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BMJ 2012; 344 doi: (Published 11 April 2012) Cite this as: BMJ 2012;344:e2602

Screening overdiagnoses 6-10 women for every death from breast cancer prevented

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Researchers estimate that breast cancer would never have become apparent in 15-25% of women diagnosed through the Norwegian screening programme. For every 2500 women invited for screening, six to 10 women are overdiagnosed, 20 cases of breast cancer are detected and treated early, and one death from breast cancer is prevented.

Previous reports have estimated overdiagnosis in breast cancer screening programmes as between 0% and 54%. Rates of overdiagnosis are difficult to estimate because breast cancer trends irrespective of screening must be taken into account and lead time (the amount of time a diagnosis by mammography precedes a clinical diagnosis) must be adjusted for. The various methods used by researchers yield very different results, and there is no universally accepted method to estimate overdiagnosis.

The current researchers took advantage of a national screening programme that was gradually implemented by geographical region over nine years. Attendance was high. Only invasive breast cancer was taken into account and carcinoma in situ was excluded from the analysis. To account for temporal trends, the incidence of breast cancer with screening was compared with historical incidence in the same region before screening was implemented, as well …

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