Dr. Phil Evans, director of the Center for Breast Care and Professor of Radiology at the University of Texas Southwestern Medical Center in Dallas, said about 40 percent of all U.S. women fall into the dense or extremely dense breast category, as classified by the American College of Radiology.
Breast density refers to the amount of fat and tissue in the breast as seen on a mammogram. A dense breast has more tissue than fat, he said.
"A challenge with mammograms is that it's often harder to distinguish healthy breast tissue from possible cancer in women with dense breast tissue, so those women -- and the radiologists who read their mammograms -- must be particularly vigilant," Evans said in a statement.
"Normal dense breast tissue is challenging because malignant tumors may have a similar mammographic appearance or the dense tissue may hide a malignancy."
This information helps women and their doctors discuss screening tests that are best for them. There are four categories of mammographic density, and the radiologist assigns each mammogram to one of the categories, Evans said.
"Your doctor should be able to tell you whether you have dense breasts based on your mammogram report," Evans said.