Breast density is one of the strongest predictors of the failure of mammography to detect cancer. About half of women younger than 50 are dense, and 1/3 of women over 50 are dense. Up to half of breast cancers cannot be seen on the mammogram in women with dense breasts. This leads to missed cancers that are discovered at later stages, resulting in more invasive treatments and a poorer prognosis. 40,000 women in the U.S. die each year from breast cancer. Some of these women had dense breasts and didn’t know it; even though they went for mammograms every year, by the time their cancers were found it was too late.
Density is determined by a mammogram. It cannot be determined with physical exam. Dense breasts have more fibrous and glandular tissue (which appears white on the films and can hide cancers, which also appear white), while breasts that are NOT dense have more fatty tissue (which appears grey and is easy to see through on the mammogram).