Estimates show that if 10,000 women age 40 were screened every year for 10 years, about four lives would be saved. In comparison, regular screening of 10,000 women age 50 would save about 37 lives. In some cases, finding a breast tumor early may mean that a woman can choose surgery that saves her breast. Also, a woman whose breast tumor is detected in its early stages may not have to undergo chemotherapy. 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.