False negatives occur when mammograms appear normal even though breast cancer is actually present. Overall, mammograms will miss approximately 20 percent of the breast cancers that are present at the time of screening. False negatives occur more often with younger women than with older women. The dense breasts of younger women contain many glands and ligaments, which make breast cancers more difficult to spot in mammograms. Most abnormalities will turn out not to be cancer.
False positives are more common in younger women than older women.
About 30 percent of women ages 40 to 49 have a false positive mammogram, as compared with about 25 percent for women age 50 and older. However, all women must undergo followup procedures when they have an abnormal mammogram. Some will require breast biopsies. Some of these cases eventually go on to become invasive cancers , but many do not. Because it is not possible to predict which cases of DCIS will progress to invasive cancer, these tissue abnormalities are commonly removed surgically.