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updated 10/7/2010 11:28:59 AM ET 2010-10-07T15:28:59

Among breast cancer patients who have an early, noninvasive form of the disease — called ductal carcinoma in situ, or DCIS — those whose breast tissue is the densest have the highest risk of facing a recurrence of their disease, a new study shows.

Women who'd previously had DCIS in one breast had three times the risk of developing cancer in the opposite breast, if their breast tissue had the greatest density among women in the study, compared with women in the study whose breast tissue was the least dense.

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Women with densest breast tissue also had about one-and-a-half times the risk of developing cancer in the same breast, and two times the risk of developing invasive cancer in either breast, than women with the least dense breast tissue.

Researchers at Kaiser Permanente Medical Care Program in California based their findings on a group of 935 women whose previous DCIS had been treated with breast-conserving surgery (not a mastectomy) between 1991 and 1997. The density of breast tissue was assessed from mammograms.

The results were in line with those of a 2007 study, which found similar increases in risk levels among women with DCIS whose breast tissue had the greatest density, according to the researchers.

"This study found real-world data, which is helpful," in showing there's a strong risk associated with denser breast tissue, said Dr. Jeffrey Tice, professor of medicine and breast cancer researcher at the University of California, San Francisco, who was not involved with the study.

The study supports prior findings, he said, and looked at more precise measurements of breast density than previous work.

The findings are important, he said, because women diagnosed with DCIS have several options for their treatment.

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"Some women want aggressive treatments," and opt for a mastectomy, he said, while others choose breast-conserving surgery. But most cases of DCIS will never become invasive, and if scientists could better predict which patients were a higher risk of later developing invasive cancers, they could help patients make better-informed decisions about their treatment, Tice said.

Increasingly, studies are finding higher breast density to be a risk factor for developing many types and stages of the disease, he said, and scientists are trying to figure out why. Something about denser breast tissue reflects a higher potential for cancer to develop, but exactly what that is remains unknown, he said.

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A woman's risk for breast cancer is most strongly associated with her age; the older she is, the more likely she is to develop the disease. But aside from age, the factor that has the greatest effect on risk is breast density.

A woman's breast density is about 60 percent to 70 percent determined by genetics, he said, citing a previous study by Canadian researchers.

Researchers are also looking for ways to improve the precision of measurements of breast density, Tice said. Mammograms, which create a two-dimensional image of the breast, are currently used to make density measurements, but scientists are investigating new techniques involving magnetic resonance imaging (MRI) and three-dimensional imaging techniques.

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