Though density generally declines with age, and the breasts become more fatty, "these changes vary from woman to woman," Vachon said. "And we know the use of postmenopausal hormone therapy can increase density."
In the other study, researchers from Georgetown University and elsewhere found that the increase in density with hormone use predicted breast cancer risk.
Drawing from the Women's Health Initiative study, which looked at hormone therapy and other health concerns, they compared 97 women on hormone replacement who developed breast cancer and 77 who did not take hormones but did develop cancer with 733 cancer-free women, some who took hormones and some who did not.
They found that 57 percent of those who did not take hormones had a decrease in density, compared with just 16 percent of those taking hormone therapy.
Breast cancer risk increased nearly fourfold for the 20 percent of women with the biggest increase in density, compared with the 20 percent of women with the lowest increase or a decrease.
When the researchers took the density information out of the analysis, hormone use was not associated with breast cancer risk, they found, leading them to conclude that the change in density accounted for the increased cancer risk.
Dr. Joanne Mortimer, a medical oncologist and director of the women's cancer programs at the City of Hope Comprehensive Cancer Center in California, said the study results were not surprising.
"We know that breast density probably correlates with the patient's own estrogen levels," she said. "The higher the estrogen, the greater the breast cancer risk."
The new research adds valuable information, she said, about a link that is accepted. Eventually, she said, experts may add density information to established risk models to produce a more precise estimate of an individual woman's risk
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