANICK (Stanford Prevention Research Center): But in the case of the clinical trial, the mammography rates were virtually identical.

AUBREY: Further, she says, more frequent mammograms observed among another group of women taking hormone therapy did not drive cancer risk either. This will certainly not be the first study to alarm women who've used some form of hormone therapy. But, Stefanick says, for women taking estrogen only after a hysterectomy, the risk of breast cancer does not seem to go up.

Dr. STEFANICK: We really do have two different situations for menopausal hormones. And the evidence that's really accumulating is for the combination of estrogen and progestin.

AUBREY: Despite the known risks, Brigham and Women's JoAnn Manson says that combination hormone therapy does remain a viable option for some women with severe menopausal symptoms, such as night sweats and interrupted sleep.

Dr. MANSON: Women should talk with their doctors about talking the lowest effective dose for the shortest duration of time necessary to treat these symptoms.

AUBREY: But Manson says, for most women, the risks of hormone therapy outweigh the benefits.

Allison Aubrey, NPR News, Washington.

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