The issue of the risks to health from radiation leaks from nuclear power stations has been significantly raised following the devastating earthquake and tsunami in Japan on March 11, 2011, and the subsequent damage to the power station at Fukushima. Both this incident and the much more serious accident at the Chernobyl nuclear plant in the Ukraine released quantities of a radioactive isotope of iodine, I-131. Iodine is easily absorbed by the thyroid, and exposure to I-131 is known to raise the risk of thyroid cancer; children are at greater risk than adults. The publication of a dose-response study of thyroid cancer incidence in a Ukrainian population exposed to I-131 following the Chernobyl disaster is therefore particularly timely.

This study, led by Alina Brenner at the National Cancer Institute, Bethesda, Maryland, USA with co-workers from other US institutes and from the Institute of Endocrinology and Metabolism in Kiev, Ukraine, was a prospective analytic cohort study of the relationship between I-131 dose and the incidence of thyroid cancer. It was designed to address some of the limitations of previous grouped dose and case-control studies. A total of 12,514 individuals who were living in the contaminated regions of the Ukraine and under 18 at the date of the accident (April 26, 1986) were studied. All participants had radioactivity measurements taken during the two months following exposure and underwent up to four thyroid cancer screening tests between 1998 and 2007.

The dose of radioactive iodine received by each individual was estimated from a combination of these measurements, diet and lifestyle data, and environmental transfer models, and it was adjusted for thyroid mass. The average (arithmetic mean) dose obtained by the individuals in the study was estimated to be 0.65 gray (Gy); for comparison, the odds ratio of developing thyroid cancer at a dose of one gray has been roughly estimated as five. The collected data were cross-classified by gender; place of residence; and iodine prophylaxis, and by variables known to be associated with differential thyroid cancer risk including smoking status and family history of thyroid disease. The excess risk of developing cancer due to exposure to radioactive iodine was estimated using a Poisson regression model.

Sixty-five cases of incident thyroid cancer were detected in the study participants, with a clear increase in cancer incidence as the estimated radiation dose increased. The best fit to the observed data was obtained using a simple linear dose-response relationship, with the best fit of all obtained using the excess relative risk (ERR) model; the excess relative risk per gray was estimated to be 1.91 (95% confidence interval: 0.43-6.34). Risk varied significantly with place of residence at the time of the incident (which is clearly correlated with dose) and with age at exposure, with younger children at higher risk than adolescents. There was no statistically significant correlation with gender; age at test; iodine prophylaxis or iodine deficiency, and no decline in excess relative risk was observed during the study period.

The excess relative risks of thyroid cancer development determined in this study are consistent with previous retrospective and ecological estimates of cancer risk arising from the Chernobyl accident. It will be useful to follow this cohort longer to determine how this risk changes over time, and further large studies will be needed if more subtle interactions between radiation exposure and some other risk factors for thyroid cancer are to be elucidated.

Article: Brenner, A.V., Tronko, M.D., Hatch, M. and 14 others (2011). I-131 Dose-Response for Incident Thyroid Cancers in Ukraine Related to the Chornobyl Accident. Environmental Health Perspectives, published online 17 March 2011. doi: 10.1289/ehp.1002674