The main goal of screening mammography is detecting early breast cancer. Early detection permits treatment of lesions, in many cases, before the cancer has had a chance to spread beyond the breast. Treatment will require removing less breast tissue.
Cancer develops through a series of sequential events.
The earlier the changes are detected, the easier it is to treat the cancer, and the better the chance of complete cure. In some cases, screening will detect pre-cancerous states. Many of the early changes of cancer look no different from the normal pattern in the breast, except that they represent a change from a prior mammogram. A prior mammogram increases the radiologist's ability to detect the smallest cancers. If prior mammograms are not available, then cancers often will be larger when they are detected.