Effects of radiation quality on interactions between oxidative stress, protein and DNA damage in Deinococcus radiodurans

Ionizing radiation damages DNA and also induces oxidative stress, which can affect the function of proteins involved in DNA repair, thereby causing repair of DNA damage to become less efficient. We previously developed a mathematical model of this potentially synergistic relationship and applied it to γ-ray exposure data on the radiation-resistant prokaryote Deinococcus radiodurans. Here, we investigate the effects of radiation quality on these processes by applying the model to data on exposures of D. radiodurans to heavy ions with linear energy transfer (LET) of 18.5–11,300 keV/μm. The model adequately describes these data using three parameters combinations: radiogenic DNA damage induction, repair protein inactivation and cellular repair capacity. Although statistical uncertainties around best-fit parameter estimates are substantial, the behaviors of model parameters are consistent with current knowledge of LET effects: inactivation cross-sections for both DNA and proteins increase with increasing LET; DNA damage yield per unit of radiation dose also increases with LET; protein damage per unit dose tends to decrease with LET; DNA and especially protein damage yields are reduced when cells are irradiated in the dry state. These results suggest that synergism between oxidative stress and DNA damage may play an important role not only during γ-ray exposure, but during high-LET radiation exposure as well.     

Biodosimetry estimate for high-LET irradiation

The purpose of this paper is to prepare for an easy and reliable biodosimeter protocol for radiation accidents involving high-linear energy transfer (LET) exposure. Human peripheral blood lymphocytes were irradiated using carbon ions (LET: 34.6 keV μm⁻¹), and the chromosome aberrations induced were analyzed using both a conventional colcemid block method and a calyculin A induced premature chromosome condensation (PCC) method. At a lower dose range (0–4 Gy), the measured dicentric (dics) and centric ring chromosomes (cRings) provided reasonable dose information. At higher doses (8 Gy), however, the frequency of dics and cRings was not suitable for dose estimation. Instead, we found that the number of Giemsa-stained drug-induced G2 prematurely condensed chromosomes (G2-PCC) can be used for dose estimation, since the total chromosome number (including fragments) was linearly correlated with radiation dose (r = 0.99). The ratio of the longest and the shortest chromosome length of the drug-induced G2-PCCs increased with radiation dose in a linear-quadratic manner (r = 0.96), which indicates that this ratio can also be used to estimate radiation doses. Obviously, it is easier to establish the dose response curve using the PCC technique than using the conventional metaphase chromosome method. It is assumed that combining the ratio of the longest and the shortest chromosome length with analysis of the total chromosome number might be a valuable tool for rapid and precise dose estimation for victims of radiation accidents.     

An approach to the assessment of risk from chronic radiation to populations of European lobster, Homarus gammarus (L.)

The basic principles underlying a four-discrete age group, logistic, growth model for the European lobster Homarus gammarus are presented and discussed at proof-of-concept level. The model considers reproduction, removal by predation, natural death, fishing, radiation and migration. Non-stochastic effects of chronic low linear energy transfer (LET) radiation are modelled with emphasis on ⁹⁹Tc, using three endpoints: repairable radiation damage, impairment of reproductive ability and, at higher dose rates, mortality. An allometric approach for the calculation of LD_50/30 as a function of the mass of each life stage is used in model calibration. The model predicts that at a dose rate of 1 Gy day⁻¹, lobster population reproduction and survival become severely compromised, leading eventually to population extinction. At 0.01 Gy day⁻¹, the survival rate of an isolated population is reduced by 10%, mainly through loss of fecundity, comparable to natural migration losses. Fishing is the main ecological stress and only dose rates in the range 0.03–0.1 Gy day⁻¹ can achieve discernible effects above it. On the balance of radiation and other ecological stresses, a benchmark value of 0.01 Gy day⁻¹ is proposed for the protection of lobster populations. This value appears consistent with available information on radiation effects in wildlife.     

Complete ablation of small squamous cell carcinoma xenografts with186Re-labeled monoclonal antibody E48

In previous studies we have shown that in mice bearing head and neck squamous cell carcinoma (HNSCC) xenografts, radioimmunotherapy (RIT) with¹⁸⁶Re-labeled MAb E48 resulted in complete regressions in one-third of the tumors (followup >150 d). MAb E48 was labeled with¹⁸⁶Re following a novel labeling procedure developed at our institute. The injected dose was 600 μCi, which was the maximum tolerated dose (MTD; <15% wt loss) in these studies. The mean size of the tumors was 140±60 mm³. To investigate whether the therapeutic efficacy of RIT in our xenograft model would be improved when treating smaller xenografts, mice bearing 2 HNSCC xenografts with a vol of 75±17 mm³ (number of mice,n=6; number of tumors,t=12) were treated with 600 μCi of¹⁸⁶Re-labeled MAb E48 IgG. All tumors completely regressed and did not regrow during followup (>150 d). In all mice, weight loss did not exceed 10%. to obtain biodistribution data, mice bearing two xenografts with a vol of 58±31 mm³ were injected with 100 μCi of¹⁸⁶Re-labeled MAb E48 IgG. The maximum uptake in blood was 26.4% injected dose/g (%ID·g⁻¹) at 2 h pi and was 53.1%ID·g⁻¹ in the tumor at d 7 pi. In normal tissues, no nonspecific accumulation was observed. Based on these biodistribution data, the absorbed cumulative radiation dose was calculated. The accumulated dose in blood and tumor was 2004 cGy and 8580 cGy, respectively. In other tissues, the dose was less than 8.1% of the dose delivered to tumor. These data implicate that RIT with¹⁸⁶Re-labeled MAb E48 may be especially suited to be used as adjuvant therapy for the treatment of head and neck cancer patients with minimal residual disease.     

Effects induced by keV low-energy ion irradiation in the nematode <Emphasis Type="Italic">Caenorhabditis elegans</Emphasis>

The nematode Caenorhabditis elegans is an excellent model organism with which to study the biological effects and mechanisms of ionizing irradiation. In this study, using C. elegans as a model, the effects of keV low-energy argon ion irradiation were investigated, by examining cuticle damage, worm survival, brood size, life span, and germ cell death. The surface etching of worm cuticle after ion impact was investigated by trypan blue staining and SEM microscopy. The degree of damage increased with ion fluence (2 × 10¹⁴ to 7 × 10¹⁴ ions cm⁻²) and energy (5–25 keV). The survival rates, as compared to vacuum control, of ion-bombarded worm larvae at different developmental stages (L1–L4) decreased with increasing ion fluence. L1 larvae were found to be more sensitive to ion bombardment than larvae at other stages. The mean brood size in ion-bombarded groups decreased with increasing ion fluence and energy. Furthermore, keV argon ions caused a significant increase in the number of apoptotic germ cells. However, average lifespan was not significantly affected.     

Different contributions of the indirect effects of γ-rays on the cytotoxicity in M10 and XRCC4 transfected M10 cells

The protective effects of dimethyl sulfoxide (DMSO) against cell killing by ¹³⁷Cs γ-rays were investigated in XRCC4-deficient cell line M10, XRCC4-complemented M10 and the parental mouse leukemia cell line L5178Y. Cell survival was determined by the colony-forming ability. M10 cells were more sensitive to γ-ray-induced cell death than L5178Y and complemented M10 cells. Cell survival was increased in both M10 and L5178Y in the presence of DMSO. However, estimation of the DMSO-protectable fraction revealed a smaller protectable fraction for M10 cells than for L5178Y cells, indicating that indirect effects contributed in a smaller extent to the cytotoxicity in M10 than that in L5178Y. This effect is due to XRCC4 deficiency, since transfection of XRCC4 cDNA into M10 cells restored the radioprotective effects of DMSO to the level seen in L5178Y. In M10 cells, the killing effects of high LET radiation (Auger electrons from ¹²⁵I-antipyrine, carbon ions with an LET of 166 keV μm⁻¹) were similar to those of low LET radiation (¹³⁷Cs γ-rays, characteristic X-rays from ¹²⁵I-bovine serum albumin). We discuss that lethal lesions produced by indirect actions in L5178Y and XRCC4-complemented M10 cells may differ, at least in part, from DNA double-strand breaks repairable by non-homologous end joining.     

Biosorption of Ni (II) by Schizosaccharomyces pombe: kinetic and thermodynamic studies

The potential of the dried yeast, wild-type Schizosaccharomyces pombe, to remove Ni(II) ion was investigated in batch mode under varying experimental conditions including pH, temperature, initial metal ion concentration and biosorbent dose. Optimum pH for biosorption was determined as 5.0. The highest equilibrium uptake of Ni(II) on S. pombe, q _e, was obtained at 25 °C as 33.8 mg g⁻¹. It decreased with increasing temperature within a range of 25–50 °C denoting an exothermic behaviour. Increasing initial Ni(II) concentration up to 400 mg L⁻¹ also elevated equilibrium uptake. No more adsorption took place beyond 400 mg L⁻¹. Equilibrium data fitted better to Langmuir model rather than Freundlich model. Sips, Redlich–Peterson, and Kahn isotherm equations modelled the investigated system with a performance not better than Langmuir. Kinetic model evaluations showed that Ni(II) biosorption process followed the pseudo-second order rate model while rate constants decreased with increasing temperature. Gibbs free energy changes (ΔG°) of the system at 25, 30, 35 and 50 °C were found as −1.47E + 4, −1.49E + 4, −1.51E + 4, and −1.58E + 4 J mol⁻¹, respectively. Enthalpy change (ΔH°) was determined as −2.57E + 3 J mol⁻¹ which also supports the observed exothermic behaviour of the biosorption process. Entropy change (ΔS°) had a positive value (40.75 J mol⁻¹ K⁻¹) indicating an increase in randomness during biosorption process. Consequently, S. pombe was found to be a potential low-cost agent for Ni(II) in slightly acidic aqueous medium. In parallel, it has been assumed to act as a separating agent for Ni(II) recovery from its aqueous solution.     

An approach for the assessment of risk from chronic radiation to populations of phytoplankton and zooplankton

A conceptual model of the effects of chronic radiation on a population of phytoplankton and zooplankton in an oceanic nutrient layer is presented. The model shows that there are distinct threshold dose rates at which the different plankton populations become unsustainable. These are 10,400 μGy h⁻¹ for phytoplankton and 125 μGy h⁻¹ for zooplankton. Both these values are considerably greater than the current screening values for protection of 10 μGy h⁻¹. The model highlights the effects of predator–prey dynamics in predicting that when the zooplankton is affected by the radiation dose, the phytoplankton population can increase. In addition, the model was altered to replicate the dose rates to the plankton of a previous ERICA Irish Sea assessment (24 μGy h⁻¹ for zooplankton and 430 μGy h⁻¹ to phytoplankton). The results showed only a 10% decrease in the zooplankton population and a 15% increase in the phytoplankton population. Therefore, at this level of dose, the model predicts that although the dose rate exceeds the guideline value, populations are not significantly affected. This result highlights the limitations of a single screening value for different groups of organisms.     

Enhancement of anti-tumor immunity specific to murine glioma by vaccination with tumor cell lysate-pulsed dendritic cells engineered to produce interleukin-12

Aim: The aim of this study was to develop an immunotherapy specific to a malignant glioma by examining the efficacy of glioma tumor-specific cytotoxic T lymphocytes (CTL) as well as the anti-tumor immunity by vaccination with dendritic cells (DC) engineered to express murine IL-12 using adenovirus-mediated gene transfer and pulsed with a GL26 glioma cell lysate (AdVIL-12/DC+GL26) was investigated. Experimentl: For measuring CTL activity, splenocytes were harvested from the mice immunized with AdVIL-12/DC+GL26 and restimulated with syngeneic GL26 for 7 days. The frequencies of antigen-specific cytokine-secreting T cell were determined with mIFN-γ ELISPOT. The cytotoxicity of CTL was assessed in a standard 51Cr-release assay. For the protective study in the subcutaneous tumor model, the mice were vaccinated subcutaneously (s.c) with 1×10⁶ AdVIL-12/DC+GL26 in the right flanks on day −21, −14 and −7. On day 7, the mice were challenged with 1×10⁶ GL26 tumor cells in the shaved left flank. For a protective study in the intracranial tumor model, the mice were vaccinated with 1×10⁶ AdVIL-12/DC+GL26 s.c in the right flanks on days −21, −14 and −7. Fresh 1×10⁴ GL26 cells were inoculated into the brain on day 0. To prove a therapeutic benefit in established tumors, subcutaneous or intracranial GL26 tumor-bearing mice were vaccinated s.c with 1×10⁶ AdVIL-12/DC+GL26 on day 5, 12 and 19 after tumor cell inoculation. Results: Splenocytes from the mice vaccinated with the AdVIL-12/DC+GL26 showed enhanced induction of tumor-specific CTL and increased numbers of IFN-γ: secreting T cells by ELISPOT. Moreover, vaccination of AdVIL-12/DC+GL26 enhanced the induction of anti-tumor immunity in both the subcutaneous and intracranial tumor models. Conclusions: These preclinical model results suggest that DC engineered to express IL-12 and pulsed with a tumor lysate could be used in a possible immunotherapeutic strategy for malignant glioma.Korea Research Foundation Grant (KRF-2004-005-E00001).     

The effect of electron and gamma irradiation on the induction of micronuclei in cytokinesis-blocked human blood lymphocytes

The effect of electrons and gamma irradiation on the induction of micronuclei in cytokinesis-blocked human peripheral blood lymphocytes was investigated to understand the relative biological effectiveness (RBE) of electrons compared with gamma rays. Blood samples were irradiated with an 8 MeV pulsed electron beam, at a mean instantaneous dose rate of 2.6 × 10⁵ Gy s⁻¹. Gamma irradiation was carried out at a dose rate of 1.98 Gy min⁻¹ using ⁶⁰Co gamma source. A dose-dependent increase in micronuclei yield was observed. The dose–response relationships for induction of micronuclei fitted well to a linear–quadratic relationship and the coefficients α and β of the dose–response curve were estimated by fitting the data using error-weighted minimum χ ² method. The RBE of 8 MeV electrons were found to be near unity as compared with gamma rays.     

Cell killing, nuclear damage and apoptosis in Chinese hamster V79 cells after irradiation with heavy-ion beams of (16)O, (12)C and (7)Li

Chinese hamster V79 cells were exposed to high LET (linear energy transfer) (16)O-beam (625keV/mum) radiation in the dose range of 0-9.83Gy. Cell survival, micronuclei (MN), chromosomal aberrations (CA) and induction of apoptosis were studied as a follow up of our earlier study on high LET radiations ((7)Li-beam of 60keV/mum and (12)C-beam of 295keV/mum) as well as (60)Co gamma-rays. Dose dependent decline in surviving fraction was noticed along with the increase of MN frequency, CA frequency as well as percentage of apoptosis as detected by nuclear fragmentation assay. The relative intensity of DNA ladder, which is a useful marker for the determination of the extent of apoptosis induction, was also increased in a dose dependent manner. Additionally, expression of tyrosine kinase lck-1 gene, which plays an important role in response to ionizing radiation induced apoptosis, was increased with the increase of radiation doses and also with incubation time. The present study showed that all the high LET radiations were generally more effective in cell killing and inflicting other cytogenetic damages than that of low LET gamma-rays. The dose response curves revealed that (7)Li-beam was most effective in cell killing as well as inducing other nuclear damages followed by (12)C, (16)O and (60)Co gamma-rays, in that order. The result of this study may have some application in biological dosimetry for assessment of genotoxicity in heavy ion exposed subjects and in determining suitable doses for radiotherapy in cancer patients where various species of heavy ions are now being generally used.     

Investigation of switch from ATM to ATR signaling at the sites of DNA damage induced by low and high LET radiation

Upon induction of DNA damage by ionizing radiation (IR), members of the phosphatidylinositol 3-kinase-like kinase family of proteins namely ataxia-telangiectasia mutated (ATM), DNA-PKcs, and ATM- and Rad3-related (ATR) maintain genomic integrity by mounting DNA damage response (DDR). Recent reports suggest that activation of ATM and ATR are oppositely regulated by the length of single stranded overhangs generated during end processing by nucleases at the break sites. These stretches of single stranded overhangs hold the clue for the transition from ATM to ATR signaling at broken DNA ends. We investigated whether differential processing of breaks induced by low and high LET radiation augments the phenomenon of switching from ATM to ATR kinase and hence a concomitant NHEJ to HR transition at the sites of DNA damage. 82-6 human fibroblasts were irradiated with 1 or 2 Gy of γ-rays and particle radiation of increasing LET in order to increase the complexity and variability of DNA double strand breaks (DSB) structures. The activation kinetics of ATM and ATR kinases along with their downstream substrates were determined utilizing Western blotting and immunofluorescence techniques. Our data provide evidence of a potential switch from ATM to ATR kinase signaling in cells treated with γ-rays at approximately 2 h post irradiation, with induction and completion of resection denoted by Rad51 foci resolution kinetics and observed with a significant decline of phosphorylated ATR kinase 8 h after IR. On the other hand, irradiation with high LET 600 MeV/u ⁵⁶Fe (180 keV/μm) and 170 MeV/u ²⁸Si (99 keV/μm) particles show a similar Rad51 foci decay kinetics, however, exhibiting prolonged resection, evident by the persistent phosphorylated ATM and ATR kinase until 24 h post irradiation. This residual effect, however, was significantly reduced for 250 MeV/u ¹⁶O particles of moderate LET (25 keV/μm) and absent for γ-rays. Hence, our results support the hypothesis that the transition from ATM to ATR signaling at DNA break sites is extended for longer periods of time, indicated by sustained resection due to the complex type of damage induced, a hallmark of high LET radiation, which may contribute to its increased biological effectiveness.     

Co-expression of Oct-4 and Nestin in human breast cancers

The aim is to investigate the clinical implications of the Oct-4 and Nestin protein in human breast cancers. A total of 346 cases including 26 fresh and 320 paraffin-embedded tumor tissues were selected for characterizing the frequency of CD44⁺CD24⁻ tumor cells by flow cytometry and the differential expression of the stem cell-related genes between CD44⁺CD24⁻ and non-CD44⁺CD24⁻ tumor cells was analyzed by PCR Array and immunofluorescence. In comparison with the non-CD44⁺CD24⁻ tumor cells, the CD44⁺CD24⁻, particularly for those with high percentage of Oct-4⁺ and Nestin⁺, tumor cells had higher tumorigenicity by forming mammospheres in vitro. More importantly, 42 (13.125%) out of 320 tumor tissues were positive for Oct-4 and Nestin staining. Universal analysis and multivariate analysis revealed that the expression of Oct-4 and Nestin was associated significantly with younger age, pathogenic degrees, lymph node metastasis and triple-negative breast cancer independently (P < 0.05) as well as shorter survival (P = 0.001). Oct-4 and Nestin were important regulators of the development of breast cancer, and Oct-4 and Nestin may be used as predictors for the prognosis of breast cancers.     

Non-parametric estimation of thresholds for radiation effects in vertebrate species under chronic low-LET exposures

Databases on effects of chronic low-LET radiation exposure were analyzed by non-parametric statistical methods, to estimate the threshold dose rates above which radiation effects can be expected in vertebrate organisms. Data were grouped under three umbrella endpoints: effects on morbidity, reproduction, and life shortening. The data sets were compiled on a simple ‘yes’ or ‘no’ basis. Each data set included dose rates at which effects were reported without further details about the size or peculiarity of the effects. In total, the data sets include 84 values for endpoint “morbidity”, 77 values for reproduction, and 41 values for life shortening. The dose rates in each set were ranked from low to higher values. The threshold TDR5 for radiation effects of a given umbrella type was estimated as a dose rate below which only a small percentage (5%) of data reported statistically significant radiation effects. The statistical treatment of the data sets was performed using non-parametric order statistics, and the bootstrap method. The resulting thresholds estimated by the order statistics are for morbidity effects 8.1 × 10⁻⁴ Gy day⁻¹ (2.0 × 10⁻⁴–1.0 × 10⁻³), reproduction effects 6.0 × 10⁻⁴ Gy day⁻¹ (4.0 × 10⁻⁴–1.5 × 10⁻³), and life shortening 3.0 × 10⁻³ Gy day⁻¹ (1.0 × 10⁻³–6.0 × 10⁻³), respectively. The bootstrap method gave slightly lower values: 2.1 × 10⁻⁴ Gy day⁻¹ (1.4 × 10⁻⁴–3.2 × 10⁻⁴) (morbidity), 4.1 × 10⁻⁴ Gy day⁻¹ (3.0 × 10⁻⁴–5.7 × 10⁻⁴) (reproduction), and 1.1 × 10⁻³ Gy day⁻¹ (7.9 × 10⁻⁴–1.3 × 10⁻³) (life shortening), respectively. The generic threshold dose rate (based on all umbrella types of effects) was estimated at 1.0 × 10⁻³ Gy day⁻¹.     

Demonstration of a radiation-induced bystander effect for low dose low LET β-particles

Radiation-induced bystander mutagenesis at a relatively low dose range was investigated using low LET β-particles in a three-dimensional cell culture model. CHO cells were labeled with 0, 0.5, 1.0 or 5.0 μCi tritiated thymidine (³HdTTP) for 12 h and subsequently incubated with A_L cells for 24 h at 11°C. The cell mixture was centrifuged to produce a spheroid of 4 × 10⁶ cells of which there was five times more A_L than CHO cells. The short-range β-particles emitted by ³HdTTP result in self-irradiation of labeled CHO cells, thus biological effects on neighboring A_L cells can be attributed to the bystander response. To evaluate such response, non-labeled bystander A_L cells were isolated from among labeled CHO cells and studied independently for survival and mutagenesis. Treatment of CHO cells with ³HdTTP resulted in a dose-dependent increase in bystander mutation incidence among neighboring A_L cells compared to controls. In addition, multiplex PCR analysis revealed the types of mutants to be significantly different from those of spontaneous origin. These data provide evidence that low dose low LET radiation can induce bystander mutagenesis in a three-dimensional model. The results of this study will address the relevant issues of actual target size and radiation quality, and are likely to have a significant impact on our current understanding of radiation risk assessment.     

The response of the early developmental stages of Laminaria japonica to enhanced ultraviolet-B radiation

The responses of the early development of Laminaria japonica collected from Kiaochow Bay in China to enhanced ultraviolet-B radiation (UV-B, 280–320 nm) were studied in the laboratory. The low UV-B radiations (11.7–23.4 J·m⁻²·d⁻¹) had no significant effects on zoospores attachment, but when the UV-B dose > 35.1 J·m⁻²·d⁻¹ the attachment decreased significantly compared with the control. Germination of embryospores was >93% under the low (11.7–35.1 J·m⁻²·d⁻¹) doses, and in the range of 78.5%–88.5% under the high (46.8–70.2 J·m⁻²·d⁻¹) UV-B doses, indicating a significant radiation effect. Under the higher UV-B exposure (35.1–70.2 J·m⁻²·d⁻¹), all of the few gametophytes formed from embryospores died 120 h post-release. After exposure to the low UV-B radiation (11.7–23.4 J·m⁻²·d⁻¹), the formation of sporophytes decreased and the female gametophyte clones increased compared with the control. However, the sex ratio and the relative growth of female gametophytes/sporophytes had not significantly changed. According to the results, enhanced UV-B radiation has a significant effect on the early development of L. japonica under laboratory conditions, suggesting that the UV-B radiation could not be overlooked as one of the important environmental factors influencing the ontogeny of macroalgae living in marine ecosystems. Supported by the Program for New Century Excellent Talents in University (Grant No. NCET-05-0597) and National Natural Science Foundation of China (Grant No. 30270258)     

Electromagnetic field effects on <Emphasis Type="Italic">Artemia</Emphasis> hatching and chromatin state

The influence of magnetic fields on hatching and chromatin state of brine shrimp, Artemia sp., was investigated. Dry Artemia cysts were exposed to a magnetic field of intensity 25 mT for 10 min. The magnetic field was applied in different variants: constant field, rotating field of different directions (right-handed and left-handed) and different magnet polarization. The effect of ultra wideband pulse radiation and microwave radiation was also investigated. The energy density on the surface of object exposed to ultra wideband pulse radiation was 10⁻², 10⁻³, 10⁻⁴, 10⁻⁵ and 10⁻⁶ W/cm², the power of microwave radiation was 10⁻⁴ and 10⁻⁵ W/cm², exposure time - 10 s. After incubation of the cysts for 48 hours in sea water the hatching percentage of Artemia from exposed cysts was higher than in controls. The number of heterochromatin granules was significantly higher in the nauplia (newborn larvae of Artemia) developed from cysts that had been exposed to magnetic and electromagnetic fields. The data obtained demonstrate an increase in percentage hatching of Artemia cysts after treatment with magnetic and electromagnetic fields and chromatin condensation in nauplia. We have also shown different effects of right-handed and left-handed rotating magnetic fields on these processes.     

Dose estimation for astronauts using dose conversion coefficients calculated with the PHITS code and the ICRP/ICRU adult reference computational phantoms

Absorbed-dose and dose-equivalent rates for astronauts were estimated by multiplying fluence-to-dose conversion coefficients in the units of Gy.cm² and Sv.cm², respectively, and cosmic-ray fluxes around spacecrafts in the unit of cm⁻² s⁻¹. The dose conversion coefficients employed in the calculation were evaluated using the general-purpose particle and heavy ion transport code system PHITS coupled to the male and female adult reference computational phantoms, which were released as a common ICRP/ICRU publication. The cosmic-ray fluxes inside and near to spacecrafts were also calculated by PHITS, using simplified geometries. The accuracy of the obtained absorbed-dose and dose-equivalent rates was verified by various experimental data measured both inside and outside spacecrafts. The calculations quantitatively show that the effective doses for astronauts are significantly greater than their corresponding effective dose equivalents, because of the numerical incompatibility between the radiation quality factors and the radiation weighting factors. These results demonstrate the usefulness of dose conversion coefficients in space dosimetry.     

Chromosome aberration measurements in mitotic and G<Subscript>2</Subscript>-PCC lymphocytes at the standard sampling time of 48 h underestimate the effectiveness of high-LET particles

The relationship between heavy-ion-induced cell cycle delay and the time-course of aberrations in first-cycle metaphases or prematurely condensed G₂-cells (G₂-PCC) was investigated. Lymphocytes of the same donor were irradiated with X-rays or various charged particles (carbon, iron, xenon, and chromium) covering an LET range of 2–3,160 keV/μm. Chromosome aberrations were measured in samples collected at 48, 60, 72, and 84 h postirradiation. Linear-quadratic functions were fitted to the data, and the fit parameters α and β were determined. At any sampling time, α values derived from G₂-cells were higher than those from metaphases. The α value derived from metaphase analysis at 48 h increased with LET, reached a maximum around 155 keV/μm, and decreased with a further rise in LET. At the later time-points, higher α values were estimated for particles with LET > 30 keV/μm. Estimates of α values from G₂-cells showed a similar LET dependence, yet the time-dependent increase was less pronounced. Altogether, our data demonstrate that heavily damaged lymphocytes suffer a prolonged G₂-arrest that is clearly LET dependent. For this very reason, the standard analysis of aberrations in metaphase cells 48 h postirradiation will considerably underestimate the effectiveness of high-LET radiation. Scoring of aberrations in G₂-PCC at 48 h as suggested by several authors will result in higher aberration yields. However, when particles with a very high-LET value (LET > 150 keV/μm) are applied, still a fraction of multiple damaged cells escape detection by G₂-analysis 48 h postirradiation.     

Changes in the human nuclear chromatin induced by ultra wideband pulse irradiation

The effects of ultra wideband pulse radiation on human cells were investigated. The density of the flow of energy on the surface of irradiated object varied from 10⁻⁶ to 10⁻² W/cm² with exposure of 10 s. It was shown that heterochromatin granule quantity in cell nuclei increased under the influence of radiation from 10⁻⁴ to 10⁻² W/cm². In some intervals the effect increased with irradiation dose. At irradiation intensity 10⁻³ W/cm² the process of heterochromatin granule formation was fully reversible after 2 h of recovery; at intensity 10⁻² W/cm² the reversion of irradiation effects was not full. The data obtained indicated the strong biological activity of ultra wideband ultra short pulse radiation.