Trends in aggregate cancer incidence rates in relation to screening and possible overdiagnosis: A word of caution

Abstract

Background Overdiagnosis in breast cancer screening is a topic of debate. Researchers often estimate trends in incidence prior to screening and project these to predict incidence during the screening epoch. Methods Data was obtained from the Cancer Registry of Norway and the Norwegian Breast Cancer Screening Programme. Using breast cancer incidence prior to screening in Norway (1976–1995), incidence trends were estimated from age-period and age-cohort models. These estimates were used to predict the incidence of breast cancer in five-year age and period groups in the screening epoch (1996–2009). Results Excess numbers of cancers in the screening age range (6,876 cancers), and deficits in women above and below the screening age range (1,947 cancers) were observed. However, only part of the observed differences between the observed and the expected incidence can be explained by screening, as evidenced by numbers of excess cancers greater than the numbers of screen-detected cancers in some age groups and time periods. Conclusion There are potential errors in estimation of overdiagnosis from screening if individual data on screening exposure and detection mode are not taken into account. For reliable estimates of overdiagnosis, it is necessary to compare excess incidence in the screening period in those actually screened with the corresponding excess in those not screened. This is the subject of ongoing research.