Mammography may not help a woman with a fast-growing or aggressive cancer that has already spread to other parts of her body before being detected. False negatives occur when mammograms appear normal even though breast cancer is actually present. False negatives are more common in younger women than in older women. The dense breasts of younger women contain many glands and ligaments, which make breast cancers more difficult to spot in mammograms. Screening mammograms miss up to 25 percent of breast cancers in women in their forties compared with about 10 percent of cancers for older women.
False positives occur when mammograms are read as abnormal, but no cancer is actually present.
Most abnormalities will turn out not to be cancer. False positives are more common in younger women than older women. About 97 percent of women ages 40 to 49 who have abnormal mammograms turn out not to have cancer, as compared with about 86 percent for women age 50 and older. But all women have to undergo followup procedures when they have an abnormal mammogram. Some of these can eventually go on to become actual cancers, but many do not.