Contrary to popular belief, annual mammograms may not save as many lives as once thought—and may even lead to substantial overdiagnosis of breast cancer, according to the results of a study published in the New England Journal of Medicine.
A mammogram is an X-ray of the breast. Screening mammography is performed in a woman without breast symptoms in order to detect breast cancer at an early stage when it is most easily treated. Different groups of experts have reached different conclusions about when mammographic screening should begin and how often it should be performed. The U.S. Preventive Services Task Force (USPSTF) recommends that routine screening of average-risk women begin at age 50 and be performed every two years. A number of other groups, including the American Cancer Society, the American College of Radiology, the American Congress of Obstetricians and Gynecologist, and the American Medical Association, recommend annual screening beginning at age 40.
Currently, mammography is the most reliable tool for screening the general population for breast cancer; however, the screening tool does have its limitations—which can include false-positive test results (a suggestion that cancer may be present when it is not); false-negative results (missed cancers); and overdiagnosis (resulting in unnecessary treatment). Put simply—overdiagnosis is the diagnosis of something that would not have resulted in clinical symptoms in a person’s lifetime.
Using data from the Surveillance, Epidemiology, and End Results (SEER) database, researchers examined trends in breast cancer detection in the U.S. from 1976 through 2008. They compared rates of breast cancer diagnoses before and after mammograms became widely used for screening and adjusted their results to exclude cancers thought to be related to the widespread use of hormone replacement therapy.
The results indicated that mammograms did identify more early-stage breast cancers; however, this identification did not necessarily prevent late-stage disease, nor did it appear to substantially reduce the rate of death from breast cancer. Here’s what the researchers found:
- The number of cases of early-stage breast cancer detected each year has doubled since the introduction of screening mammography. By the numbers, this translates to an increase of 122 cases per 100,000 women.
- At the same time, the rate at which women are initially diagnosed with late-stage breast cancer has decreased only slightly—by 8 percent, or dropping by 8 cases from 102 to 94 per 100,000.
- Only 8 of the 122 additional early-stage cancers were expected to progress to advanced disease.
- Over the 30-year period of the study, death rates from breast cancer in women over the age of 40 dropped by 28 percent; however, breast cancer death rates dropped 42 percent among women under the age of 40 (a group that does not routinely undergo screening mammography).
Based on the data, the researchers reached several conclusions:
- They estimated that breast cancer was overdiagnosed in 1.3 million U.S. women over the past 30 years.
- They estimated that in 2008, breast cancer was overdiagnosed in more than 70,000 women—or one-third of all breast cancers diagnosed.
- Screening mammography has only slightly reduced the rate at which women present with advanced cancer.
- Screening mammography has only a small effect on the rate of death from breast cancer—in fact, decreased mortality appears to be the result of improved treatment, not screening.
The researchers concluded that screening mammography might cause more harm than good because it leads to substantial overdiagnosis. A large number of women may be receiving treatment for early-stage breast cancer that would never have caused any symptoms; however, at this point, it’s impossible to determine which cancers are harmless and which are more serious. The researchers support the USPSTF guidelines that delay annual mammography screening until age 50 and test every other year until age 75.
Of course, the issue of screening mammography is not black and white. Women who are considered high risk—either because of a family history or a genetic predisposition—should adhere to stricter screening guidelines, which include beginning screening earlier and repeating it at more frequent intervals.
The bottom line—one size does not fit all when it comes to breast cancer screening mammography, but this new data indicates that average-risk women may want to wait until age 50 to begin screening.
Bleyer A, Welch HG. Effect of three decades of screening mammography on breast-cancer incidence. New England Journal of Medicine. 2012; 367: 1998-2005.