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

Researchers estimate that breast cancer would never have become apparent in 15 - 25% of women diagnosed through the Norwegian screening programme. For every 2 500 women invited for screening, 6 - 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 9 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 as with concurrent incidence of breast cancer in countries not yet included in the screening programme. Two different methods were also used to adjust for lead time.

Since 2005, all Norwegian women aged 50 - 69 years have been invited for mammography every 2 years.

Kalager M, et al. Ann Intern Med 2012;156:491-499.