The impact of organised screening programmes on the stage-specific incidence of breast cancer in some Italian areas

Abstract

The aim of this study was to examine the effects of mammographic screening programmes on stage-specific incidence of breast cancer. The study compared prescreening and screening periods in seven areas in Italy, primarily evaluating the first screening round. All 17617 breast cancers (16554 invasive, 1063 in situ) registered in women aged 40-79 years between 1988 and 1999 were analysed through age-standardised rates and Poisson regression models. For all areas, independent of the baseline rates, the introduction of screening increased incidence for invasive cancers overall and, more markedly, for early cancers (screening/prescreening ratio: range 1.07-1.47 and 1.23-1.82, respectively), modifying the pattern of age-specific rates. The multiple regression analysis showed that the percentage of cases diagnosed at screening explained most of the increase; a residual effect of diagnosis period (screening versus prescreening) suggested a role for 'spontaneous' early detection in ages outside of the screening programme. Advanced cases did not show consistent variations across the registries for those aged 40-79 years (range: 0.91-1.21), whereas a more coherent picture was observed for those aged 50-69 years. In one area, a moderate reduction in the number of 'advanced' cases in the second screening period was observed. For all stages, the age-specific incidence rates of cases diagnosed outside of the screening programme for the age groups 50-69 years were lower than the corresponding rates in the prescreening period, suggesting a shift from the usual clinical services to the screening programme. Our results confirmed the increase in early-stage cancers occurring at the start of screening, and substantially explained the rise in breast cancer incidence. In addition, our study confirms the importance of cancer registries in monitoring the effect of breast cancer screening and the validity, for this purpose, of the linkage between cancer registries and screening programme databases.