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Screening mammography overdiagnoses women with breast cancer

Clinical Question:
How often does mammography identify breast cancer that was never destined to become clinically apparent?

Bottom Line:
The concept of overdiagnosis is getting a lot of play in books, magazines, and news articles because it occurs with most screening tests. Sometimes a screening test identifies a disease that would otherwise never have become apparent in a patient's lifetime -- they would have died of something else, blissfully ignorant. It is not the same as a false positive result; the positive results are true, but they aren't helpful and may be harmful. Overdiagnosis makes treatment look better than it actually is because the treated patients were not destined to be harmed by the disease. Be prepared for the "Isn't it better to know?" questions. Probably not, it turns out, since overdiagnosed patients are exposed to all the risks of treatment without the ability to benefit. In this study, performed in Norway, between 1 in 4 and 1 in 7 women who had mammography-identified breast cancer were overdiagnosed. Unfortunately, at this stage, we don't know who will be overdiagnosed. (LOE = 1b)

Kalager M, Adami HO, Bretthauer M, Tamimi RM. Overdiagnosis of invasive breast cancer due to mammography screening: Results from the Norwegian Screening Program. Ann Intern Med 2012;156(7):491-499.  [PMID:22473436]

Study Design:
Cohort (prospective)



Overdiagnosis is a tricky concept, based on the premise that people can die of one illness, disease, or accident while having other diseases or illnesses that, at the time of death, were not clinically apparent and found only if autopsy was performed. Since the ultimate trial -- a randomized study of screening versus no screening, following everyone up until they died -- cannot be done, overdiagnosis has to be estimated from large studies comparing groups that were screened and groups that weren't. These researchers studied data from Norway collected between 1996 to 2005, comparing breast cancer rates in counties in which screening was implememented vs counties that did not have screening. As screening is implemented, the number of breast cancer diagnoses should go up at first and then decline; if it doesn't, it's because of overdiagnosis. Using 2 different methods of analysis, the estimated rate of overdiagnosis ranged from 18% to 25% for method one and 15% to 20% using method two. In other words, between 1 in 4 and 1 in 7 women diagnosed with breast cancer would have died -- due to something else -- without ever knowing they had breast cancer had they not been screened. These women were exposed to all the risks and problems of treatment without a chance to benefit. Translated to screening, 6 to 10 women were overdiagnosed for every 2500 women screened.


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