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The researchers reviewed about 463,000 mammograms performed on nearly 330,000 U.S. women aged 40 to 89. Breast cancer was eventually diagnosed in 2,223 of the women. The investigators analyzed the results of the mammograms, which were taken from 1996 through 1998, then looked at each mammogram's result and subsequent breast cancer diagnoses in relation to each woman's age, her breast density, and whether she was taking hormone therapy. With the large amount of data, the researchers were able to assess the individual and combined effects of the three factors.
The study showed that mammograms were more accurate for women whose breasts were less dense and more fatty, and were more accurate in older women. Of the factors examined, "breast density had the strongest association" with accurate mammograms, says study author Rachel Ballard-Barbash, M.D., M.P.H., associate director of the Applied Research Program in NCI's Division of
Cancer Control and Population Sciences. Dense breast tissue may decrease mammography's accuracy by obscuring or mimicking tumors.
"The main thing we've discovered is that mammography performs differently among women with different characteristics," says the study's lead author, Dartmouth Medical School's Patricia A. Carney, Ph.D. "This finding is contrary to the popularly-held belief that mammography works the same for everyone."
Researchers are studying several approaches to improving mammography's accuracy. In the meantime, Carney and her colleagues recommend that women with dense breasts and those taking hormone therapy be told that increased
breast density may obscure results and additional imaging may be needed to follow up on suspicious findings.
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