When a patient has dense breast tissue, the radiologist can be limited when interpreting the mammogram. Small masses can hide within the dense tissue, limiting cancer detection. This is called the masking effect. Women with dense tissue are at increased risk for interval cancer, which is defined as a cancer that presents as a palpable mass during the interval between recommended rounds of screening (one year in the U.S., but up to 3 years in other countries). When breast cancer is detected at screening, women with dense tissue tend to have cancers that are larger and more likely a higher stage with positive lymph nodes.
Breast density and the associated masking of cancer has become a political issue in the United States. Connecticut became the first state to enact legislation requiring that women receive notification about their breast density, and the limitations it can cause, with their mammography results. This movement has spread and resulted in similar laws in almost all 50 U.S. states.