Epidemiological evidence for two components of cervical cancer.

Incidence rates for cervical cancer were analyzed for Finland in 1953 to 1974. The age-specific incidence rates fit an age-incidence curve based on the assumption of two epidemiological components. The time trends give support to the hypothesis that one of the components occurs at a lower age and is preceded by a carcinoma in situ stage. The risk of cervical cancer originating from the component occurring in younger women showed a decreasing trend which correlated with the organized mass screening programme. The other component occurred later in life and was not influenced by the screening programme. It was estimated that the proportion of the cases in the earlier category decreased from 53% in the 1950s to 17% in 1973 to 1974 of all cases of cervical cancer diagnosed in the Finnish population.