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It has generally been assumed that the neutron and γ-ray absorbed doses in the data from the life span study (LSS) of the Japanese A-bomb survivors are too highly correlated for an independent separation of the all solid cancer risks due to neutrons and due to γ-rays. However, with the release of the most recent data for all solid cancer incidence and the increased statistical power over previous datasets, it is instructive to consider alternatives to the usual approaches. Simple excess relative risk (ERR) models for radiation-induced solid cancer incidence fitted to the LSS epidemiological data have been applied with neutron and γ-ray absorbed doses as separate explanatory covariables. A simple evaluation of the degree of independent effects from γ-ray and neutron absorbed doses on the all solid cancer risk with the hierarchical partitioning (HP) technique is presented here. The degree of multi-collinearity between the γ-ray and neutron absorbed doses has also been considered. The results show that, whereas the partial correlation between the neutron and γ-ray colon absorbed doses may be considered to be high at 0.74, this value is just below the level beyond which remedial action, such as adding the doses together, is usually recommended. The resulting variance inflation factor is 2.2. Applying HP indicates that just under half of the drop in deviance resulting from adding the γ-ray and neutron absorbed doses to the baseline risk model comes from the joint effects of the neutrons and γ-rays-leaving a substantial proportion of this deviance drop accounted for by individual effects of the neutrons and γ-rays. The average ERR/Gy γ-ray absorbed dose and the ERR/Gy neutron absorbed dose that have been obtained here directly for the first time, agree well with previous indirect estimates. The average relative biological effectiveness (RBE) of neutrons relative to γ-rays, calculated directly from fit parameters to the all solid cancer ERR model with both colon absorbed dose covariables, is 65 (95 %CI: 11; 170). Therefore, although the 95 % CI is quite wide, reference to the colon doses with a neutron weighting of 10 may not be optimal as the basis for the determination of all solid cancer risks. Further investigations into the neutron RBE are required, ideally based on the LSS data with organ-specific neutron and γ-ray absorbed doses for all organs rather than the RBE weighted absorbed doses currently provided. The HP method is also suggested for use in other epidemiological cohort analyses that involve correlated explanatory covariables.

The effective dose of combined spectrum energy neutrons and high energy spectrum gamma-rays in A-bomb survivors in Hiroshima and Nagasaki has long been a matter of discussion. The reason is largely due to the paucity of biological data for high energy photons, particularly for those with an energy of tens of MeV. To circumvent this problem, a mathematical formalism was developed for the photon energy dependency of chromosomal effectiveness by reviewing a large number of data sets published in the literature on dicentric chromosome formation in human lymphocytes. The chromosomal effectiveness was expressed by a simple multiparametric function of photon energy, which made it possible to estimate the effective dose of spectrum energy photons and differential evaluation in the field of mixed neutron and gamma-ray exposure with an internal reference radiation. The effective dose of reactor-produced spectrum energy neutrons was insensitive to the fine structure of the energy distribution and was accessible by a generalized formula applicable to the A-bomb neutrons. Energy spectra of all sources of A-bomb gamma-rays at different tissue depths were simulated by a Monte Carlo calculation applied on an ICRU sphere. Using kerma-weighted chromosomal effectiveness of A-bomb spectrum energy photons, the effective dose of A-bomb neutrons was determined, where the relative biological effectiveness (RBE) of neutrons was expressed by a dose-dependent variable RBE, RBE(gamma, D (n)), against A-bomb gamma-rays as an internal reference radiation. When the newly estimated variable RBE(gamma, D (n)) was applied to the chromosome data of A-bomb survivors in Hiroshima and Nagasaki, the city difference was completely eliminated. The revised effective dose was about 35% larger in Hiroshima, 19% larger in Nagasaki and 26% larger for the combined cohort compared with that based on a constant RBE of 10. Since the differences are significantly large, the proposed effective dose might have an impact on the magnitude of the risk estimates deduced from the A-bomb survivor cohort.

The analysis of chromosomal aberrations in peripheral blood of radiation accident victims is an established method of biological dosimetry. The dose estimate on the basis of an in vitro calibration curve is straightforward when the radiation exposure is homogeneous and the analysis not delayed. In recent years three radiation accidents occurred, where the irradiation or sampling conditions precluded a simple estimation of the dose. During the Georgian accident soldiers carried in their pockets small sources of 137Cs leading to partial and protracted body exposures. During the Tokai-mura accident, three employees involved in the process of 235U enrichment were exposed to very high doses of gamma rays and neutrons. During the Bialystok accident, five patients with breast cancer undergoing radiotherapy were exposed to a single dose of electrons which reached about 100 Gy. In the present paper the approaches chosen to estimate, by cytogenetic methods, the doses absorbed by the people involved in the accidents are described., (Copyright 2003 S. Karger AG, Basel)

The first criticality accident in Japan occurred in a uranium processing plant in Tokai-mura on September 30, 1999. The accident, which occurred while a large amount of enriched uranyl nitrate solution was being loaded into a tank, led to a chain reaction that continued for 20 h. Two workers who were pouring the uranium solution into the tank at the time were heterogeneously exposed to neutrons and gamma rays produced by nuclear fission. Analysis of dose distributions was essential for the understanding of the clinical course observed in the skin and organs of these workers. We developed a numerical simulation system, which consists of mathematical human models and Monte Carlo radiation transport programs, for analyzing dose distributions in various postures and applied the system to the dose analysis for the two workers. This analysis revealed the extreme heterogeneity of the doses from neutrons and gamma rays in the skin and body, which depended on the positions and postures of the workers. The detailed dose analysis presented here using color maps is indispensable for an understanding of the biological effects of high-dose exposure to a mixed field of neutrons and gamma rays as well as for the development of emergency treatments for victims of radiation exposure.

Purpose: To investigate the dynamics of chromosome aberrations in the blood cells of three workers severely exposed to neutrons and gamma-rays in a criticality accident that occurred in Tokai-mura, Japan, in 1999., Materials and Methods: The change with time of the frequency of' chromosome aberrations in the three patients was examined using a new analysis to score drug-induced prematurely condensed ring chromosomes (PCC-R) and a conventional meta-phase analysis., Results: The frequencies and cellular distributions of PCC-R, dicentrics and rings did not change significantly among the samples obtained at 9-48h after the accident while the first depletion of lymphocytes occurred. The distributions of these aberrations in the cells of two patients showed a slight overdispersion compared with a Poisson distribution reflecting neutron and non-uniform exposures. The dose response curve of rings paralleled that of dicentrics, but not PCC-R. The half-lives of PCC-R (8.5 months) and of rings (8.7 months) were shorter than that of dicentrics (13.5 months)., Conclusions: In the three patients of the Tokai-mura accident, lymphocytes in the circulating and extravascular pools had reached equilibrium at 9h, and highly damaged lymphocytes did not selectively move away from the circulatory system during the first rapid depletion of lymphocytes after exposure. Data on the in vivo half-life of PCC-R as well as dicentrics and rings obtained in the present study may be useful for retrospective dosimetry.

A previous investigation has uncoupled the solid cancer risk coefficient for neutrons from the low dose estimates of the relative biological effectiveness (RBE) of neutrons and the photon risk coefficient, and has related it to two more tangible quantities, the excess relative risk (ERR1) due to an intermediate reference dose D1 = 1 Gy of gamma-rays and the RBE of neutrons, R1, against this reference dose. With tentatively assumed RBE values between 20 and 50 and in terms of organ-averaged doses--rather than the usually invoked colon doses--the neutron risk factor was seen to be in general agreement with the current risk estimate of the International Commission on Radiation Protection (ICRP). The present assessment of the risk coefficient for gamma-rays incorporates--in terms of the unchanged A-bomb dosimetry system, DS86--this treatment of the neutrons, but is otherwise largely analogous to the evaluation of the A-bomb data for the ICRP report and for the recent report of the United Nations Scientific Committee on the effects of ionizing radiation, UNSCEAR. The resulting central estimate of the lifetime attributable risk (LAR) for solid cancer mortality is 0.043/Gy for a working population (ages 25-65), and is nearly the same whether the age at exposure or the attained age model is used for risk projection. For a population of all ages 0.042/Gy is obtained with the attained age model and 0.068/Gy with the age at exposure model. The values do not include a dose and dose rate effectiveness factor (DDREF), and they are only half as large as the new UNSCEAR estimates of 0.082/Gy (attained age model and all ages) and 0.13/Gy (age at exposure model and all ages). The difference is only partly due to the more explicit treatment of the neutrons. It reflects also the fact that UNSCEAR has converted ERR into LAR in a way that differs from the ICRP procedure, and that it has summed the overall risk coefficient for solid tumor mortality and incidence from separate estimates for eight solid tumor categories, whereas the present study employs a combined computation for all solid tumors and uses the ICRP procedure for the conversion of ERR into LAR. The appendix gives results for the solid cancer incidence data.

In the criticality accident which occurred on 30 September 1999 at a uranium conversion facility in Tokai-mura, Japan, three workers were severely exposed to neutron and gamma-ray irradiation. Preliminary estimations of doses from blood properties and 24Na concentration in blood were 16-20, 6-10 and 1-4 gamma-ray gray-equivalent (gammaGyEq) respectively for the three workers. For apparent dose estimation, neutron-induced radionuclides in biological materials such as blood, hair and urine were measured. Accordingly, we detected 32p in urine samples. The concentration ratios of 32P in the urine for the three workers showed a similar tendency to those of 24Na in blood. This result indicated that the radioactivity of 32P in urine could be used to estimate the neutron exposure level.

The depth profile of 152Eu activity induced in a large granite stone pillar by Hiroshima atomic bomb neutrons was calculated by a Monte Carlo N-Particle Transport Code (MCNP). The pillar was on the Motoyasu Bridge, located at a distance of 132 m (WSW) from the hypocenter. It was a square column with a horizontal sectional size of 82.5 cm x 82.5 cm and height of 179 cm. Twenty-one cells from the north to south surface at the central height of the column were specified for the calculation and 152Eu activities for each cell were calculated. The incident neutron spectrum was assumed to be the angular fluence data of the Dosimetry System 1986 (DS86). The angular dependence of the spectrum was taken into account by dividing the whole solid angle into twenty-six directions. The calculated depth profile of specific activity did not agree with the measured profile. A discrepancy was found in the absolute values at each depth with a mean multiplication factor of 0.58 and also in the shape of the relative profile. The results indicated that a reassessment of the neutron energy spectrum in DS86 is required for correct dose estimation.

While it is recognized that neutrons contributed to the excess cancer incidence and mortality among the atomic bomb survivors in Hiroshima, there is no possibility to deduce the magnitude of this contribution from the data. This remains true even if the neutron doses in the dosimetry system DS86 are corrected upwards in line with recent neutron activation measurements. In spite of this fact, important information can be obtained in the form of an inverse relation of the risk coefficients for gamma-rays and neutrons. Such an interrelation must apply because the observed excess incidence or mortality is made up of a gamma-ray and a neutron component; increased attribution to neutrons decreases the attribution to photons. Computations with the uncorrected and the corrected DS86 are performed for the mortality and the incidence of solid tumors combined. They refer to doses up to 2 Gy and employ the constant relative risk model and a linear-quadratic dose dependence with variable ratio-the neutron relative biological effectiveness (RBE) at low doses-of the linear component for neutrons and gamma-rays. In line with past analyses, no quadratic component is obtained with the uncorrected DS86. but it is seen, even in these calculations, that the assumption of increased neutron RBEs does not translate into proportional increases of the risk coefficients of neutrons, because it leads to substantially reduced risk estimates for gamma-rays. Calculations with the corrected dosimetry bring out this reciprocity even more clearly. High values of the neutron RBE reduce-in line with recent suggestions by Rossi and Zaider-the risk estimates for gamma-rays substantially. Even a purely quadratic dose relation for gamma-rays is consistent with the data; it requires no major increase of the nominal risk coefficients for neutrons over the currently assumed values. The cancer data from Hiroshima can still provide 'prudent' risk estimates for photons, but with the corrected DS86, they do not prove that there is a linear component in the dose dependence for photons.

Two granite and two concrete core samples were obtained within 500 m from the hypocenter of the Hiroshima atomic bomb, and the depth profile of 152Eu was measured to evaluate the incident neutron spectrum. The granite cores were obtained from a pillar of the Motoyasu Bridge located 101 m from the hypocenter and from a granite rock in the Shirakami Shrine (379 m); the concrete cores were obtained from a gate in the Gokoku Shrine (398 m) and from a pillar top of the Hiroshima bank (250 m). The profiles of the specific activities of the cores were measured to a depth of 40 cm from the surface using low background germanium (Ge) spectrometers. According to the measured depth profiles, relaxation lengths of incident neutrons were derived as 13.6 cm for Motoyasu Bridge pillar (granite), 12.2 cm for Shirakami Shrine core (granite), and 9.6 cm for concrete cores of Gokoku Shrine and Hiroshima Bank. In addition, a comparison of the granite cores in Hiroshima showed good agreement with Nagasaki data. Present results indicates that the depth profile of 152Eu reflects incident neutrons not so high but in the epithermal region.