Reliability of underlying incidence rates for estimating the effect and efficiency of screening for breast cancer.

The process of setting screening performance targets requires an estimate of what the incidence of breast cancer would have been in the population invited for screening if there had not been a screening programme. Before the introduction of the National Health Service breast screening programme in 1988 the incidence of breast cancer was already increasing in the population targeted for screening. To establish the incidence before screening the most recent complete data from all the regional cancer registries were collated. An age-period model was constructed to predict what these incidence rates might now have been if the screening programme had not been introduced. The model predicted that if prescreening trends continued (between 1987 and 1995) underlying incidence over this period would increase by 2.3 per 10,000 in women aged 50-54, 2.6 per 10,000 in women aged 55-59, and 2.9 per 10,000 in women aged 60-64. If the prescreening trends have continued then the use of a universal rate, applied across all calendar years and age groups, would seem to be inappropriate when setting targets.