A New method to evaluate radioiodine activity in thyroid by the spectroscopy measurements using a simplified phantom incorporated with simulations

数値計算と簡易ファントムによる実測を組み合わせたエネルギー分析型甲状腺モニタの校正方法の開発

吉富 寛   ; 西野 翔   ; 高橋 聖 ; 谷村 嘉彦   

Yoshitomi, Hiroshi; Nishino, Sho; Takahashi, Masa; Tanimura, Yoshihiko

A new portable thyroid monitoring system equipped with spectrometers has been developed. To evaluate radioiodine activity in thyroid, it should be properly calibrated. The calibration of a thyroid monitor has been widely performed using a neck phantom with a thyroid shaped volume source. However, they cannot reflect the anatomical feature of the thyroid in detail. For the precise assessment, the difference of the counting efficiency between the calibration and the measurements of the wide variety of the subjects should be corrected. However, it is not practical to manufacture various volume sources reflecting anatomical feature precisely. To solve this problem, we have developed a novel method to evaluate radioiodine activity in thyroid precisely by the measurements using a simplified physical phantom in conjunction with numerical simulations. The primary photon fluence around the neck were firstly evaluated by Monte Carlo calculations using mathematical phantoms. Secondly, the combinations of an acrylic phantom with a Ba point source were selected by calculations so as to provide the equivalent photon fluence varying the phantom size and the source location. By applying the corresponding physical phantom for the calibration of the thyroid monitor, the same counting efficiency as measured in the realistic situation was obtained.