Seventeen years of organised breast screening in Denmark has not reduced the incidence of advanced tumours but has markedly increased the incidence of non-advanced tumours and ductal carcinoma in situ, research published in the Annals of Internal Medicine has shown.1
The researchers calculated that one in three breast tumours detected in women aged 50 to 69 was probably overdiagnosed and said that screening mammography did not seem to have delivered its promised benefit of earlier diagnosis.
Organised breast cancer screening programmes began in various regions of Denmark at different times and covered only 20% of the population. This allowed the researchers, from the Nordic Cochrane Centre in Copenhagen, to compare the size of diagnosed breast tumours in different regions with and without screening in the same period. Tumours were classed as advanced if they were bigger than 20 mm and as non-advanced if they were 20 mm or smaller.
The study found that screening was not associated with a lower incidence of advanced tumours. The incidence of non-advanced tumours was higher in screening periods than in non-screening periods (incidence rate ratio 1.49 (95% confidence interval (1.43 to 1.54)).
Overdiagnosis occurs when mammography detects small tumours that may never affect patients’ health during their lifetime. To estimate overdiagnosis the researchers analysed trends in advanced and non-advanced cancer in the screen...