Radiation Dose Reconstruction of the exposed population in the Marshall Island

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From 1946 to 1958, 66 times of American nuclear weapons tests were conducted at Bikini and Enewetak atolls in the Marshall Islands, which located the mid-pacific. consequently, many of the Marshallese people potentially exposed to the radio-active fallout of the tests. The fallout included high level of radio-active Cesium and Iodine. These radio-active agents were taken by the Marshallese through their digestive and respiratory systems: internal radiation exposure. It was well established risk factor for the thyroid cancer to expose to radio-iodine, so that we started the follow-up study to the thyroid cancer occurrence among the Marshallese population from 1993. The preliminary results of initial phase of the study(i.e. cross sectional) were follows: (i) the prevalence of the thyroid cancer among the people who was born before the test was as high as 1.5%, (ii) there was possible dose-response relationship between the radiation exposure dose and the prevalence of thyroid cancer. In addition to relatively small cancer cases, since the available dose estimate at that time was very rough, this dose-response relationship would be not statistically significant. We conducted more detail dose reconstruction to get more study power in this study. Three types of dose reconstruction were performed: (i) thyroid dose based on those of highly exposed people of Rongelap and Utirik from the Bravo test in 1954, (ii) dose based on residual radioactive Cesium of each atoll NOT taken into account radiation rate, (iii) that taken into account radiation rate. Using dose estimates type i, there was significant dose-response between thyroid dose and thyroid cancer prevalence among people exposed more than 50 cGy. Thyroid cancer incidence will be evaluated using these new dose reconstructions.