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Mammography results in substantial overdiagnosis of breast cancer, concludes study

BMJ 2012; 345 doi: (Published 21 November 2012) Cite this as: BMJ 2012;345:e7910
  1. Jacqui Wise
  1. 1London

Screening mammography has resulted in the overdiagnosis of breast cancer in 1.3 million women in the United States in the past 30 years, concludes a new study.

Overdiagnosis means that tumors are detected on screening that would never lead to clinical symptoms, so women undergo unnecessary surgery, radiotherapy, and chemotherapy. The study, in the New England Journal of Medicine,1 estimated that more than 70 000 women in the US were overdiagnosed in 2008, representing almost a third of all cases of breast cancer diagnosed in women aged 40 or over.

This latest research adds to the growing debate about the merits of screening mammography. Only a few weeks ago an expert panel concluded that every year 4000 women in the United Kingdom underwent treatment for breast cancer that would not have harmed them.2

Using US national trend data from 1976 to 2008, the researchers found that despite substantial rises in the number of cases of early stage breast cancer detected, screening mammography only marginally reduced the rate at which women presented with advanced cancer.

The introduction of screening mammography resulted in a doubling in the number of cases of early stage breast cancer that were detected each year, an absolute increase of 122 cases per 100 000 women. At the same time the rate at which women presented with late stage cancer fell by 8%, an absolute decrease of eight cases per 100 000 women.

Although the rate of death from breast cancer fell considerably over the same period, the authors said that this was largely due to the result of better treatment, not screening. Among women aged over 40, deaths from breast cancer fell from 71 to 51 per 100 000 women, a 28% decrease.

Because the absolute reduction in deaths (20 per 100 000 women) was larger than the absolute reduction in the number of cases of late stage cancer (eight per 100 000), the contribution of early detection to the decrease in the number of deaths must be small, said the researchers. And the small reduction in cases of late stage cancer was confined to regional (largely node positive) disease, which can now often be treated successfully.

The authors, Archie Bleyer of St Charles Health System, Oregon, and H Gilbert Welch of the Dartmouth Institute for Health Policy and Clinical Practice, Hanover, New Hampshire, concluded: “Our study raises serious questions about the value of screening mammography. It clarifies that the benefit of mortality reduction is probably smaller, and the harm of overdiagnosis probably larger, than has been previously recognized.”

They continued: “Although no one can say with any 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.”


Cite this as: BMJ 2012;345:e7910


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