Breast density refers to the relative amount of fat pictured here in yellow versus connective and epithelial tissues pictured in pink.

Many of the women who believe that their lives were saved by mammography screenings actually had so called IDLE cancers (indolent lesions of epithelial origin), which would not have progressed before they died of other causes. Meanwhile, the very aggressive cancers, which are most likely to prove fatal, tend to present as interval cancers that appear between screenings.