Doctors know that women who have dense breasts have as much as six times the risk of breast cancer as those who have less dense breasts. But they haven’t been quite sure why.

New research presented today at the San Antonio Breast Cancer Symposium offers possible clues: Biopsies of healthy women showed differences in the cells of dense and nondense tissue that may contribute to the development of tumors.

Mayo Clinic docs performed mammograms and biopsies on 60 women ages 45-85 with no history or symptoms of breast cancer. Data that's come back from about half of them showed that dense breast tissue was composed of 6 percent epithelial cells, which line the milk glands and ducts, compared to just 1 percent in the nondense tissue. Some 64 percent of the dense tissue was made up of stroma, or connective tissue, compared to 20 percent in the nondense tissue. And dense tissue was comprised of 30 percent fat, versus 80 percent in nondense tissue.

It's too soon to say why those differences may be significant for cancer development, but here are some potential reasons, says Karthik Ghosh, an assistant professor of medicine at the Mayo Clinic: Breast tumors tend to originate in the epithelium, so having more of those cells may up a woman's risk of cancer. And areas surrounding the stroma produce growth factors that may stimulate the epithelium to turn cancerous, she adds.

Ghosh's colleague, Celine Vachon, found more expression of the aromatase enzyme, which converts the hormone androgen into estrogen, in the dense breast tissue. Estrogen drives most breast cancers, so drugs known as aromatase inhibitors are a common treatment choice.

If larger studies confirm their findings, doctors may be able to use density to determine how well such treatments are working, Ghosh says.

"It's intriguing," she says. "Can we use density as a marker when treating a patient with aromatase inhibitors, that if density is decreasing, we know we're on the right track?"

Breast cancer is the most common tumor among U.S. women, after skin cancer; an estimated 182,460 will be diagnosed this year and 40,480 will die of it, according to the National Cancer Institute.

"Rockets Against Breast Cancer" by Erik Charlton, via Flickr