Protracted exposure to 134Cs and 137Cs gives substantial contribution to long-term thyroid absorbed dose after nuclear power plant accidents.

Thyroid dose estimations after nuclear power plant (NPP) accidents are traditionally based on internal uptake of radioiodine, mainly 131I, either by instrumental measurements of thyroid uptake or by ecological estimations based on geographical dispersion of the radioiodine cloud, demographics, and food habits. However, it has been shown that 134Cs and 137Cs in some cases can be the dominant contributors to the thyroid dose over long time following NPP accidents. Based on an ecological model using Swedish-specific parameters of the radioactive fallout from the Chernobyl accident in 1986, estimations of the protracted (30 years) thyroid absorbed dose were made for the population in northern Sweden (2.2 million inhabitants in 1986). The internal dose contribution was estimated from both the short-lived nuclides-mainly 131I (T½,phys = 8.1 d) in dairy milk and from inhalation-and nuclides with longer half-lives-134Cs (T½,phys = 2.06 y) and 137Cs from aggregate ecological transfer of radiocesium in foodstuff (T½,phys = 30.2 y). The external radiation dose to the thyroid was based on air-borne measurements of the ground deposition of 137Cs, combined with absorbed dose contribution of short-lived radionuclides and with correction for shielding from residential buildings and snow cover. The total thyroid absorbed dose from 1986 to 2015 ranged from 0.06 to 15.5 mGy (mean 2.0 mGy) among subjects in the study population. The calculated mean thyroid absorbed dose the first year was 0.7 mGy, where radioiodine accounted for ~0.3 mGy. The protracted thyroid absorbed dose after 30 years was 0.3 mGy (15%) from 131I, and 1.7 mGy (85%) from internal and external 134Cs and 137Cs taken together. Hence, the estimated mean absorbed dose contribution from radiocesium was higher than for radioiodine (131I) both in the first year and in the consecutive 30 years. Furthermore, the 30-year external absorbed dose (1.2 mGy) dominates over the internal absorbed dose (0.8 mGy) to the thyroid. This finding is of relevance for low-dose exposure epidemiological studies of thyroid cancer which previously have focused solely on radioiodine.