Impact of uncertainties in exposure assessment on estimates of thyroid cancer risk among Ukrainian children and adolescents exposed from the Chernobyl accident.

The 1986 accident at the Chernobyl nuclear power plant remains the most serious nuclear accident in history, and excess thyroid cancers, particularly among those exposed to releases of iodine-131 remain the best-documented sequelae. Failure to take dose-measurement error into account can lead to bias in assessments of dose-response slope. Although risks in the Ukrainian-US thyroid screening study have been previously evaluated, errors in dose assessments have not been addressed hitherto. Dose-response patterns were examined in a thyroid screening prevalence cohort of 13,127 persons aged <18 at the time of the accident who were resident in the most radioactively contaminated regions of Ukraine. We extended earlier analyses in this cohort by adjusting for dose error in the recently developed TD-10 dosimetry. Three methods of statistical correction, via two types of regression calibration, and Monte Carlo maximum-likelihood, were applied to the doses that can be derived from the ratio of thyroid activity to thyroid mass. The two components that make up this ratio have different types of error, Berkson error for thyroid mass and classical error for thyroid activity. The first regression-calibration method yielded estimates of excess odds ratio of 5.78 Gy(-1) (95% CI 1.92, 27.04), about 7% higher than estimates unadjusted for dose error. The second regression-calibration method gave an excess odds ratio of 4.78 Gy(-1) (95% CI 1.64, 19.69), about 11% lower than unadjusted analysis. The Monte Carlo maximum-likelihood method produced an excess odds ratio of 4.93 Gy(-1) (95% CI 1.67, 19.90), about 8% lower than unadjusted analysis. There are borderline-significant (p = 0.101-0.112) indications of downward curvature in the dose response, allowing for which nearly doubled the low-dose linear coefficient. In conclusion, dose-error adjustment has comparatively modest effects on regression parameters, a consequence of the relatively small errors, of a mixture of Berkson and classical form, associated with thyroid dose assessment.

Radiation dosimetry for highly contaminated Belarusian, Russian and Ukrainian populations, and for less contaminated populations in Europe.

The explosions at the Chernobyl Nuclear Power Plant (CNPP) in Ukraine early in the morning of 26 April 1986 led to a considerable release of radioactive materials during 10 d. The cloud from the reactor spread many different radionuclides, particularly those of iodine (131I) and cesium (134Cs and 137Cs), over the majority of European countries, but the greatest contamination occurred over vast areas of Belarus, the Russian Federation and Ukraine. As the major health effect of Chernobyl is an elevated thyroid cancer incidence in children and adolescents, much attention has been paid to the thyroid doses resulting from intakes of 131I, which were delivered within 2 mo following the accident. The thyroid doses received by the inhabitants of the contaminated areas of Belarus, Russia, and Ukraine varied in a wide range, mainly according to age, level of ground contamination, milk consumption rate, and origin of the milk that was consumed. Reported individual thyroid doses varied up to approximately 40,000 mGy, with average doses of a few to 1,000 mGy, depending on the area where people were exposed. In addition, the presence in the environment of long-lived 134Cs and 137Cs has led to a relatively homogeneous exposure of all organs and tissues of the body via external and internal irradiation, albeit at low rates. Excluding the thyroid doses, the whole-body (or effective) dose estimates for the general population accumulated during 20 y after the accident (1986-2005) range from a few millisieverts (mSv) to some hundred mSv with an average dose of approximately 10 mSv in the contaminated areas of Belarus, Russia, and Ukraine. In other European countries, both the thyroid and the effective doses are, on average, much smaller.