The results support the notion that external risk coefficients are different from internal risk coefficients following exposure to a mixed radiation field. The total risk coefficients [3.0xl0 (#4.5x1 ) cancers per person-rad-year at risk, 10-year latent period, and 2.3x10 (+3.5x10 ~) cancers per person-rad-year at risk, 5-year latent period] are similar to the literature values (1,2) for this age distribution and for external exposure » The literature values are 2.1x10° for a 10-year latent period and 1.9x10 for a S-year latent period. However, if the risk is examined as a function of age or as a function of dose, differences are encountered. For example, the ratio of the risk coefficient for external exposure to the risk coefficient for internal exposure, in the <10 year age group, is 2.5 (0.38 to 4.6). In the 10- to 18-year age group, this risk coefficient ratio is 9.40 (0.22 to 2.6). Small group size, in this study, and the uncertainties reported in studies on medical and fallout exposures make it difficult to establish relative risks of thyroid cancer from internal and external radiation doses to the thyroid. The possible synergistic effect of internal and external exposures and the modifying factors such as high TSA levels and nonuniform irradiation of thyroid cells compiicate the biological tnterpretation of the risk. In this study, different age groups correspond to different dose levels, and very high dose to the thyroid may be a significant modifying factor. Because of the high interest in evaluating human sensitivity to I, continued efforts are needed to obtain data and to conduct analyses that will establish better estimates of risk coefficients than are now available. It is not likely that data for the Marshallese exposures will contribute to the answer to that important question. ACKNOWLEDGMENT The authors fully appreciate the efforts of John Baum, Associate Head for Research in the Safety and Environmental Protection Division, for his review of this manuscript. This work was performed by members of Brookhaven National Laboratory, Associated Universities, Incorporated, under contract No. DE~ACO2-76CHO0016 with the United States Department of Energy, and under contract No. YOL-CP40503 with the National Cancer Institute. REFERENCES 1. Committee on the Biological Effects of Ionizing Radiation, The Effects on Populations of Exposure to Low Levels of Ionizing Radiation: 1980, National Research Council Report, 1980. 2. National Academy Press, Washington, DC, Recommendations of the National Council on Radiation Protection and Measurements, Induction of Thyroid Cancer by Ionizing Radiation, NCRP Report No.80, Bethesda, MD, 1985. 3. Lessard, E.T., R.P. Miltenberger, R.A. Conard, $.V. Musolino, J.R. Naidu, A. Moorthy, and C.J. Schopfer, Thyroid Absorbed Dose for People at Rongelap, Utirik and Sifo on March 1, 1954, Brookhaven National Laboratory Report, Upton, NY, BNL 51882, 1985. 29