Appendix A Paper presented at the Fourth International Radiopharmaceutical Dosimetry Symposium, Oak Ridge, Tennessee, November 1985. THYROID CANCER IN THE MARSHALLESE: RELATIVE RISK OF SHORT-LIVED INTERNAL EMITTERS AND EXTERNAL RADIATION EXPOSURE Lessard, E.T.,? Brill, A.B.,? and Adams, weH.? Brookhaven National Laboratory 4safety & Environmental Protection Division Medical Department Upton, NY 11973 ABSTRACT In a study of the comparative effects of internal versus external irradiation of the thyroid in young people, we determined that the dose from internal irradiation of the thyroid with short-lived internal emitters produced several times less thyroid cancer than did the same dose of radiation given externally. We determined this finding for a group of 85 Marshall Islands children, who were less than 10 years of age at the time of exposure and who were accidentally exposed to internal and external thyroid radiation at an average level of 1400 rad. The assumed risk coefficient for children, from external radiation alone, was derived from 1) values in The Effects on Populations of Exposure to Low Levels of Ionizing Radiation: 1980, National Academy Press, 2) values in Report of the Ad Hoc Working Group to Develop Radioepidemiological Tables, National Institutes of Health, and 3) values in Induction of Thyroid Cancer by Ionizing Radiation, National Council on Radiaction Protection, Report 80. The risk from internal irradiation was computed from dose, health effect results which were reported in a recent BNL study, The and an estimate of the external risk coefficient based on other studies. external risk coefficient ranged between 2.5 and 4.9 cancers per million person-rad-years at risk, and thus, from our computations, the internal risk coefficient for the Marshallese children was estimated to range between 1.0 and 1.4 cancers per million person-rad-years at risk. In contrast, for individuals more than 10 years of age at the time of exposure, the dose from internal irradiation of the thyroid with short-lived internal emitters produced several times more thyroid cancer than did the same dose of radiation given externally. The external risk coefficients for the older age groups were reported in the above literature to be in the range of 1.0 to 3.3 cancers per million person-rad-years-at risk. We computed internal risk coefficients of 3.3 to 8.1 cancers per million person-rad-years at risk for adolescent and adult groups. This higher sensitivity to cancer induction in the exposed adolescents and adults, its different from that seen in other exposed groups. The small number of cancers (9} in the exposed population and the influence of increased levels of TSH, nonuniform irradiation of the thyroid, and thyroid cell killing at high dose make it difficult to draw firm conclusions from these studies. 19