Clouding the issue: The breast's milk glands and connective tissue account for most of the density; fat makes a breast less dense. In a study of 2,224 women published in The New England Journal of Medicine earlier this year, doctors found that those with dense tissue in 25 to 49 percent of their breasts were twice as likely to develop cancer than women with mostly fat tissue; women with 50 to 74 percent were 2.5 times more likely; and women with 75 percent or more were about five times more likely. The findings confirm that density ranks with age and genetics as one of the major risk factors for breast cancer, says Norman Boyd, MD, a senior scientist at the Ontario Cancer Institute and lead author of the study.
For the most part, density is inherited, and it declines with age. Having children later in life—or not at all—will preserve density, and hormone replacement therapy will increase it. You can find out how dense your breasts are with a mammogram. Radiologists generally score breasts in one of four categories that roughly correlate to Boyd's classifications: fatty (less than 25 percent dense tissue), scattered (25 to 49 percent), heterogeneous (50 to 74 percent), or extremely dense (more than 75 percent). "We eyeball it," says Roxanne Lorans, MD, a radiologist at the Mayo Clinic in Arizona. "But we're pretty close to the mark."
While every mammogram report includes a density score, few doctors share it with their patients, says Karla Kerlikowske, MD, professor of medicine, epidemiology, and biostatistics at the University of California, San Francisco. "At this point, we're still educating physicians." In the meantime, patients should ask for their score.
Protecting yourself: Women worried about their breast density should consider getting digital mammograms, says Susan Love, MD, president and medical director of the Dr. Susan Love Research Foundation. With a standard mammogram, dense tissue appears light, the same as tumors, so it can mask the cancer. "It's like trying to find a polar bear in the snow," says Love. According to results reported in 2005 from the Digital Mammographic Imaging Screening Trial, digital mammograms are better than film-screen mammograms at detecting tumors in dense breasts.
Also be diligent about monthly breast self-exams. You can decrease your risk of breast cancer in general by exercising, limiting alcohol, and watching your weight.
One thing to avoid is panic. The average 50-year-old woman has about a 2.5 percent chance of getting breast cancer in the next ten years. If she has density in more than half the breast, the number climbs to about 5 percent. "Even though density is a strong risk factor," says Boyd, "the majority of women with high levels will not get the disease."