Screening effects in risk studies of thyroid cancer after the Chernobyl accident |
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Authors: | Jan Christian Kaiser P Jacob M Blettner S Vavilov |
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Institution: | 1.Helmholtz-Zentrum München, German Research Centre for Environmental Health,Institute of Radiation Protection,Neuherberg,Germany;2.Institut für Medizinische Biometrie,Epidemiologie und Informatik Johannes Gutenberg-Universit?t Mainz,Mainz,Germany;3.Radiation Protection Institute,Academy of Technological Sciences,Kiev,Ukraine |
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Abstract: | In this article scenarios have been developed, which simulate screening effects in ecological and cohort studies of thyroid
cancer incidence among Ukrainians, whose thyroids have been exposed to 131I in the aftermath of the Chernobyl accident. If possible, the scenarios were based on directly observed data, such as the
population size, dose distributions and thyroid cancer cases. Two scenarios were considered where the screening effect on
baseline cases is either equal to or larger than that of radiation-related thyroid cancer cases. For ecological studies in
settlements with more than ten measurements of the 131I activity in the human thyroid in May–June 1986, the screening bias appeared small (<19%) for all risk quantities. In the
cohort studies, the excess absolute risk per dose was larger by a factor of 4 than in the general population. For an equal
screening effect on baseline and radiation-related cancer (Scenario 1) the excess relative risk was about the same as in the
general population. However, a differential screening effect (Scenario 2) produced a risk smaller by a factor of 2.5. A comparison
with first results of the Ukrainian–US-American cohort study did not give any indication that a differential screening effect
has a marked influence on the risk estimates. The differences in the risk estimates from ecological studies and cohort studies
were explained by the different screening patterns in the general population and in the much smaller cohort. The present investigations
are characterized by dose estimates for many settlements which are very weakly correlated with screening, the confounding
variable. The results show that under these conditions ecological studies may provide risk estimates with an acceptable bias. |
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