To do so, Kerlikowske’s team tracked outcomes for about 200,000 U.S. women in the national Breast Cancer Surveillance Consortium, a database aimed at assessing breast cancer screening. About 18,000 of the women had varying stages of breast cancer, while the remaining 184,000 did not.
The study participants had their breast densities recorded and were split into four categories: category A, breasts made up almost entirely of fat; category B, breasts scattered with dense tissue but mostly fat; category C, those with moderately dense tissue; and category D, breasts where dense tissue makes up at least 75 percent of the breast.
The research team then looked over a range of known breast cancer risk factors — a family history of disease, a woman’s prior history of benign breast lesions, her breast density and having a first baby after the age of 30 — to determine the effect of each on cancer risk.
Overall, breast density was the most prevalent risk factor for breast cancer, the UCSF team reported.
Based on its calculations, Kerlikowske’s team estimated that about 39 percent of breast cancers in younger, premenopausal women could have been prevented if those in the two higher tissue density categories had been shifted to a lower density category. About a 26 percent reduction in breast cancers for older, postmenopausal women was also estimated, using the same calculations.
Unfortunately, there’s not much a woman can do about the density of her breast tissue, the researchers noted.
“Treatment with tamoxifen, an estrogen hormone blocker, is the only intervention currently known that substantially reduces breast density, and thus reduces breast cancer risk,” Engmann explained.
“However, tamoxifen can have serious side effects and is generally only recommended for women at high risk of breast cancer, with guidance from their physician. Our study highlights the need for new interventions to reduce breast density for women at average risk,” she added.
Simply gaining excess weight — even though it tends to add fatty tissue to breasts — would not lower a woman’s risk for breast cancer. That’s because the UCSF study, along with prior research, found that obesity also raises breast cancer risk in older, postmenopausal women.