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ABSTRACTAir dose rates within a range of 80 km from the Fukushima Dai-ichi Nuclear Power Plant measured via car-borne surveys using the KURAMA systems from 2011 to 2018 were analyzed. The ratio of the air dose rates relative to the first car-borne survey was 0.08 over a period of seven years, indicating a much more rapid decrease than that estimated by the physical decay of radiocesium (0.3). Differences in the decreasing trends of the air dose rates among the land-use categories and evacuation order areas were examined. The ecological half-lives were evaluated according to the land-use categories in each evacuation order area. To model the acceleration of the decrease in the air dose rates observed in the evacuation order area, we modified the bi-exponential curve formula of ecological half-life and introduced the two-group model. The ecological half-life of the fast components in the evacuation order area of ‘zone in preparation for the lifting of the evacuation order’ evaluated using the two-group model after 2013 were 0.5–1 y, which were much shorter than those before 2013 (2–3 y).

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released the most significant quantity of radiocesium into the environment since Chernobyl, and detailed measurements over the initial 5 years provide new insights into fluvial redistribution of radiocesium. We found that the high initial activity concentration of 137Cs-bearing suspended sediment in rivers was followed by a steep exponential decline (λ1) which extended to approximately 1 year after the accident, while the rate of initial decline in radiocesium activity concentration in water was an order of magnitude higher than rates measured after Chernobyl. Fluvial transport of 137Cs to the ocean from the Abukuma river totaled 12 TBq between June 2011 and August 2015 and almost all this radiocesium (96.5%) was transported in the particulate form. The primary sources of 137Cs were paddy fields, farmland, and urban areas [plaque-forming unit (PFU)], discharging 85% of the exported 137Cs from 38% of the watershed area. After 1 year, activity concentrations were lower and exhibited a more gradual secondary decline (λ2) which was associated with reduced radiocesium losses from PFU areas, while forest areas continue to represent more stable contaminant stores.

ABSTRACTTo analyze radiation protection strategies and rehabilitation programs in Fukushima, prediction models have been developed for ambient dose equivalent rate distributions within the 80 km-radius around the Fukushima Daiichi nuclear power plant. The prediction models characterized by ecological half-lives of radioactive caesium for land-use, enable Fukushima residents to obtain distribution maps of ambient dose equivalent rates after the Fukushima Daiichi nuclear power plant accident. Model parameters such as the ecological half-lives for the short-term component and the fractional distribution of short-term component were evaluated using ambient dose equivalent rates through car/vehicle-borne surveys. It was found that the ecological half-lives among land-use differ only slightly, whereas the fractional distributions of the short-term component are clearly dependent on land-use. In addition, uncertainties concerning predictions of ambient dose equivalent rates arising from variability in model parameters were assessed using Monte Carlo simulations. Long-term changes of ambient dose equivalent rates were predicted for different land-use areas. Distribution maps of ambient dose equivalent rates for the next 30 years after the accident, created by the prediction models are expected to be useful for follow-up of the radiological situation since they provide information on the space variation of the ambient dose equivalent rates in inhabited areas.

Conversion coefficients of radionuclide deposition density to the ambient dose equivalent rate at 1 m height above ground were calculated for exponentially distributed sources in the ground. First, Monte Carlo transport simulations assuming exponential distributions in the ground were performed to obtain ambient dose equivalent for mono-energetic gamma-ray sources having different relaxation depths; next, on the basis of the simulated data, conversion coefficients for radionuclides were composed considering recent nuclear decay data. The ambient dose equivalent rates were then compared to the effective dose rates for reference adults and a new-born baby as well as to air kerma rates quoted from previous studies. It was confirmed that the ambient dose equivalent sufficiently overestimates effective doses, independently of age, for sources exponentially distributed in the ground. Furthermore, the air kerma was found to also overestimate the effective doses for all ages in the same conditions. In order to verify the computed conversion coefficients, the ratio of ambient dose equivalent to air kerma obtained by simulation was compared to the ratios measured at hundreds of locations in Japan which have been contaminated with radioactive cesium after the accident at the nuclear power plant in Fukushima Prefecture, Japan, in 2011; a good agreement was observed.

AbstractJapan Atomic Energy Agency has developed several Japanese voxel phantoms and performed simulations using the developed phantoms mainly for radiation protection purposes. In recent years, the techniques have been applied to other research fields including medical fields. This paper summarizes recent research results concerning the Japanese phantoms. The main conclusions of the results are as follows. First, according to comparison of organ doses in the Japanese voxel phantoms to those in Rex and Regina for external photon exposure, it was confirmed that on average, organ doses are slightly larger for Japanese phantoms because of the difference in body size; however, the difference is very small. Second, organ doses were found to be obviously different between standing and lying postures both in external and internal exposures because of the change in body structures; however, in terms of effective doses, the effect is limited. Third, the alpha version of a dose calculation system for remotely supporting X-ray therapy through the Internet was completed.

AbstractKnowledge of interspecies scaling of organ doses from experimental animals such as mice to humans is important in the preclinical evaluation of new radiopharmaceuticals. Interspecies scaling factors should be reliably determined since the dose-response relationships in mice would be translated to those in humans. To obtain reliable interspecies scaling factors of organ doses from mice to humans, absorbed fractions (AFs) are needed for sophisticated models on both mice and humans. In the present study, self-AFs for photons and electrons in the spleen, kidneys, and liver of both a mouse and humans were evaluated using Monte Carlo simulations. For the mouse and human models, voxel phantoms based on computed tomography were used. The sources were assumed to be monoenergetic in the energy range 10 keV to 4 MeV and to be uniformly distributed in the spleen, kidneys, and liver. Interspecies scaling factors were determined using the results of the self-AFs for the voxel mouse and voxel human. Consequently, interspecies scaling factors were found to be dependent upon energy emitted in the source organ. It was found that the scaling factor for the photon self-AF, which is corrected by the cube root of the organ mass, shows a similar trend as a function of energy with the scaling factor for the electron self-AF.

A Japanese adult female voxel (volume pixel) phantom (JF) was developed using CT images of a healthy female volunteer. The height (152 cm) and weight (44 kg) of JF are smaller than the averages of Japanese adult female. The voxel size of JF is 0:98 × 0:98 × 1mm3. Therefore, the shapes of small or complicated organs, such as thyroid and stomach, are distinctly reproduced. The shapes of female-specific organs, such as ovary and uterus, in JF are also realistically represented compared with those of a previously developed phantom. These results suggest that JF enables the accurate evaluation of organ doses due to diverse exposures to Japanese adult females. Photon-specific absorbed fractions (SAFs) for liver or stomach as target organs and for kidney as a source organ were calculated using JF and compared with those of other phantoms. It was concluded that the SAFs calculated using JF and other phantoms depend on the organ distances from source to target.

Purpose: As the conventional graphical user interface (GUI) associated with DOSXYZnrc or BEAMnrc is unable to define specific structures such as gross tumor volume (GTV) on computed tomography (CT) data, the quantitative analysis of doses in the form of dose-volume histograms (DVHs) is difficult. The purpose of this study was to develop an interface that enables us to analyze the results of DOSXYZnrc output with a commercial radiation treatment planning (RTP) system and to investigate the validity of the system. Materials and methods: Interface software to visualize three-dimensional radiotherapy Monte Carlo (MC) dose data from DOSXYZnrc on the XiO RTP system was developed. To evaluate the interface, MC doses for a variety of photon energies were calculated using the CT data of a thorax phantom and a uniform phantom as well as data from patients with lung tumors. Results: The dose files were analyzed on the XiO RTP system in the form of isodose distributions and DVHs. In all cases, the XiO RTP system perfectly displayed the MC doses for quantitative evaluation in the form of differential and integral DVHs. Conclusion: Three-dimensional display of DOSXYZnrc doses on a dedicated RTP system could provide all the existing facilities of the system for quantitative dose analysis.

Abstract. The radiation-induced process of strand breaks on pBR322 plasmid DNA in aqueous solution for different energy electrons was studied by Monte Carlo simulation. Assumptions of induction mechanisms of single- and double-strand breaks (SSBs and DSBs) used in the simulation are that SSB is induced by OH or H reaction with DNA and that DSB is induced by two SSBs on the opposite strands within 10 bp. Dose-response relationships of SSBs and DSBs were demonstrated for monoenergetic electrons of 100 eV, 10 keV, 1 keV and 1 MeV, and the yields of SSB and DSB were calculated. The dose-response relationships of SSBs and DSBs can be fitted by linear and linear-quadratic functions, respectively. The ratio of quadratic to linear components of DSB induction changes due to the electron energy. A high contribution of the linear component is observed for 1 keV electrons in the dose range below 160 Gy. The yields of SSBs and DSBs for all examined electron energies lie well within the experimental data when the probability of strand-break induction by OH and H is assumed to be around 0.1-0.2. The yield of SSBs has a minimum at 1 keV, while the yield of DSBs has a maximum at 1 keV in the examined energies. The strand breaks are formed most densely for 1 keV electrons.

Conversion from tooth enamel dose to organ doses was analyzed to establish a method of retrospective individual dose assessment against external photon exposure by electron spin resonance (ESR) dosimetry. Dose to tooth enamel was obtained by Monte Carlo calculations using a modified MIRD-type phantom with a teeth part. The calculated tooth enamel doses were verified by measurements with thermo-luminescence dosimeters inserted in a physical head phantom. Energy and angular dependences of tooth enamel dose were compared with those of other organ doses. Additional Monte Carlo calculations were performed to study the effect of human model on the tooth enamel dose with a voxel-type phantom, which was based on computed tomography images of the physical phantom. The data derived with the modified MIRD-type phantom were applied to convert from tooth enamel dose to organ doses against external photon exposure in a hypothesized field, where scattered radiation was taken into account. The results indicated that energy distribution of photons incident to a human body is required to evaluate precisely an individual dose based on ESR dosimetry for teeth.

Absorbed dose to tooth enamel was quantitatively correlated to organ doses by Monte Carlo calculations using the Electron Gamma Shower Code Version 4 for the Electron Spin Resonance (ESR) dosimetry using teeth. A region for teeth was newly added to a mathematical human model. Kerma coefficients for whole tooth and enamel part were prepared to estimate the dose to tooth enamel. Experiments were also carried out with a physical head phantom, which is made of tissue equivalent materials. Tooth samples and thermo-luminescence dosimeters (TLDs) of CaSO4crystal were set at the teeth position in the head phantom. Additional Monte Carlo calculations were performed to verify the results of the experiments by using a Voxel-type phantom reconstructed from computed tomographic (CT) images of the physical phantom. The obtained data are to be useful for retrospective dose assessments in past radiation events by the ESR dosimetry with teeth.

We have developed a new shaped NaI(Tl) scintillation detector for the direct measurement of “effective dose, (E)” or “effective dose equivalent, (HE)” for external photon exposures. These quantities related to human risk are generally difficult to be evaluated directly from measurements, because these depend not only on the energy distribution but also on the angular distribution of the gamma ray field. The present work, however, solved the problem concerning the angular dependence, by considering a detector shape that satisfies the angular response similar to human body's. The optimum dimensions of NaI(Tl) crystal for HEand for Ewere determined respectively by a Monte Carlo simulation. The detector subject to HEwas manufactured and tested. The angular characteristics of the developed detector indicate a fairly good coincidence with those of the theoretical values, HE. Together with the dose conversion method, so called “G(E)Function method”, it was made possible to directly measure effective dose (equivalent).

The organ doses for γ rays from typical environmental sources were determined with Monte Carlo calculations using anthropomorphic phantoms having different body sizes. It has been suggested that body weight is the predominant factor influencing organ doses for environmental γ rays, regardless of sex and age. A weight function expressing organ doses for environmental γ rays was introduced. This function fitted well with the organ doses calculated using the different phantoms. The function coefficients were determined mathematically with the least squares method. On the assumption that this function was applicable to organ doses for human bodies with diverse characteristics, the variances in organ doses due to race, sex, age and difference in body weight of adults were investigated. The variations of organ doses due to race and sex were not significant. Differences in body weight were found to alter organ doses by a maximum of 10% for γ rays over 100 keV, and 20% for low-energy γ rays. The doses for organs located deep inside a body, such as ovaries, differed between a newborn baby and an adult by a maximum factor of 2 to 3. For y rays over 100 keV, the variation was within a factor of 2 for all organs. The organ doses for adolescents more than 12 years agreed within 15% with those of the average adult.

Kidney dosimetry in mice is of considerable significance in preclinical studies of new radiopharmaceuticals. There has been a gradual accumulation of kidney dosimetry data in mouse studies. However, a comprehensive understanding of the kidney dosimetry is still lacking. To conduct reliable kidney dosimetry, there exists a need to evaluate self-absorbed fractions (self-AFs) for sophisticated models of the kidneys. In the present study, self-AFs for photons and electrons in the kidneys of a voxel mouse were evaluated using Monte Carlo simulations. The sources were assumed to be monoenergetic in the energy range 10 keV-4 MeV, and be uniformly distributed in the kidneys. The self-AFs for the kidneys of the voxel mouse were compared with those for voxel humans. In addition, self-S values (μGy/MBq·s) for 18F and 90Y of potential interest in the kidney dosimetry were assessed using the self-AFs. It was found that the photon self-AFs for the voxel mouse are significantly smaller than those for the voxel humans and that the electron self-AFs for the voxel mouse are consistent with those for the voxel humans in the energy range 10-100 keV, followed by a sharp fall. The self-S values for the voxel mouse were found to be much larger than those for the voxel humans.

A single cell can form a colony, and ionizing irradiation has long been known to reduce such a cellular clonogenic potential. Analysis of abortive colonies unable to continue to grow should provide important information on the reproductive cell death (RCD) following irradiation. Our previous analysis with a branching process model showed that the RCD in normal human fibroblasts can persist over 16 generations following irradiation with low linear energy transfer (LET) γ-rays. Here we further set out to evaluate the RCD persistency in abortive colonies arising from normal human fibroblasts exposed to high-LET carbon ions (18.3 MeV/u, 108 keV/µm). We found that the abortive colony size distribution determined by biological experiments follows a linear relationship on the log–log plot, and that the Monte Carlo simulation using the RCD probability estimated from such a linear relationship well simulates the experimentally determined surviving fraction and the relative biological effectiveness (RBE). We identified the short-term phase and long-term phase for the persistent RCD following carbon-ion irradiation, which were similar to those previously identified following γ-irradiation. Taken together, our results suggest that subsequent secondary or tertiary colony formation would be invaluable for understanding the long-lasting RCD. All together, our framework for analysis with a branching process model and a colony formation assay is applicable to determination of cellular responses to low- and high-LET radiation, and suggests that the long-lasting RCD is a pivotal determinant of the surviving fraction and the RBE.