Density Danger
Women with dense breasts have a greater likelihood of cancer
When Pam Schmid got a baseline mammogram in her 30s, the radiologist remarked that she had dense breasts. And that was that. "I remember thinking, 'I don't know what that means,'" says the Raleigh, N.C., wellness coach, now 49. Eventually, it meant a belated discovery of cancer. In 2003, Schmid felt a lump in her breast, and it turned out she had several tumors.
After a grueling series of treatments-mastectomy, chemotherapy, radiation, and hormonal therapy-Schmid is now in remission. But she wonders if understanding the implications of breast density at that first appointment might have led to an earlier diagnosis. Doctors are only now widely realizing that a high proportion of dense, nonfatty tissue in a woman's breasts trails only her age and rare mutations in the BRCA genes at upping the odds of developing cancer. "Beyond a doubt, it's an important risk factor," says Norman Boyd, an epidemiologist at the Ontario Cancer Institute in Toronto. In a study published last month in the New England Journal of Medicine, Boyd and his colleagues reported that when at least 75 percent of a woman's breast tissue is dense, she has about five times as high a chance of developing breast cancer as someone with very little dense tissue. That's a scary stat, given that about a quarter of women are believed to hit at least the 50 percent mark.
So there's a new sense of urgency about uncovering explanations. Until now, many physicians have believed dense tissue to be dangerous only because it can cloak tumors on a mammogram. How, exactly, might it lead to tumors? Scientists don't know, though they have some ideas. Dense tissue (which shows up as the light areas on a mammogram) contains different types of connective tissue and epithelial cells, which support and line the body's organs. Epithelial cells throughout the body give rise to most tumors; in the case of dense breasts, researchers suspect that some process involving connective tissue helps nascent tumors take root and grow. (Collagen, a type of connective tissue found in tumors, is found in greater proportion in dense breasts.)
Hormones found in fatty tissue could play a role, too, says James Cerhan, an epidemiologist at the Mayo Clinic College of Medicine. It may in fact be a dialogue between dense and fatty tissues that increases the risk of cancer. Locating the genes responsible for breast density, researchers hope, will provide clues about the mechanism that produces cancer-and a better understanding of why some women have dense breasts and others don't.
Next steps. It's way too soon to say whether altering a woman's breast density might lower her risk of cancer, but researchers are exploring ways to do just that. Since density changes in response to levels of sex hormones-decreasing after a woman gives birth and as she ages and reaches menopause-it's possible that drugs might make a difference, too. It's already clear, for example, that hormone therapy that includes both estrogen and progesterone increases density by stimulating the growth of the epithelium and connective tissues. Tamoxifen reduces density by shrinking those tissues. Meanwhile, a study released last week led by Melinda Irwin, a researcher at the Yale School of Public Health, found that exercise reduced density in obese postmenopausal breast cancer survivors. There's also some evidence associating greater density with consumption of fat and alcohol, but those links have yet to be established.
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