Study: Mammograms often lead to cancer overdiagnosis

Breast cancer screening often results in overdiagnosis of the disease, according a study published this week in the Annals of Internal Medicine. What's more, the potential for overdiagnosis is greater in the U.S. than in other nations, researchers say, considering many women begin such screenings at age 40, 10 years before European doctors generally begin such tests.

This latest study, which examines a screening program used in Norway from 1996 to 2005, determines that between 15 and 25 percent of breast cancer cases are overdiagnosed. According to lead study author Mette Kalager, a visiting scientist with the Harvard School of Public Health, when patients with tumors don't show symptoms, those tumors often will still receive treatment; in such cases, she says, doctors can't tell if the tumor ever had the potential to mature, Medscape Medical News reports.

Specifically, for every 2,500 women asked to participate in screening, six to 10 were overdiagnosed; 39,888 patients with invasive breast cancer were part of the study.

Because screening programs for women in the U.S. tend to start at age 40, rather than age 50 as is the case in Europe, the potential for overdiagnosis is greater, say Joann Elmore, M.D.,  and Suzanne Fletcher, M.D. Elmore in an editorial about the study. Elmore is from the University of Washington School of Medicine, while Fletcher represents Harvard Medical School and Harvard Pilgrim Health Care Institute in Boston.

"The underlying tenet of cancer screening has saved many lives," they write. "However, studies are beginning to challenge the certainty that finding cacner early is always better. ... Unfortunately, mammography screening programs cannot distinguish between fatal and harmless breast cancer."

A similar study published in January 2010 found that women who undergo routine breast cancer screenings in Britain tend to be overdiagnosed and subject to unnecessary treatment.

To learn more:
- here's the study's abstract (subscription required)
- read the accompanying editorial (subscription required)
- check out the Medscape Medical News article

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