Forty years of repeated screening: the significance of carcinoma in situ.

Two cohorts of women born in 1914-18 and 1929-33 who participated in a cervical screening programme have been followed for over 40 years. Age-specific incidence rates of squamous carcinoma of the cervix by rank of smear and length of interval between smears are reported. The younger cohort, who had undergone more frequent screening, had lower rates of invasive disease. From these incidence rates, estimates of false-negative rates and regression rates for carcinoma in situ have been made. The false-negative rate was estimated to be about 15%. Regression seemed more frequent in younger than in older women. For the younger cohort it was estimated to be 72% and in the older 47%. A comparison of the rates of in situ carcinoma with those of invasive disease suggests that the screening of the younger cohort reduced the rate of invasive disease to at least one-half or one-third of what it would have been if screening had commenced later. Rates of disease appear less dependent on age than previously thought and are consistent with causation by an infective agent.