In Finland women are invited to cervical cancer screening every five years between the ages 30—60, with some municipalities also inviting 25 and 65-year-olds. Thus a woman may go through up to 9 routine screens during her lifetime. If borderline abnormalities are detected, additional follow-up screening is recommended. The purpose of this study was to determine the cumulative probability of having any cytological abnormalities by the age of 64 in organized cervical cancer screening. The focus of attention was particularly on the difference between borderline and more severe abnormalities. We also analyzed whether previous abnormalities were associated with the risk of detecting new ones.
Individual screening histories during 1991—2012 were collected for all women from the Mass Screening Registry of the Finnish Cancer Registry. Analysis of cumulative probabilities was restricted to cohorts born in 1950-1965 since they had the most comprehensive follow-up data with 1 207 017 routine screens and 88 143 follow-up screens among 364 487 women. The most severe result within a five-year screening round was treated as the outcome, detected either by routine or follow-up screening. Probabilities by age were estimated using logistic regression with a GEE approach which accounts for individual-level correlation.
The probability of experiencing any abnormality at least once by the age of 64 was 34.0% (95% CI: 33.3-34.6%). This was considerably high compared to the proportion of results warranting referral (5.4%, 95% CI: 5.0-5.8%) or results with histologically confirmed findings (2.2%, 95% CI: 2.0-2.4%). Previous occurrences were associated with an increased risk of detecting new ones, specifically in older cohorts.
The difference in the magnitude between mild and more severe results detected by the programme refers to a notable overdiagnosis of borderline results which may increase costs and have psychological downstream consequences. Improvement of diagnostic criteria concerning borderline abnormalities should reduce the problem. The overall detection of pre-cancerous lesions was very low, indicating that a proportion of pre-cancerous lesions might be treated outside the programme.