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Detection of Breast Cancer by Screening Mammography Does Not Always Provide a Benefit

Making informed decisions about cancer screening involves understanding the benefits and the limitations of the test being used. No test is perfect, but in the case of recommended screening tests such as mammography the benefits are believed to outweigh the risks for most people. Nevertheless, it’s important not to overstate the benefits and to have realistic expectations about what a screening test can do. A study recently published in the Archives of Internal Medicine added to this discussion by exploring how often mammograms save a woman’s life. The question is more complex than it first appears.    

A mammogram is an X-ray of the breast. A screening mammogram is a mammogram that is performed in a woman without breast symptoms. The goal of screening mammography is 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 recommends that routine screening of average-risk women begin at age 50 and be performed every two years. The American Cancer Society recommends annual screening beginning at age 40. Women may wish to talk with their doctor about the approach that’s right for them. Women should also be aware that these screening recommendations do not apply to women with breast lumps or other breast symptoms; breast symptoms should be evaluated regardless of a woman’s age. 

For women who survive breast cancer that was detected by screening mammography, it’s natural to assume that the mammogram is responsible for their survival. This will sometimes be true: in some cases, early detection of cancer will allow women to be successfully treated for a cancer that would otherwise have taken their life. In other cases, however, detection by mammography will not alter the outcome of their cancer.  For example, some women will have cancer that would have been equally treatable even if it had been detected later, after symptoms developed. Improvements in treatment may make this scenario more likely. Other women may have very slow-growing cancers that will never affect their health during their lifetime, and that don’t actually need to be detected. Detection of these very slow-growing cancers is sometimes referred to as “overdiagnosis.” 

To explore how frequently screen detection of breast cancer is life-saving, researchers analyzed data from a large, national cancer registry (the National Cancer Institute’s Surveillance, Epidemiology, and End Results program). 

  • Among 50-year-old women with breast cancer detected by screening mammography, 13% will have their life saved by the mammogram. This estimate assumes that mammography reduces breast cancer mortality by 20%. If mammography has a smaller effect on breast cancer mortality (which is possible), the likelihood of screen detection being life-saving would be lower than 13%.  
  • Regardless of the age of the woman (40, 50, 60, or 70) or the mortality reduction provided by mammography (5 to 25%), the probability that screen detection saved a woman’s life never exceeded 25%. 

The researchers conclude that most women with breast cancer detected by screening mammography have not had their life saved by screening. They were more likely to have been diagnosed early without the early detection affecting their breast cancer outcome, or to have been overdiagnosed. 

These results do not mean that women should avoid being screened. Rather, the results provide additional information about what women can reasonably expect from mammographic screening. Mammography can save lives, but it’s not the final answer to the problem of breast cancer.  

Reference: Welch HG, Frankel BA. Likelihood that a woman with screen-detected breast cancer has had her “life saved” by that screening. Archives of Internal Medicine. Early online publication October 24, 2011. 

Posted October 27, 2011