Environmental Exposure to Radioactive Iodine and Thyroid Disease

Abstract

Eew topics engender more vigorous debate than low-dose or chronic radiation effects on Fhealth, especially when exposures are due to the release of radioactive materials to the environment. Such releases have occurred as a result of nuclear weapons explosions, nuclear experiments and operations, and nuclear reactor accidents. Concerns regarding effects to the thyroid from environmental radiation probably originated with the atomic blasts in Hiroshima and Nagasaki, and continued with the testing of ever more powerful atomic weapons during the Cold War years. The deliberate reactor releases at the Hanford Works in the state of Washington and accidents involving nuclear power plants, especially the Chernobyl catastrophe in the Ukraine, have further fanned the debate. There has been particular concern about the atmospheric nuclear testing carried out at the Nevada test site. This concem led to a series of studies of thyroid disease and leukemia in populations residing in states surrounding the test site. Of special interest is the potential effect of iodine-131 (13'I) on thyroid disease among those exposed during early childhood. To address this question, a cohort was assembled in 1965 consisting of children attending junior and senior high school in Washington County, Utah. This cohort has been examined every 5 years, including thyroid screening, collection of medical histories, and interviews of study subjects and their parents. 2 Extensive efforts to estimate individual doses of 1311 have been undertaken. Reports from this study have been published at various stages to describe both the dose reconstructions and assignment and results of the epidemiologic follow up (phases I and 11).3-6 The report by Lyons and colleagues' in this issue of EPIDEMIOLOGY is a revision (phase IIR) of the second epidemiologic follow-up study6 and the third in the series describing the dosimetry and epidemiologic results of the Utah cohort. This revision was prompted by problems uncovered during preparations to imple ment phase III of the study. While gearing up for the new effort, a number of deficiencies were discovered, including corrupted data files, coding errors, software and hardware issues, dose-estimation errors, and problems of exposure and outcome misclassification. 1,2 The researchers also used this opportunity to address another problem in the phase II results, namely that thyroid disease diagnoses had been assigned after exposures were known.18 In addition, the new analysis took advantage of new information from the National Cancer Institute (NCI) on estimated 1311 thyroid doses to the U.S. populace from nuclear weapons tests fallout.7 The current report (phase IIR) corrects the deficiencies just mentioned and imple ments additional quality controls. The report revises the dose estimates (including application of the aforementioned NCI population dose estimates) to subjects who resided outside of Utah and surrounding states and who had previously been assigned doses of zero. Finally, the report uses data from rescreening subjects and assigns diagnoses using updated diagnostic criteria by clinicians who were blind as to exposure, residence, and other characteristics of the study subjects. 2 To the authors' credit, they present results from both the original and the revised analyses.