Effect of 131I biodistribution on measurements using a scanning whole-body counter

To validate the thyroid dose assessment performed at Nagasaki University after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, the influence of 131I biodistribution on the direct in-vivo counting measurements by a scanning whole-body counter with two NaI(Tl) detectors was evaluated by a Monte Carlo simulation. The peak efficiency for gamma rays from 131I was calculated for the Bottle-Manikin Absorption (BOMAB) phantom and International Commission on Radiological Protection voxel phantom with a uniform radionuclide distribution in the body and a localized distribution in the thyroid. The ratio of scan peak efficiencies for the BOMAB phantom with the uniform and localized distribution was 1.17 (localized/uniform), while that for the voxel phantom was 1.00. For the voxel phantom, a shielding by the head and tissues covering thyroid reduced the scan peak efficiency for the localized source, resulting in a negligible difference in peak efficiency between the source distributions. The assumption of a completely localized thyroid source is suggested to result in an underestimation within approximately 4% in residual activity assessment, considering 131I biodistribution calculated using ratios of physicochemical forms reported after the FDNPP accident, which represents a scenario with the maximum uncertainty. Consequently, the influence of 131I biodistribution on the scanning whole-body counting was negligibly small under the present conditions, partly due to the detector arrangement with two NaI(Tl) detectors positioned above and below the subject.