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. 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. Younger women have a higher proportion of DCIS than older women. Approximately 45 percent of breast cancers detected by screening mammograms in women ages 40 to 49 are DCIS compared with about 20 to 30 percent of those detected in women age 50 and older.