Effect of Screening on the Risk Estimates of Socio DemographicFactors on Cervical Cancer - A Large Cohort Study from RuralIndia

Background: Prospective cohort studies to determine cofactors with oncogenic HPV- infections for cervicalcancer are very rare from developing countries and such data are limited to the few screening trials. Largescreening trials provide such data as a by product. Some of the cases are prevented by screening and do notsurface as invasive cancers at all. Also, pre-invasive lesions are detected almost entirely by screening. Screeningcauses selection bias if attendance in or effectiveness of screening is correlated with the risk factors. The aim ofthis study was to quantify the influence of screening on risk factors for cervical cancer. Materials and
Methods:Our material stems from a rural cohort of 80,000 women subjected to a randomised screening trial. The effect ofscreening on the incidence of cervix cancer was estimated with reference to socio-demographic and reproductiverisk factors of cervical cancer. We compared these risks with the incidence of cancer in the randomised controlpopulation by the same determinants of risk.
Results: The results in the screening arm compared to the controlarm showed that the women of low SES and young age were benefitting more than those of high SES and oldage. The relative risk by age (30-39 vs 50-59) was 0.33 in the control arm and 0.24 in the screening arm. Therelative risk by education (not educated vs educated) was 2.8 in the control arm and 1.8 in the screening arm.The previously married women did not benefit (incidence 113 and 115 per 100,000 women years in control vsscreening arms) whereas the effect was substantial in those married (86 vs 54).
Conclusions: The results incontrols were consistent with the general evidence, but results in attenders and nonattenders of the screening armshowed that screening itself and self-selection in attendance and effectiveness can influence the effect estimates ofrisk factors. The effect of cervical cancer screening programmes on the estimates of incidence of cervical cancercauses bias in the studies on etiology and, therefore, they should be interpreted with caution.