Effects of screening on cervical cancer incidence and mortality in New South Wales implied by influences of period of diagnosis and birth cohort.

STUDY OBJECTIVES Cervical cancer incidence and mortality in NSW during 1972-1996 is examined under counterfactual assumptions to estimate the number of new cervical cancer cases averted and deaths avoided, with projections to 2006. SETTING Cervical cancer incident cases and deaths in NSW for 1972-96 were obtained from the NSW Central Cancer Registry, Sydney, Australia. DESIGN Data were analysed by age-period-cohort (APC) modelling, using Poisson regression. Projection of incidence to 2006 was based on a linear trend for period effects. A counterfactual scenario was constructed assuming stable period effects (1972-74), but modelled cohort effects. Modelled rates were converted to cases and deaths (using mortality:incidence ratios for cervical cancer), and compared with actual data to estimate cancers prevented and deaths averted due to screening. RESULTS Rising cohort effects with recency of birth were found after controlling for age and period of diagnosis, and declining period effects were identified after controlling for age and birth cohort. The estimated cumulated number of new cases of cervical cancer prevented during 1972-1996 was 3440. The cumulated number of averted deaths over 1972-1996, derived from incident cases, was estimated to be 1610 (including actual declines in the M/I ratio). With no change in the M/I ratio from 1972, estimated cumulated mortality averted due to cervical cancer for 1972-1996 was 1210 deaths. CONCLUSIONS Cervical screening has prevented a substantial number of new cases of cervical cancer and deaths. In addition, secondary prevention and improved treatment has contributed further to cervical cancer deaths averted.