THURSDAY, April 16 (HealthDay News) -- For years, researchers have known that breast density is almost as important as age in predicting who will develop breast cancer.

But now they're discovering how the density of a woman's breast tissue can also predict how she will respond to cancer treatment and whether her cancer will recur.

The denser a woman's breasts, the less fat they have, explained Diana Buist, an epidemiologist at Group Health Center for Health Studies in Seattle. "If you have a dense breast, it's harder to see breast tumors on a mammogram."

Women with dense breasts are more likely to have abnormal mammograms, Buist added. "Density on a mammogram is white, cancer on a mammogram is white," she said, so that makes it more difficult to detect signs of cancer and can prompt reports of possible abnormalities.

About 10 percent to 15 percent of women have low-density breasts, another 10 percent to 15 percent have very dense breasts, and the rest have breasts with a density somewhere in the middle, said Dr. Karla Kerlikowske, a professor of medicine, epidemiology and biostatistics at the University of California, San Francisco.

But the average woman may not have a clue whether her breasts are dense, she added.

For one thing, Kerlikowske said, it's not as easy to measure density as it is to measure cholesterol, for instance. "In the last 10 years, a lot of researchers have focused on how to measure it better, and what does it mean to have that density," she said.

Kerlikowske was part of a research team that discovered that high breast density predicts local, but not distant, recurrence of cancer after a lumpectomy and radiation for invasive breast cancer. The finding was published in the January issue of the International Journal of Radiation Oncology Biology Physics.

Another group of researchers found that changes in breast density during treatment with tamoxifen, a drug used to lower breast cancer risk, help predict how well the drug is working. Their research was presented in December at a breast cancer symposium in San Antonio, Texas.

Just how much high-density boosts breast cancer risk, though, is something that experts can only roughly estimate, Kerlikowske said. She said that women with very high-density breasts have about a fourfold increase in risk compared with women who have very low-density breasts.

And though density often decreases with age, especially after menopause, it remains a risk factor -- and one women can't do much about, Kerlikowske said.

Some experts have suggested that women with high-density breasts might consider a more frequent surveillance schedule. But she said there's not sufficient evidence to suggest, at least yet, that that would be a good idea. "We are trying to study that now," she said.

Women with dense breasts who are younger than 50 and premenopausal, though, might benefit from having digital rather than traditional mammograms, Kerlikowske said. "We are looking at different ways to measure breast density other than mammograms," she said, because "you can't keep having [frequent] mammograms," for safety reasons.

Buist said that older women who don't want to increase the density of their breasts could opt not to take combined hormone therapy, noting that "most women get an increase in density with that."

But beyond that, the two agreed, there's little additional advice doctors can offer women with dense breasts at this time.

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