Re: Risk of Thyroid Cancer After Exposure to ¹³¹ I in Childhood

Extract

In his editorial, Boice ( 1 ) summarized “what is new” about radiogenic thyroid cancer in children as a result of the April 1986 Chernobyl nuclear accident, as presented by Cardis et al. ( 2 ) . One of their puzzling findings is the fact that potassium iodide given to children months to years after exposure reduced the cancer risk by threefold. Cardis et al. hypothesize that the reduction in risk might be the result of shrinkage in the size of the thyroid due to prolonged administering of dietary iodine supplements.

Neither Cardis et al. nor Boice considered the fact that the release of ¹³¹ I (half-life = 8 days) into the environment from the molten fuel rods was accompanied by large amounts of ¹²⁹ I (half-life = 16 million years), probably in higher proportion than that released by the Hanford, WA, plutonium production process ( 3 ) .

In a report that suggests excess cancers, including thyroid cancers, among a population of residents downwind of the reactors in Hanford, WA, during periods of large releases of radioiodine into the atmosphere (downwinders), Grossman et al. ( 4 ) referred to several studies that show that the commonly assumed biological clearance half-time for iodine (80 days from the thyroid and 12 days for the rest of the body) ( 3 ) is inconsistent with several well-documented observations of much longer iodine retention times in tissue. Therefore, in contrast to statements that contributions to dose from uptake of ¹²⁹ I by the thyroids of residents downwind of Hanford, WA, were negligible ( 3 ) , we suggested that a long retention time, combined with the considerably higher relative biological effectiveness of the very low-energy radiation from the radioactive decay of ¹²⁹ I compared with ¹³¹ I emissions may contribute substantially to damage of thyroid tissue, including cancer induction ( 4 ) .