Radiation exposure and thyroid cancer incidence among Hiroshima and Nagasaki residents.

The dependence of thyroid cancer incidence on radiation exposure level is examined with the use of tumor registry follow-up data on a life-span cohort of 98,610 Hiroshima and Nagasaki residents. The sample includes 112 clinically evident thyroid cancer cases: 62 in Hiroshima and 50 in Nagasaki. A clear, predominantly linear, increase in thyroid cancer incidence corresponds to increasing levels of gamma radiation to the thyroid gland, whereas neutron exposure could not be shown to contribute further to thyroid cancer risk. The relative risk associated with gamma ray exposure is particularly high among persons who were less than 30 years old at the time of radiation exposure. Limitations on the incidence and dosimetry data are discussed, as well as the ability of the Cox regression method to accommodate some of these limitations.