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バイスタンダー効果を考慮した内部被ばくと外部被ばくの生物学的効果比

Estimation of biological effectiveness of internal exposure relative to external exposure considering bystander effect

佐藤 達彦   ; 浜田 信行*; 真辺 健太郎   

Sato, Tatsuhiko; Hamada, Nobuyuki*; Manabe, Kentaro

福島第一原子力発電所事故後、$$^{137}$$Cs, $$^{134}$$Cs, $$^{131}$$Iの内部被ばくによるリスクは、公衆の関心事となっている。そこで、粒子輸送計算コードPHITSを用いて、それらのRIが細胞核,細胞質及び細胞外に局在した場合の放射線挙動を詳細に解析し、細胞内及び細胞核内におけるミクロ吸収線量分布を計算した。また、662keV光子の外部被ばくに対しても同様のシミュレーションを実施し、外部被ばくと比べた内部被ばくの生物学的効果比(RBE)を、RI不均一効果、飛跡構造効果、及びバイスタンダー誘発効果の3つの観点から評価した。その結果、それらRIによる内部被ばくと662keV光子の外部被ばくによるミクロ吸収線量分布はほぼ一致し、そのRBEは最大でも1.04程度であることが分かった。以上のことから、マイクロドジメトリの観点から、$$^{137}$$Cs, $$^{134}$$Cs, $$^{131}$$Iの内部被ばくによるリスクは、バイスタンダー効果など最新の生物学的知見を考慮しても、同じ吸収線量レベルの$$gamma$$線外部被ばくとほぼ同等であると結論づけられた。

The risk of internal exposure to $$^{137}$$Cs, $$^{134}$$Cs, and $$^{131}$$I is of great public concern after the accident at the Fukushima-Daiichi Nuclear Power Plant. The relative biological effectiveness (RBE, defined herein as effectiveness of internal exposure relative to the external exposure to $$gamma$$-rays) is occasionally believed to be much greater than unity due to insufficient discussions on the difference of their microdosimetric profiles. We therefore performed a Monte Carlo particle transport simulation in ideally aligned cell systems to calculate the probability densities of absorbed doses in subcellular and intranuclear scales for internal exposures to electrons emitted from $$^{137}$$Cs, $$^{134}$$Cs, and $$^{131}$$I, as well as the external exposure to 662 keV photons. The RBE due to the high ionization density and that for the bystander effect were very close to 1, because the microdosimetric probability densities were nearly identical between the internal exposures and the external exposure from the 662 keV photons. On the other hand, the RBE due to the RI inhomogeneity largely depended on the intranuclear RI concentration and cell size, but their maximum possible RBE was only 1.04 even under conservative assumptions. Thus, it can be concluded from the microdosimetric viewpoint that the risk from internal exposures to $$^{137}$$Cs, $$^{134}$$Cs, and $$^{131}$$I should be nearly equivalent to that of external exposure to $$gamma$$-rays at the same absorbed dose level, as suggested in the current recommendations of the International Commission on Radiological Protection.

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