Thyroid cancer rates and 131I doses from Nevada atmospheric nuclear bomb tests

Background: We examined data on death from thyroid cancer across the continental United States and data on incidence from selected areas of the country for evidence of an association between this disease and exposure to radioactive iodine (I-131) from nuclear tests in Nevada in the 1950s, Methods: Analyses involving 4602 thyroid cancer deaths (1957-1994) and 12 657 incident cases of thyroid cancer (1973-1994) were performed. Excess relative risks (ERRs) per Gray (Gy) of radiation were estimated by relating age-, calendar year-, sex-, and county-specific rates to estimates of dose to the thyroid that take age at exposure into account. Results: Analyses of cumulative dose yielded negative ERRs that were not statistically significant. An association was suggested for dose received by children under 1 year of age for both mortality data (ERR per Gy = 10.6; 95% confidence interval [CI] = -1.1 to 29) and incidence data (ERR per Gy = 2.4; 95% CI = -0.5 to 5.6); no association was found for dose received at older ages. For mortality data, but not incidence data, there was an elevated ERR in the 1950-1959 birth cohort of 12.0 (95% CI = 2.8 to 31) per Gy, Conclusions: Risk of thyroid cancer from exposure to I-131 from atmospheric nuclear tests did not increase with cumulative dose or dose received at ages 1-15 years, but associations were suggested for individuals exposed under 1 year of age and for those in the 1950-1959 birth cohort, The absence of increased risk from dose received at ages 1-15 years is not consistent with studies of children exposed to external radiation sources. This inconsistency may result from the limitations and biases inherent in ecologic studies, including the error introduced when studying a mobile population. These problems preclude making a quantitative estimate of risk due to exposure; however, given such limitations, it is perhaps remarkable that any evidence of the effects of 131I emerges from this study.