Concern about the carcinogenic effects of exposure to radioiodine on the thyroid gland is motivated by three major factors. First, evidence has accumulated that the thyroid gland is uniquely sensitive to the effects of radiation. There is some evidence that measurable increases in thyroid cancer can occur with external doses of radiation as low as 0.1 sievert (Sv) (10 rem). A finite risk at low doses of that magnitude is consistent with risks for other solid cancers reported for the Japanese atomic-bomb survivors (Pierce and others 1996). Second, the cow-milk-man pathway described in the NCI (1997a) report and discussed in Chapter 2 of this report provides a mechanism by which radioiodines in the environment can be greatly concentrated in the human food chain. Finally, because most of the radiation dose is from ingested or inhaled radioiodine, the radiation dose to the thyroid is 500-1,000 times greater than is the largest radiation dose to other organs in the body.
For several reasons, persons exposed to I-131 as children are uniquely at risk for carcinogenic effects. First, children drink more milk relative to their body size than do adults. Second, the same amount or a higher fraction of internalized iodine is concentrated in the smaller thyroid glands of children; therefore the radiation dose to the thyroid in children is higher than it is in adults. Finally, studies of children whose thyroid glands were exposed to external radiation suggest a strong inverse relationship between age at exposure and the carcinogenic effects of radiation on the thyroid. Over the age of 15, little increase in thyroid cancers has been observed. Below the age of 15, thyroid cancer increased by a factor of approximately 2 for every 5 years' decrease in age. Not only is the frequency of malignant nodules increased by thyroid irradiation, but benign nodules also occur with greater than usual frequency after irradiation (Wong and others 1996).
Stable iodine and its radioactive isotopes are water-soluble and readily absorbed, either from the gastrointestinal tract after ingestion or through the lungs