Assessing the Risk of Thyroid Cancer from Exposure to 131I in Children: Application of Bayes' Theorem to Indirect Evidence. F. Owen Hoffman, SENES Oak Ridge, Inc., 102 Donner Dr., Oak Ridge, TN 37830; and Stan Kaplan, 3678 Vigilance Dr., Rancho Palos Verdes, CA 90275
Epidemiological data sets on the relationship between a dose of 131I to the human thyroid and the risk of thyroid cancer are presently insufficient to demonstrate a clear quantitative dose/response. The reason for this limitation is that most medical applications of 131I in humans have been on adults and individuals in their mid to late teens. It has recently been established that the radiation induction of thyroid cancer in individuals exposed in this age group is very low regardless of the dose received to the thyroid. Recent epidemiological studies on populations exposed to x-rays and gamma rays have demonstrated however, that a relationship between radiation dose and excess thyroid cancer is most evident when exposures occur in early childhood; with the most sensitive population subgroup being female children exposed under the age of five. Indirect evidence that thyroid cancer may be caused by childhood exposure to 131I is suggested by epidemiological investigation of cohorts of children exposed to Nevada Test Site fallout, releases of 131I from the Chernobyl accident, and laboratory experiments on prepubescent female rats. Each of these studies however, have been individually criticized. The Utah cohort, due to its low population size, exhibited only a small number of health outcomes. The Chernobyl studies exhibit problems with the accurate estimation of thyroid dose and the possibility of additional exposure to shorter lived isotopes of 131I. The laboratory animals produced different types of thyroid cancers than those commonly induced by radiation in humans. This presentation will demonstrate the application of Bayes’ theorem in a systematic analysis on all available evidence to produce a subjective probability distribution for the cancer risk induced by a given dose of 131I to the thyroid of children potentially exposed within the United States.