title: Effect of screening on the risk estimates of socio demographic factors on cervical cancer - a large cohort study from rural India.
abstract: BACKGROUND
  Prospective cohort studies to determine cofactors with oncogenic HPV- infections for cervical cancer are very rare from developing countries and such data are limited to the few screening trials. Large screening trials provide such data as a by product. Some of the cases are prevented by screening and do not surface as invasive cancers at all. Also, pre-invasive lesions are detected almost entirely by screening. Screening causes selection bias if attendance in or effectiveness of screening is correlated with the risk factors. The aim of this study was to quantify the influence of screening on risk factors for cervical cancer.
  
  
  METHODS
  Our material stems from a rural cohort of 80,000 women subjected to a randomised screening trial. The effect of screening on the incidence of cervix cancer was estimated with reference to socio-demographic and reproductive risk factors of cervical cancer. We compared these risks with the incidence of cancer in the randomised control population by the same determinants of risk.
  
  
  RESULTS
  The results in the screening arm compared to the control arm showed that the women of low SES and young age were benefitting more than those of high SES and old age. The relative risk by age (30-39 vs 50-59) was 0.33 in the control arm and 0.24 in the screening arm. The relative 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 vs screening arms) whereas the effect was substantial in those married (86 vs 54).
  
  
  CONCLUSIONS
  The results in controls were consistent with the general evidence, but results in attenders and nonattenders of the screening arm showed that screening itself and self-selection in attendance and effectiveness can influence the effect estimates of risk factors. The effect of cervical cancer screening programmes on the estimates of incidence of cervical cancer causes bias in the studies on etiology and, therefore, they should be interpreted with caution.
authors.affiliation: Achutha Menon Centre for Health Science Studies, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, India. jissa@sctimst.ac.in
authors.forename: Jissa Vinoda
authors.initials: JV
authors.lastname: Thulaseedharan
authors.forename: Nea
authors.initials: N
authors.lastname: Malila
authors.forename: Matti
authors.initials: M
authors.lastname: Hakama
authors.forename: Pulikottil Okuru
authors.initials: PO
authors.lastname: Esmy
authors.forename: Mary
authors.initials: M
authors.lastname: Cherian
authors.forename: Rajaraman
authors.initials: R
authors.lastname: Swaminathan
authors.forename: Richard
authors.initials: R
authors.lastname: Muwonge
authors.forename: Rengaswami
authors.initials: R
authors.lastname: Sankaranarayanan
mesh_terms: D000328:Adult; D000367:Age Factors; D015982:Bias; D055088:Early Detection of Cancer; D004522:Educational Status; D005260:Female; D020073:Gravidity; D006801:Humans; D015994:Incidence; D007194:India / Q000453:epidemiology; D017533:Marital Status; D008875:Middle Aged; D010298:Parity; D012307:Risk Factors; D012424:Rural Population / Q000706:statistics & numerical data*; D012923:Social Class; D002583:Uterine Cervical Neoplasms / Q000175:diagnosis* / Q000453:epidemiology*