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Numerical Simulation Based on Individual Voxel Phantoms for a Sophisticated Evaluation of Internal Doses Mainly From 131I in Highly Exposed Workers Involved in the TEPCO Fukushima Daiichi NPP Accident

Kunishima, Naoaki1,2; Tani, Kotaro1; Kurihara, Osamu1; Kim, Eunjoo1; Nakano, Takashi1; Kishimoto, Riwa1; Tsuchiya, Hiroki1; Omatsu, Tokuhiko1; Tatsuzaki, Hideo1; Tominaga, Takako1; Watanabe, Sadahiro1,2; Ishigure, Nobuhito1; Akashi, Makoto3

doi: 10.1097/HP.0000000000000995
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As a response to the Tokyo Electric Power Company’s Fukushima Daiichi nuclear power plant accident in 2011, seven TEPCO workers whose exposure doses were expected to be >250 mSv (a tentative dose limit stipulated by the Japanese central authority) attended Japan’s National Institute for Radiological Sciences for additional internal dose measurements. The National Institute for Radiological Sciences examination revealed that these workers’ internal doses came mainly from their intake of the radionuclide 131I during emergency operations. In this study, we performed numerical simulations based on individual volume-pixel (voxel) phantoms of six of the seven workers for a more sophisticated evaluation of their internal doses, taking into account the individual thyroid size and other specific parameters. The voxel phantoms were created from magnetic resonance imaging scan images. As a result, the individual thyroid volumes ranged from 6.5 to 28.2 cm3 and were considerably smaller than the reference value (~20 cm3) adopted in the International Commission on Radiation Protection’s dosimetric model for four of the six subjects. Compared to the original estimates of the thyroid absorbed dose, our preliminary evaluation revealed values that were increased by approximately 3-fold or decreased by 30% at maximum. A wide difference in the individual thyroid size would be one of the significant modifiers in the current dose estimation of subjects of the ongoing epidemiological study project. The present simulations also provided evidence that the direct thyroid measurements by the National Institute for Radiological Sciences to determine the workers’ 131I thyroid contents were sufficiently accurate.

1National Institutes for Quantum and Radiological Sciences and Technology, National Institute of Radiological Sciences, Chiba, Japan;

2Self-Defense Forces Central Hospital, Tokyo, Japan;

3National Institutes for Quantum and Radiological Sciences and Technology, Chiba, Japan.

The authors declare no conflicts of interest.

For correspondence contact Osamu Kurihara, Director of Radiation Measurement and Dose Assessment, National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Sciences and Technology, 4-9-1 Anagawa, Inage-ku, Chiba-city, Chiba, 263-8555, Japan, or email at kurihara.osamu@qst.go.jp.

(Manuscript accepted 14 September 2018)

© 2019 by the Health Physics Society