Microdosimetric Analysis Confirms Similar Biological Effectiveness of External Exposure to Gamma-Rays and Internal Exposure to 137Cs, 134Cs, and 131I

Microdosimetric Analysis Confirms Similar Biological Effectiveness of External Exposure to Gamma-Rays and Internal Exposure to 137Cs, 134Cs, and 131I Tatsuhiko Sato1*, Kentaro Manabe1, Nobuyuki Hamada2 1 Research Group for Radiation Protection, Japan Atomic Energy Agency (JAEA), Shirakata Shirane 2-4, Tokai, Ibaraki, Japan, 2 Radiation Safety Research Center, Nuclear Technology Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1 Iwado-kita, Komae, Tokyo, Japan Abstract The risk of internal exposure to 137Cs, 134Cs, and 131I 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 c-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 137Cs, 134Cs, and 131I, as well as the external exposure to 662 keV photons. The RBE due to the inhomogeneous radioactive isotope (RI) distribution in subcellular structures and the high ionization density around the particle trajectories was then derived from the calculated microdosimetric probability density. The RBE for the bystander effect was also estimated from the probability density, considering its non-linear dose response. 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 137Cs, 134Cs, and 131I should be nearly equivalent to that of external exposure to c-rays at the same absorbed dose level, as suggested in the current recommendations of the International Commission on Radiological Protection. Citation: Sato T, Manabe K, Hamada N (2014) Microdosimetric Analysis Confirms Similar Biological Effectiveness of External Exposure to Gamma-Rays and Internal Exposure to 137Cs, 134Cs, and 131I. PLoS ONE 9(6): e99831. doi:10.1371/journal.pone.0099831 Editor: Jian Jian Li, University of California Davis, United States of America Received February 2, 2014; Accepted May 19, 2014; Published June 11, 2014 Copyright: ß 2014 Sato et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by Japan Atomic Energy Agency. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: accident in Fukushima) was not extensively discussed. This is because these RIs emit relatively high energy electrons and photons, and because the scientific community considers that their RBE is 1. Nevertheless, the public occasionally believes that the risk from internal exposure is much greater than that from external exposure even for the intake of 137Cs, 134Cs, and 131I, albeit no supportive scientific evidence. Such belief comes, at least in part, from the lack of a detailed analysis of the contribution of the track-structure and RI-inhomogeneity effects to the RBE for the intake of these RIs, except for the RI-inhomogeneity effect of 131 I [8]. The contribution of these effects may not be negligible, if these RIs are selectively located inside cell nucleus. An animal study has suggested that 12–21% of 137Cs are localized in cell nuclei [9]. We therefore set out to quantitatively analyze the contribution of the track-structure and RI-inhomogeneity effects for the intake of 137Cs, 134Cs, and 131I, and for this, a microdosimetric simulation was performed using the Particle and Heavy Ion Transport code System (PHITS) version 2.64 [10]. The RBE for internal exposure to these RIs was then derived as a function of cell size and the fraction of the RI distributed in cell nuclei. In Introduction The risk of internal radiation exposure is of great public concern after the accident at the Fukushima-Daiichi nuclear power plant [1,2]. This is partially because the risks from internal exposure can differ from those from external exposure to c-rays at the same absorbed dose level, i.e., their relative biological effectiveness (RBE, defined herein as effectiveness of internal exposure relative to the external exposure to c-rays) is not always 1. For example, there is evidence that RBEs for the intake of a, low-energy b, and Auger-electron emitters are greater than 1 [3,4]. In general, the high ionization density around the trajectories of a particles and low-energy electrons as well as the inhomogeneous radioactive isotope (RI) distribution in subcellular structures are considered to explain the higher RBE values. These are referred to hereafter as the track-structure and RI-inhomogeneity effects, respectively. A number of studies have been carried out to estimate the RBE for the intake of a, low-energy b, and Auger-electron emitters [5– 7], as these results are well summarized in Report of the Committee Examining Radiation Risks of Internal Emitters [4]. In contrast, the RBE for the intake of 137Cs, 134Cs, and 131I (major contributors to the internal exposure dose from the nuclear PLOS ONE | www.plosone.org 1 June 2014 | Volume 9 | Issue 6 | e99831 RBE of Internal Exposure to Cs-137, Cs-134, and I-131 addition, the RBE for the bystander effect (biological effect caused by signaling from irradiated to non-irradiated cells) [11,12] was also discussed based on the inhomogeneity of absorbed dose among cell nuclei, since the bystander effect may play an important role in the risk estimation of low-dose internal exposure owing to its non-linear dose response [13,14]. The results of the simulation, together with the maximum possible RBE from the dosimetric viewpoint, are presented in this paper. Data Analysis The calculated PDs of z in each cell nucleus, fi(z), were used to estimate the RBE due to the RI-inhomogeneity and bystander effects. On the other hand, the calculated dose PDs as a function of y, di(y), were used to estimate the RBE due to the track-structure effect by combining the Q(y) relationship, which is the radiation quality factor expressed as a function of y (...truncated)