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Thyroid cancer has increased in the adult populations of countries moderately affected by Chernobyl fallout.

BACKGROUND
The incidence of thyroid carcinoma increased among children affected by Chernobyl fallout. Less evidence exists for a corresponding effect in adolescents and adults. The Cancer Registry of the Czech Republic provides an opportunity to study various determinants of the occurrence of thyroid cancer.


MATERIAL/METHODS
Anonymous population-based incidence data on thyroid carcinoma of the Czech Republic from 1976 to 1999 were obtained from the Czech Statistical Office (CSO) and the Institute of Health Information and Statistics (IHIS). This study covers 247 million person-years. Linear logistic regression models allowing for possible changes in slope (change-points) are suggested for the trends of incidence proportions.


RESULTS
From 1976 to 1999 a uniform annual increase of 2.0% per year was found in the directly age-standardized thyroid cancer incidence proportion (95%-CI: 1.3-2.7, p<0.0001). From 1990 on, we observed an additional significant increase in the thyroid cancer incidence of 2.6% per year (95%-CI:1.2-4.1, p=0.0003). This effect (change-point) is essentially independent of age but dependent on gender: females 2.9% per year (95%-CI: 1.3-4.7, p=0.0006), males 1.8% per year (95%-CI: -1.0-4.7,p=0.2127). The estimated minimum latency period for the population as a whole is 4 years.


CONCLUSIONS
Although the Czech Republic received only a relatively moderate amount of radioactive fallout,an unexpected uniformly accelerated increase of thyroid cancer in all age categories is seen from 1990 onwards. Therefore one should look carefully at collective dose and at the group of persons low in individual organ dose but high in number.