Estimation of Radiation Doses for a Case-control Study of Thyroid Cancer Among Ukrainian Chernobyl Cleanup Workers

Thyroid doses were estimated for 607 subjects of the case-control study of thyroid cancer nested in the cohort of 150,813 male Ukrainian cleanup workers, who were exposed to radiation as a result of the 1986 Chernobyl nuclear power plant accident. Individual thyroid doses due to external irradiation, inhalation of (131)I and short-lived radioiodine and radiotellurium isotopes ((132)I, (133)I, (135)I, (131m)Te and (132)Te) during the cleanup mission, and intake of (131)I during residence in contaminated settlements were calculated for all study subjects, along with associated uncertainty distributions. The average thyroid dose due to all exposure pathways combined was estimated to be 199 mGy (median dose of 47 mGy; range: 0.15 mGy to 9.0 Gy), with averages of 140 mGy (median of 20 mGy; range: 0.015 mGy to 3.6 Gy) from external irradiation during the cleanup mission, 44 mGy (median of 12 mGy; range: ~0 mGy to 1.7 Gy) due to (131)I inhalation, 42 mGy (median of 7.3 mGy; range: 0.001 mGy to 3.4 Gy) due to (131)I intake during residence, and 11 mGy (median of 1.6 mGy; range: ~0 mGy to 0.38 Gy) due to inhalation of short-lived radionuclides. Internal exposure of the thyroid gland to (131)I contributed more than 50% of the total thyroid dose in 45% of the study subjects. The uncertainties of the individual stochastic doses were characterized by a mean GSD of 2.0, 1.8, 2.0 and 2.6 for external irradiation, inhalation of (131)I, inhalation of short-lived radionuclides and residential exposure, respectively. The models used for dose calculations were validated against instrumental measurements done shortly after the accident. Results of the validation showed that thyroid doses could be estimated retrospectively for Chernobyl cleanup workers two-three decades after the accident with a reasonable degree of reliability.