FIGURE 7-4 Distribution of study-specific estimates of ERR/Gy for thyroid cancer according to level of average thyroid dose. NOTE: The estimate from populations exposed to ¹³¹I is excluded.

about 30 years after exposure but was still elevated at 40 years.

Three studies provided data on exposure protraction or fractionation (thymus, tinea capitis, and Michael Reese); analyses indicate that a small nonsignificant decrease in risk may be related to exposure fractionation in these studies.

A meta-analysis of hyperthyroidism studies provides a risk estimate of thyroid cancer in relation to ¹³¹I exposure in childhood (Shore 1992). The ERR estimate from that study is 0.3 Gy⁻¹, lower than that from studies of external exposures, but based on only two exposed cases. This study therefore provides little information about the risk of thyroid cancer in relation to exposure to this nuclide. Studies of the effects of ¹³¹I exposure later in life are reviewed in the preceding section, although no dose-related estimate of risks have been provided. These studies, taken together, provide little evidence of an increased risk of thyroid cancer following ¹³¹I exposure after childhood.