Analysis Suggests Screening Mammography Results in Substantial Overdiagnosis with Small Effect on Mortality

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An analysis of Surveillance, Epidemiology, and End Results (SEER) data from 1976 through 2008 “suggests that whatever the mortality benefit, breast-cancer screening involved a substantial harm of excess detection of additional early-stage cancers that was not matched by a reduction in late-stage cancers,” according to a report in The New England Journal of Medicine. “This imbalance indicates a considerable amount of overdiagnosis involving more than 1 million women in the past 3 decades—and, according to our best-guess estimate, more than 70,000 women in 2008 (accounting for 31% of all breast cancers diagnosed in women 40 years of age or older),” the authors wrote.

The introduction of mammography has been associated with a large increase in early-stage breast cancer detected each year, from 112 to 234 cancers per 100,000 women—an absolute increase of 122 cancers per 100,000 in the United States. There has also been a decrease in the cases of late-stage breast cancer—from 102 to 94 cases per 100,000.

“With the assumption of a constant underlying disease burden, only 8 of the 122 additional early-stage cancers diagnosed were expected to progress to advanced disease. After excluding the transient excess incidence associated with hormone-replacement therapy and adjusting for trends in the incidence of breast cancer among women younger than 40 years of age, we estimated that breast cancer was overdiagnosed (ie, tumors were detected on screening that would never have led to clinical symptoms) in 1.3 million U.S. women in the past 30 years,” the analysts explained.

Serious Questions

“Our study raises serious questions about the value of screening mammography,” the authors concluded. “It clarifies that the benefit of mortality reduction is probably smaller, and the harm of overdiagnosis probably larger, than has been previously recognized. And although no one can say with certainty which women have cancers that are overdiagnosed, there is certainty about what happens to them: They undergo surgery, radiation therapy, hormonal therapy for 5 years or more, chemotherapy, or (usually) a combination of these treatments for abnormalities that otherwise would not have caused illness. Proponents of screening should provide women with data from a randomized screening trial that reflects improvements in current therapy and includes strategies to mitigate overdiagnosis in the intervention group. Women should recognize that our study does not answer the question ‘Should I be screened for breast cancer?’ However, they can rest assured that the question has more than one right answer.”

Bleyer A, Welch HG: N Engl J Med 367:1998-2005, 2012.




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