New research may explain why breast cancer tends to be more aggressive in women with denser breast tissue. Breast cancer cells grown in dense, rigid surroundings step up their invasive activities, Vanderbilt-Ingram Cancer Center investigators report in the journal Current Biology.

The findings suggest a cellular mechanism for the correlation between human breast tissue density and tumor aggressiveness. Women with increased breast density on mammograms have an increased risk for both developing breast cancer and having breast cancers with invasive characteristics.

This connection between breast density and cancer aggressiveness has begged the question of which comes first. Is the tissue denser because the tumor is more aggressive (and recruits cells that "lay down" more matrix), or is the tumor more aggressive because the tissue is denser?

"Our study shows that if you have a dense, rigid matrix, the cells will be more aggressive and invasive; it's a direct effect," said Alissa Weaver, M.D., Ph.D., assistant professor of Cancer Biology and lead author of the study.