Cervical cancer prevented by screening: Long-term incidence trends by morphology in Norway

Both major morphologic types of cervical cancer, squamous cell carcinoma (SCC) and adenocarcinoma (AC), are causally related to persistent infection with high-risk human papillomavirus (hrHPV), but screening has primarily been effective at preventing SCC. We analysed incidence trends of cervical cancer in Norway stratified by morphologies over 55 years, and projected SCC incidence in the absence of screening by assessing the changes in the incidence rate of AC. The Cancer Registry of Norway was used to identify all 19,530 malignancies in the cervix diagnosed in the period 1956-2010. The majority of these (82.9%) were classified as SCCs, 10.5% as ACs and the remaining 6.6% were of other or undefined morphology. By joint-point analyses of a period of more than five decades, the average annual percentage change in the age-standardised incidence was -1.0 (95%CI: -2.1-0.1) for cervical SCC, 1.5 (95%CI:1.1-1.9) for cervical AC and -0.9 (95%CI: -1.4 to -0.3) for cervical cancers of other or undefined morphology. The projected age-standardised incidence rate of cervical SCC in Norway, assuming no screening, was 28.6 per 100,000 woman-years in 2010, which compared with the observed SCC rate of 7.3 corresponds to an estimated 74% reduction in SCC or a 68% reduction due to screening in the total cervical cancer burden. Cytology screening has impacted cervical cancer burden more than suggested by the overall observed cervical cancer incidence reduction since its peak in the mid-1970s. The simultaneous substantial increase in cervical adenocarcinoma in Norway is presumably indicative of an increase in exposure to HPV over time. What's new? Estimates indicate that cervical cancer incidence has decreased by half since the expansion of cytology screening in the 1970s in Norway. The decline has been attributed to the prevention of squamous cell carcinoma. This study shows, however, that screening has impacted cervical cancer burden beyond the overall observed reduction in incidence. Analysis of more than 50 years of incidence trends by cancer morphology reveals that the actual proportion of cervical cancers prevented by screening in Norway was 68%, considerably larger than previous figures. The difference is attributed to an increase in the background risk of cervical adenocarcinoma.