Karger, C. P., Debus, J., Peschke, P., Münter, M. W., Heiland, S. and Hartmann, G. H. Dose–Response Curves for Late Functional Changes in the Normal Rat Brain after Single Carbon-Ion Doses Evaluated by Magnetic Resonance Imaging: Influence of Follow-up Time and Calculation of Relative Biological Effectiveness. Radiat. Res. 158, 545–555 (2002).

This study investigated late effects in the brain after irradiation with carbon ions using a rat model. Thirty-six animals were irradiated stereotactically at the right frontal lobe using an extended Bragg peak with maximum doses between 15.2 and 29.2 Gy. Dose–response curves for late changes in the normal brain were measured using T1- and T2-weighted magnetic resonance imaging (MRI). Tolerance doses were calculated at several effect probability levels and times after irradiation. The MRI changes were progressive in time up to 17 months and remained stationary after that time. At 20 months the tolerance doses at the 50% effect probability level were 20.3 ± 2.0 Gy and 22.6 ± 2.0 Gy for changes in T1- and T2-weighted MRI, respectively. The relative biological effectiveness (RBE) was calculated on the basis of a previous animal study with photons. Using tolerance doses at the 50% effect probability level, RBE values of 1.95 ± 0.20 and 1.88 ± 0.18 were obtained for T1- and T2-weighted MRI. A comparison with data in the literature for the spinal cord yielded good agreement, indicating that the RBE values for single-dose irradiations of the brain and the spinal cord are the same within the experimental uncertainty.

Sachs, R. K., Arsuaga, J., Vázquez, M., Hlatky, L. and Hahnfeldt, P. Using Graph Theory to Describe and Model Chromosome Aberrations. Radiat. Res. 158, 556–567 (2002).

A comprehensive description of chromosome aberrations is introduced that is suitable for all cytogenetic protocols (e.g. solid staining, banding, FISH, mFISH, SKY, bar coding) and for mathematical analyses. “Aberration multigraphs” systematically characterize and interrelate three basic aberration elements: (1) the initial configuration of chromosome breaks; (2) the exchange process, whose cycle structure helps to describe aberration complexity; and (3) the final configuration of rearranged chromosomes, which determines the observed pattern but may contain cryptic misrejoinings in addition. New aberration classification methods and a far-reaching generalization of mPAINT descriptors, applicable to any protocol, emerge. The difficult problem of trying to infer actual exchange processes from cytogenetically observed final patterns is analyzed using computer algorithms, adaptations of known theorems on cubic graphs, and some new graph-theoretical constructs. Results include the following: (1) For a painting protocol, unambiguously inferring the occurrence of a high-order cycle requires a corresponding number of different colors; (2) cycle structure can be computed by a simple trick directly from mPAINT descriptors if the initial configuration has no more than one break per homologue pair; and (3) higher-order cycles are more frequent than the obligate cycle structure specifies. Aberration multigraphs are a powerful new way to describe, classify and quantitatively analyze radiation-induced chromosome aberrations. They pinpoint (but do not eliminate) the problem that, with present cytogenetic techniques, one observed pattern corresponds to many possible initial configurations and exchange processes.

Holley, W. R., Mian, I. S., Park, S. J., Rydberg, B. and Chatterjee, A. A Model for Interphase Chromosomes and Evaluation of Radiation-Induced Aberrations. Radiat. Res. 158, 568–580 (2002).

We have developed a theoretical model for evaluating radiation-induced chromosomal exchanges by explicitly taking into account interphase (G₀/G₁) chromosome structure, nuclear organization of chromosomes, the production of double-strand breaks (DSBs), and the subsequent rejoinings in a faithful or unfaithful manner. Each of the 46 chromosomes for human lymphocytes (40 chromosomes for mouse lymphocytes) is modeled as a random polymer inside a spherical volume. The chromosome spheres are packed randomly inside a spherical nucleus with an allowed overlap controlled by a parameter Ω. The rejoining of DSBs is determined by a Monte Carlo procedure using a Gaussian proximity function with an interaction range parameter σ. Values of Ω and σ have been found which yield calculated results of interchromosomal aberration frequencies that agree with a wide range of experimental data. Our preferred solution is one with an interaction range of 0.5 μm coupled with a relatively small overlap parameter of 0.675 μm, which more or less confirms previous estimates. We have used our model with these parameter values and with resolution or detectability limits to calculate yields of translocations and dicentrics for human lymphocytes exposed to low-LET radiation that agree with experiments in the dose range 0.09 to 4 Gy. Five different experimental data sets have been compared with the theoretical results. Essentially all of the experimental data fall between theoretical curves corresponding to resolution limits of 1 Mbp and 20 Mbp, which may reflect the fact that different investigators use different limits for sensitivity or detectability. Translocation yields for mouse lymphocytes have also been calculated and are in good agreement with experimental data from 1 cGy to 10 cGy. There is also good agreement with recent data on complex aberrations. Our model is expected to be applicable to both low- and high-LET radiation, and we include a sample prediction of the yield of interchromosomal rejoining in the dose range 0.22 Gy to 2 Gy of 1000 MeV/nucleon iron particles. This dose range corresponds to average particle traversals per nucleus ranging from 1.0 to 9.12.

Durante, M., George, K., Wu, H. and Cucinotta, F. A. Karyotypes of Human Lymphocytes Exposed to High-Energy Iron Ions. Radiat. Res. 158, 581–590 (2002).

Chromosomal aberrations were analyzed using multicolor fluorescence in situ hybridization (mFISH) in human peripheral blood lymphocytes after in vitro exposure to γ rays or accelerated ⁵⁶Fe ions (1 GeV/nucleon, 145 keV/μm) at Brookhaven National Laboratory (Upton, NY). Doses of 0.3 and 3 Gy were used for both radiation types. Chromosomes were prematurely condensed by a phosphatase inhibitor (calyculin A) to avoid the population selection bias observed at metaphase as a result of the severe cell cycle delays induced by heavy ions. A total of 1053 karyotypes (G₂ and M phases) were analyzed in irradiated lymphocytes. Results revealed different distribution patterns for chromosomal aberrations after low- and high-LET radiation exposures: Heavy ions induced a much higher fraction of cells with multiple aberrations, while the majority of the aberrant cells induced by low doses of γ rays contained a single aberration. The high fraction of complex-type exchanges after heavy ions leads to an overestimation of simple-type asymmetrical interchanges (dicentrics) from analysis of Giemsa-stained samples. However, even after a dose of 3 Gy iron ions, about 30% of the cells presented no complex-type exchanges. The involvement of individual chromosomes in exchanges was similar for densely and sparsely ionizing radiation, and no statistically significant evidence of a nonrandom involvement of specific chromosomes was detected.

Salomaa, S., Lindholm, C., Tankimanova, M. K., Mamyrbaeva, Z. Z., Koivistoinen, A., Hulten, M., Mustonen, R., Dubrova, Y. E. and Bersimbaev, R. I. Stable Chromosome Aberrations in the Lymphocytes of a Population Living in the Vicinity of the Semipalatinsk Nuclear Test Site. Radiat. Res. 158, 591–596 (2002).

Translocation analysis using FISH (fluorescence in situ hybridization) chromosome painting was performed to evaluate the magnitude of exposure to ionizing radiation among the human population living close to the Semipalatinsk nuclear test site in Kazakhstan. We studied two generations of people living in villages that were in the path of the radioactive cloud from the first Soviet surface nuclear test performed in August 1949 and from later tests. The older generation (P₀) lived in the area at the time of testing, and the younger generation (F₁) was exposed to smaller doses from the residual fallout and later tests. In both P₀ and F₁ generations, similar translocation frequencies were observed in persons living in either the Semipalatinsk area or a noncontaminated area. Assuming translocation stability in peripheral blood lymphocytes over several decades, these findings suggest that on average, the magnitude of exposure of this cohort in the Semipalatinsk area has been considerably smaller than that reported in the literature. Previously reported doses of the order of 1−4.5 Gy (mean 2.9 Gy in the P₀ generation) cannot be confirmed by the present data.

Leenhouts, H. P., Brugmans, M. J. P., Andersson, M. and Storm, H. H. A Reanalysis of Liver Cancer Incidence in Danish Patients Administered Thorotrast Using a Two-Mutation Carcinogenesis Model. Radiat. Res. 158, 597–606 (2002).

In recent years, a two-mutation carcinogenesis (TMC) model has been used to analyze epidemiological data and estimate the radiation risks at low doses for the organs affected. Here the TMC model was used to reanalyze the liver cancer incidence in the Danish population in general and in patients administered Thorotrast, and to estimate the radiation risks for the liver. The data for 807 patients for whom sufficient data on the injected volumes of Thorotrast were available were used in this reanalysis. These data were combined with data on liver cancer incidence in the Danish population as the baseline or background incidence. Because males and females show different baseline liver cancer incidences, separate fits were made for males and females. The fits showed that the radiation effect could be ascribed entirely to the radiation dependence of the first mutation rate of the TMC model, which was higher for females than for males. The second mutation rate was not significantly dependent on dose. The radiation risks for the liver were calculated on the basis of the model parameters. These risks for lifetime exposures are about the same for males and females and are between a factor of 2 and 10 higher than current estimates. The discrepancy between the model results and previous risk estimates probably arises because the model calculations give more complete lifetime radiation risk estimates. For short-term exposures of the liver to ionizing radiation, the maximum radiation-induced excess liver cancer risk per unit dose applies to exposures at the age of about 10; exposures at ages above 35 have a radiation effect of less than approximately 15% of this maximum.

Heidenreich, W. F., Luebeck, E. G., Hazelton, W. D., Paretzke, H. G. and Moolgavkar, S. H. Multistage Models and the Incidence of Cancer in the Cohort of Atomic Bomb Survivors. Radiat. Res. 158, 607–614 (2002).

The analyses in this paper show that a number of biologically based models describe cancer incidence among the A-bomb survivors equally well. However, these different models can predict very different temporal patterns of risk after irradiation. No evidence was found to support the previous claim of Pierce and Mendelsohn that excess cancer risks for the solid tumors depend only upon attained age and not on age at exposure or time since exposure. Although the A-bomb survivor cohort is the largest epidemiological data set for the study of radiation and cancer, it is not large enough to discriminate among various possible carcinogenic mechanisms. Unfortunately for hypothesis generation, the data appear to be consistent with a number of different mechanistic interpretations of the role of radiation in carcinogenesis.

Vanhaelewyn, G. C. A. M., Sadlo, J., Matthys, P. F. A. E. and Callens, F. J. Comparative X- and Q-Band EPR Study of Radiation-Induced Radicals in Tooth Enamel. Radiat. Res. 158, 615–625 (2002).

Human tooth enamel blocks and powders that were either unheated or heated prior to X irradiation at room temperature were investigated by means of Q-band electron paramagnetic resonance (EPR). It was found that the EPR spectra of unheated human tooth enamel consist mainly of two different anisotropic CO₂⁻ signals, as was suggested previously from an X-band study of analogous samples. In the present study, the two CO₂⁻ radical contributions could be differentiated convincingly by comparing the anisotropic Q-band spectra of heated and unheated enamel blocks. One type of CO₂⁻ is probably located in the bulk of the apatitic microcrystallites that constitute the enamel, and it appears in both heated and unheated samples. The other type is presumably located in an intercrystallite position and appears mainly in the unheated samples. Clear differences between g values in the Q-band spectra of heated and unheated enamel suggest that the CO₂⁻ radicals in the bulk exhibit larger g anisotropy than those in intercrystallite positions. Isotropic CO₂⁻ signals and contributions that may be from CO₃³⁻ and CO⁻ radicals have also been detected. However, the present work focuses mainly on the CO₂⁻ signals and discusses potential and/or real difficulties that may be encountered in applications of EPR dosimetry using calcified tissues.

Mothersill, C. and Seymour, C. B. Bystander and Delayed Effects after Fractionated Radiation Exposure. Radiat. Res. 158, 626–633 (2002).

Human immortalized keratinocytes were exposed to a range of single or fractionated doses of γ rays from ⁶⁰Co, to medium harvested from donor cells exposed to these protocols, or to a combination of radiation and irradiated cell conditioned medium (ICCM). The surviving fractions after direct irradiation or exposure to ICCM were determined using a clonogenic assay. The results show that medium harvested from cultures receiving fractionated irradiation gave lower “recovery factors” than direct fractionated irradiation, where normal split-dose recovery occurred. The recovery factor is defined here as the surviving fraction of the cells receiving two doses (direct or ICCM) separated by an interval of 2 h divided by the surviving fraction of cells receiving the same dose in one exposure. After treatment with ICCM, the recovery factors were less than 1 over a range of total doses from 5 mGy–5 Gy. Varying the time between doses from 10 min to 180 min did not alter the effect of ICCM, suggesting that two exposures to ICCM are more toxic than one irrespective of the dose used to generate the response. In certain protocols using mixtures of direct irradiation and ICCM, it was possible to eliminate the bystander effect. If bystander factors are produced in vivo, then they may reduce the sparing effect of the dose fractionation.

Mirzaie-Joniani, H., Eriksson, D., Johansson, A., Löfroth, P-O., Johansson, L., Riklund Åhlström, K and Stigbrand, T. Apoptosis in HeLa Hep2 Cells is Induced by Low-Dose, Low-Dose-Rate Radiation. Radiat. Res. 158, 634–640 (2002).

Radioimmunotherapy with radiolabeled antibodies may cause inhibition of the growth of epithelial tumors, despite low total radiation doses and comparatively low radiosensitivity of epithelial tumor cells. The induction of apoptosis by low-dose radiation, such as delivered in radioimmunotherapy, was investigated in vitro in human HeLa Hep2 carcinoma cells. The cultured cells were exposed to defined radiation doses from a ⁶⁰Co radiation therapy source. The radiation source delivered 0.80 ± 0.032 (mean ± SD) Gy/min and the cells were given total doses of 1, 2, 5, 10 and 15 Gy. Using fluorescein-labeled Annexin V, followed by flow cytometry and DNA ladder analysis, apoptotic cells were detected and quantified. Radiation doses below 2 Gy did not cause any significant increase in apoptosis. Compared to control cells, apoptosis was pronounced after 5–10 Gy irradiation and was correlated to the radiation dose, with up to 42 ± 3.5% of the cells examined displaying apoptosis. Clonogenic assays confirmed significantly decreased viability of the cells in the interval 2 to 10 Gy with low-dose-rate radiation, 60 ± 2% compared to 2 ± 2%. Lethal effects on the tumor cells were also evaluated by an assay of the cytotoxic effects of the release of ⁵¹Cr. Significant cytotoxicity, with up to 64 ± 6% dead cells, was observed at 5 Gy. Similar results were obtained when the dose rate was reduced to 0.072 ± 0.003 Gy/min (mean ± SD). In the case of the ¹³⁷Cs source, the dose rate could be reduced to 0.045 Gy/h, a level comparable to radioimmunotherapy, which induced significant apoptosis, and was most pronounced at 72–168 h postirradiation. It can be concluded that in vitro low-dose and low-dose-rate radiation induces apoptosis in epithelial HeLa Hep2 cells and thus may explain a mechanism by which pronounced inhibition of growth of HeLa Hep2 tumors at doses used in radioimmunotherapy has been obtained previously.

Yi, M-J., Park, S-H., Cho, H-N., Chung, H. Y., Kim, J-I., Cho, C-K., Lee, S-J. and Lee, Y-S. Heat-Shock Protein 25 (Hspb1) Regulates Manganese Superoxide Dismutase through Activation of Nfkb (NF-κB). Radiat. Res. 158, 641–649 (2002).

We previously demonstrated that overexpression of HSP25 (now known as Hspb1) conferred increased resistance to ionizing radiation (Radiat Res. 154, 421–428, 2000). In the present study, L929 cells overexpressing Hspb1 were shown to have increased expression of the manganese superoxide dismutase gene (now known as SOD2) and its enzyme activity. To elucidate Hspb1-induced pathways leading to activation of these antioxidant enzymes, the production of the tumor necrosis factor alpha (Tnf) and interleukin 1 beta (Il1b) genes was examined. Increased expression of Tnf and Il1b resulting from Hspb1 overexpression was detected by RT-PCR. Increased activation of Nfkb (degradation of Ikb, a member of the Nfkb family) was also found in Hspb1-overexpressing cells. When treated with Tnf, Nfkb activation and SOD2 gene expression were increased more by Hspb1 overexpression. Moreover, transfection with the Hspb1 antisense gene abrogated all of the Hspb1-mediated phenomena. To further elucidate the exact relationship between induction of SOD2 and Nfkb activation, a dominant negative I-kBα (now known as Nfkb1a) construct was transfected into Hspb1-overexpressing cells. The dominant negative Nfkb1a inhibited Hspb1-mediated SOD2 gene expression. In addition, Hspb1-mediated radioresistance was blocked by dominant negative Nfkb1a transfection. When the SOD2 gene was transfected into L929 cells, a somewhat increased radioresistance was detected by a clonogenic survival assay compared to control cells. Hspb1 produced Tnf and Il1b and facilitated SOD2 gene expression through Nfkb activation, possibly resulting in Hspb1-mediated radioresistance.

Alard, J. P., Bodez, V., Tchirkov, A., Arnold, J., Nénot, M. L., Crespin, S., Rapp, M., Verrelle, P. and Dionet, C. Simulation of Neutron Interactions at the Single-Cell Level. Radiat. Res. 158, 650–656 (2002).

We recently reported that the exposure of cancer cells to 14 MeV neutrons at a very low dose rate (0.8 mGy min⁻¹) produced a marked increase in cell killing at 5 cGy, followed by a plateau in survival and chromosomal damage. Simulation of the energy deposition events in irradiated cells may help to explain these unusual cell responses. We describe here a Monte Carlo simulation code, Energy Deposition in Cells Irradiated by Neutrons (EDCIN). The procedure considered the experimental setup and a hemispheric cell model. The simulation data fitted the dosimetric measurements performed using tissue-equivalent ionization chambers, Geiger-Müller counters, fission chambers, and silicon diodes. The simulation showed that 80% of the energy deposited in a single cell came from the interactions of neutrons outside the cell and only 20% came from neutron interactions inside the cell. Thus the “external” interactions that result in the production of recoil protons and secondary electrons may induce most of the biological damage, which may be repaired efficiently at low dose rate. The repair process may be triggered from a threshold level of damage, which would explain an initial increase cell death due to unrepaired sublethal damage, and then may compensate for induced damage, resulting in the plateaus.

Meesungnoen, J., Jay-Gerin, J-P., Filali-Mouhim, A. and Mankhetkorn, S. Low-Energy Electron Penetration Range in Liquid Water. Radiat. Res. 158, 657–660 (2002).

Monte Carlo simulations of electron tracks in liquid water are performed to calculate the energy dependence of the electron penetration range at initial electron energies between 0.2 eV and 150 keV, including the subexcitation electron region (<7.3 eV). Our calculated electron penetration distances are compared with available experimental data and earlier calculations as well as with the results of simulations using newly reported amorphous ice electron scattering cross sections in the range ∼1–100 eV.