Thyroid cancer risk in the population around the Nevada Test Site.

Potential thyroid cancer risk in Utah populations due to weapons testing fallout has been examined in two ways. Thyroid cancer incidence rates for Utah for the years 1973-77 were compared to those of the reporting areas of the Surveillance, Epidemiology and End Results Registry. In addition, numbers of lifetime thyroid cancer cases and distribution of probability of causation values for those cases have been estimated for a population of about 23,000 who were in the fallout area of three major shots at the Nevada Test Site during the 1950s. A computer program designed to estimate numbers of expected and excess cases occurring in the remaining lifetime after exposure was used to make the estimates. The estimates of internal dose distribution within various age at exposure groups were made using dose estimates developed by the Off-Site Radiation Exposure Review Project. The potential human risk from 131I at low doses is far from certain. The upper bound risk must be considered to be near that for low doses of acutely delivered external radiation to the thyroid. A practical upper limit for risk from low-dose 131I was taken to be one per million per rad-yr for males and four per million per rad-yr for females (although absolute upper limits would need to be higher). The upper bound estimate of excess cases in the lifetime of the population is about 13, compared to an estimate of about 58 cases expected to occur in the population on the basis of ordinary incidence. If excess risk is assumed to increase linearly with dose, and not to change with age, 12 of 13 excess cases fall into the group aged 18 and below at the time the dose occurred, and 11 of 13 excess cases fall into the group exposed to 10 rad or more.