EVects 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 ageperiod-cohort (APC) modelling, using Poisson regression. Projection of incidence to 2006 was based on a linear trend for period eVects. A counterfactual scenario was constructed assuming stable period eVects (1972–74), but modelled cohort eVects. 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 eVects with recency of birth were found after controlling for age and period of diagnosis, and declining period eVects 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. (J Epidemiol Community Health 2001;55:782–788) Cervical cancer is a significant gynaecological cancer in women in most populations, and is preventable by regular screening by exfoliative cytology using the Papanicolaou technique. Screening identifies cervical intraepithelial neoplasia (CIN), which conveys a risk of development into invasive carcinoma; these lesions can then be removed or ablated, which reduces population incidence of invasive cervical carcinoma. Screening can also detect early asymptomatic invasive carcinoma and produce lower case fatality and improved survival. 2 Declines in cervical cancer mortality in populations over time is a consequence of declines in both incidence and case fatality. CIN and invasive carcinoma are consequences of causes that operate over years, often decades. The most important cause is probably infection with human papilloma virus (HPV), which is spread by sexual contact. Cervical cancer incidence and mortality shows generational (birth cohort) trends as successive cohorts are diVerentially aVected by exposure to causative influences, especially to the number of sexual partners during young adulthood, the primary source of infection with HPV. Strong birth cohort eVects are characteristic of the epidemiology of most cancers because cancer is usually a consequence of long term cumulative exposures from youth that derive from structural aspects of the physical, sociocultural and economic environment, which change slowly over generations. Age, period, and cohort (APC) models, as well as age-period and age-cohort models, Figure 1 Cervical cancer incidence in New South Wales 1972–96. EVects of age and period of diagnosis and birth cohort, and projections. 1