It is unclear why a woman may reduce her breast density or even why one woman has denser breasts than another, Vachon said. Factors may include genetics, drugs or environmental factors.

"We need to understand why women are changing," she said. "Our study doesn't have enough events to decipher what the reason is for the changes."


HORMONES THICKEN THE PLOT

There is, however, confirmed data that show women who undergo the postmenopausal treatment of estrogen and progestin therapy (EPT) are at a higher risk of developing breast cancer. Using this information, researcher Celia Byrne of the Lombardi Comprehensive Cancer Center at Georgetown University studied breast density in both women who use EPT and those who do not.

Researchers looked at data from the Women's Health Initiative, which is a data set of tens of thousands of women. They studied about 1,000 women from the initiative for this particular research.

The team reported an association between EPT and breast cancer risk, which supports the WHI findings from several years ago, Byrne said. She found a 24 percent increased risk, explained by the change in breast density.

Byrne and her team of researchers found that 57 percent of women in the placebo group - those who did not receive EPT - had a decline in mammographic density, compared with only 16 percent of women in the EPT group. Of the women in the placebo group, 47 percent had a "modest" increase in density compared with 85 percent of women in the EPT group.

Further, an important finding showed that with every 1 percent increase in breast density, there was a 3 percent increase in breast cancer risk, Byrne explained.

"That's significant," she said, explaining that a 2 percent density increase means a 6 percent risk increase.

Moving forward, Byrne said this research can be used clinically in studying risk factors for breast cancer.

"This illustrates the idea that density can be used as intermediate marker," she said. "It could be used more clinically."

FOR YOUNG, DENSE BREASTS, A SCREENING ALTERNATIVE?

For women who do not undergo mammography or young women who seek density screening without high exposure to radiation, research found that measuring breast density with dual energy X-ray absorptiometry (DXA) could offer a low-radiation option.

DXA is mainly used to evaluate bone density and total body composition, and it has a reputation for low-radiation exposure.

Gertraud Maskarinec, a professor of epidemiology at the Cancer Research Center at the University of Hawaii, Honolulu, found that there is an "agreement between breast density as determined by mammograms and the use of DXA," she said in a statement.

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