Stage distribution at first and repeat examinations in breast cancer screening

Objectives To investigate observed stage distributions at first and repeat screenings. To compare the observed outcomes with expected values based on simulation modelling, varying the assumptions about the natural history of the disease. Methods-An overview is made of observed data on stage distribution at first and repeat screenings and the difference between those distributions is summarised in a Gini coefficient. Four possible explanations for the observations are considered, two of these are worked out as Miscan simulation models, and the outcomes are compared with observations. Results-Often the reported stage distributions at repeat screenings are not or only slightly more favourable than at first screenings and, in the ones that are more favourable, the difference is relatively small. If, in the Miscan model,it is assumed that there is no correlation between the duration of preclinical breast cancer in consecutive tumour size categories and that there is a strong influence of latent cancers, it is not possible to reproduce the observed outcomes. Conclusions-The two modelled explanations are not sufficient. Decreasing sensitivity seems an unlikely explanation for the discrepancy in many screening programmes. The possibility that the observations may be explained because false reassurance has been given should be seriously considered and investigated.