Effect of a mass screening program on the risk of cervical cancer.

In Finland, the organized mass screening program for the early detection of cervical cancer covers, with few exceptions, all women between the ages of 30 and 55 every fifth year. On the basis of material originating from the Finnish Cancer Registry it was estimated that the probability for a woman aged 30-59 to experience frankly invasive cervical cancer was 0.010 before the screening program, and 0.002 after the first screening. The corresponding probability estimate was 0.022 for pre-invasive lesions subjected to operative treatment. On the assumption that the same relationship applies beyond this age group it was estimated that from 28-39% of the pre-invasive cases progress to invasive cervical cancer, and that 21% of the frankly invasive cases are preceded by a pre-invasive stage of shorter duration than the time period between the screenings, or have no preclinical stage.