Lung dosimetry of inhaled radon progeny in mice

マウスにおける吸入ラドン子孫核種の肺線量評価

迫田 晃弘   ; 石森 有  ; 深尾 光佑*; 山岡 聖典*; 片岡 隆浩*; 光延 文裕*

Sakoda, Akihiro; Ishimori, Yuu; Fukao, Kosuke*; Yamaoka, Kiyonori*; Kataoka, Takahiro*; Mitsunobu, Fumihiro*

ラドン子孫核種への曝露に対する生体応答が動物実験で検討されてきたが、マウスの肺の吸収線量は評価例がない。本研究では、マウスにおける吸入ラドン子孫核種の肺線量を評価した。すなわち、粒子の気道沈着,除去、及びターゲット組織へのアルファ線の沈着エネルギー割合を体系的に計算した。参考のため、ラットとヒトに対しても同様に行った。まず、気管支,気管支,胞領域,全肺について、平衡等価ラドン濃度1Bq m(平衡係数0.4、非付着成分比0.01)の下で、単分散ラドン粒子径に対する吸収線量を評価した。この結果に基づき、(1)標準的な鉱山環境、及び(2)先行動物実験の曝露条件における吸収線量を求めた。鉱山環境におけるマウス,ラット,ヒトの全肺の線量は34.8, 20.7, 10.7nGy(Bq m)h、同様に動物実験条件においては16.9, 9.9, 6.5nGy(Bq m)hであった。両曝露条件において、マウスの数値はラットの約2倍、ヒトの約3倍であった。ラットとヒトの数値を文献値と比較すると、許容範囲内で一致しており、本研究でマウスに適用したモデルが妥当であると示唆された。今後は、マウスへの適用モデルをより高度化し、解剖学的・生理学的・環境パラメータが線量へ及ぼす影響を検討することが望まれる。

Biological response of exposure to radon progeny has long been investigated, but there are few studies in which absorbed doses in lungs were estimated if laboratory animals were used. The present study is the first attempt to calculate the doses of inhaled radon progeny for mice. For reference, the doses for rats and humans were also computed with the corresponding models. Lung deposition of particles, their clearance, and energy deposition of alpha particles to sensitive tissues were systematically simulated. Absorbed doses to trachea and bronchi (BB), bronchioles and terminal bronchioles (bb), alveolar-interstitial (AI) regions, and whole lung were first provided as a function of monodisperse radon-progeny particles with an equilibrium equivalent radon concentration of 1 Bq m-3 (equilibrium factor: 0.4 and unattached fraction: 0.01). Based on the results, absorbed doses were then calculated for (1) a reference mine condition and (2) a condition previously used for animal experiments. It was found that the whole lung doses for mice, rats and humans were 34.8, 20.7 and 10.7 nGy (Bq m) h for the mine condition, respectively, while they were 16.9, 9.9 and 6.5 nGy (Bq m) h for the animal experimental condition. In both cases, the values of mice are about 2 times higher than those of rats, and about 3 times higher than those of humans. Comparison of our data on rats and humans with those published in the literature shows an acceptable agreement, suggesting the validity of the present modeling for mice. In the future, a more sophisticated dosimetric study of inhaled radon progeny in mice would be desirable to demonstrate how anatomical, physiological and environmental parameters can influence absorbed doses.