The Utah Thyroid Cohort Study: analysis of the dosimetry results.

Above ground testing of nuclear weapons at the Nevada Test Site (NTS) during the 1950s created radioactive fallout that was dispersed into the atmosphere and deposited over a large geographical area of the U.S. One area believed to have received a considerable amount of exposure to radioiodines (131I and 133I) in the fallout was southwest Utah and southeast Nevada. This paper describes the estimates of doses to the thyroid for a cohort of 3,545 subjects who were children during the atmospheric testing period. This group of children was examined for thyroid disease during 1965-1970 and again in 1985-1986. The cohort was made up of children who lived in three counties in 1965: Washington County, Utah; Lincoln County, Nevada; and Graham County, Arizona (originally thought to be an unexposed group). Pathway analysis was used in the dosimetry, considering exposures through the ingestion of milk and vegetables, inhalation of iodine during the passage of the fallout cloud, and external exposure. Specific data were obtained on diet (including sources and levels of milk and vegetables consumed, residence history, and lifestyle) by interviewing the parents or nearest living relative of subjects. The final dosimetry file for each member of the cohort contained specific doses to the thyroid glands and uncertainties (reported as geometric standard deviations, GSD) related to each dose estimate. The mean absorbed dose to the thyroid for subjects living in Washington County, Utah, was 170 mGy; for Lincoln County, Nevada, 50 mGy; and for those living in Graham County, Arizona, 13 mGy. The maximum dose to any subject was 4,610 mGy. There were 10 subjects who had doses greater than 1 Gy. The majority of uncertainty values calculated in this study were GSD values between 2.0 and 4.0. The results of the dosimetry were combined with the results of clinical examinations of the cohort to determine if a causal relationship exists between dose to thyroid from NTS generated radioactive iodines and the incidence of thyroid disease.