Duration of preclinical cervical cancer and reduction in incidence of invasive cancer following negative pap smears.

Abstract

An investigation has been made into the differences between estimates for the duration of preclinical cervical cancer resulting from two types of studies. A median duration of 5-10 years was suggested by the observed build-up of incidence of invasive cervical cancer after one or more negative smears. Much longer median durations of more than 15 years have been reported from fitting statistical models to screening data. We developed one of these statistical models and fitted it to clinical incidence and screening data from British Columbia, which resulted in estimated mean durations of 12 years for pre-invasive stages, and 4-5 years for screen-detectable stages. The model is used to predict the build-up of the incidence of invasive cancer after one and after two negative smears. The model predictions appear to correspond closely to the observed incidence trends following negative smears. The apparent contradiction between model estimates and observed data is explained by recognizing that many of the women who have had negative smears will have further Pap smears, resulting in earlier diagnosis of invasive cervical cancers and thus an apparent faster build-up of the incidence. When the impact of further Pap smears is neglected, the data suggest that the risk of invasive cancer following one or more negative smears returns to close to prescreening levels within 6-10 years. This is an overestimation of the risk of clinical invasive cancer. In the case of cessation of screening it will take longer before the incidence of clinical cancer will increase. Where there is continuous screening the screen-detected cancers have a relatively favourable prognosis, thus contributing less to the serious morbidity and mortality risks associated with invasive cancer. This should be taken into account in making comparisons with the prescreening situation.