Morphometric studies of heavy ion damage in the brains of rodents.

As an approach to determining the relative biological effectiveness (RBE) of each of five different heavy ions for the mammalian brain, histological preparations of brains from mice exposed to various HZE particles at different doses and primary LETinfinity values were examined by means of semi-automated image analysis for volume changes in specific regions of the olfactory bulb. The mice were irradiated at 100 days of age and euthanatized about 500 days (16 months) later. Exposures were: 60Co gamma photons (LETinfinity = 1-2 keV/micrometer), 4He (LETinfinity = 6 keV/micrometer), 12C (LETinfinity = 80 keV/micrometer), 20Ne (LETinfinity = 150 keV/micrometer), 56Fe (LETinfinity = 180 keV/micrometer), and 40Ar (LETinfinity = 650 keV/micrometer). Animals receiving particle radiation were exposed in an extended Bragg peak region except for iron where the plateau region was used. The zones measured in the olfactory bulb were 1) the external plexiform layer (zone) and 2) an internal region consisting of the granule cells, internal plexiform layer, and layer of mitral cells. These studies indicated that volume changes did indeed occur, not only in absolute terms but also when expressed as the ratio of the structures to each other and to the bulb as a whole. Although this study is exploratory in character, the data obtained may nevertheless contribute to a determination of risk factors due to late effects from HZE articles.     

Cellular and subcellular effect of heavy ions: a comparison of the induction of strand breaks and chromosomal aberration with the incidence of inactivation and mutation.

Radiobiological effects of heavy charged particles are compared for a large variety of ions from Helium to Uranium and energies between 1 and 1000 MeV/u which correspond to LET values between 10 and 16000 keV/micrometers. The different cross section for the induction of strand breaks and chromosomal aberrations as well as for inactivation and mutation induction exhibit striking similarities when compared as function of the linear energy transfer (LET). At LET values below 100 keV/micrometers all data points of one specific effect form one single curve as a function of LET, independent of the atomic number of the ion. In this LET range, the biological effects are independ from the particle energy or track structure and depend only on the energy transfer. Therefore, LET is a good parameter in this regime. For LET values greater than 100 keV/micrometers, the curves for the different ions separate from the common curve in order of increasing atomic numbers. In this regime LET is no longer a good parameter and the physical parameters of the formation of particle tracks are important. The similarity of the sigma-LET curves for different endpoints indicates that the 'hook-structure' is produced by physical and chemical effects which occur before the biologically relevant lesions are formed. However, from the existing data of biological effects, it can be concluded that the efficiencies for cell killing are always smaller than those extrapolated from X-ray data on the basis of the energy deposition only. Therefore, cells which are directly hit by an HZE particle are not killed and undergo a finite risk of mutation and transformation.     

Effects of prolonged exposure of lettuce seeds to HZE particles on orbital stations.

In a study of the biological effects of cosmic HZE particles, lettuce (Lactuca sativa) seeds were flown on the orbital stations Salyut 6 and 7 for varying periods of time (from 40 to 457 days). The dependence of the biological damage on flight duration, physical parameters and the fact of passage of an HZE particle through the seed was estimated using the criterion of the frequency of aberrant cells. The arrangement of the flight biological container Biobloc made it possible to trace the location of tracks of individual HZE particles with Z > or = 6 and LET 200 keV/um. In seeds hit by HZE particles, for all exposure times, a statistically significant much higher yield of aberrant cells and also of cells containing multiple chromosome aberrations was observed than in the control material. The frequency of aberrant cells is markedly higher (by a factor of 1,5) in seeds hit than in non-hit ones. The changes of the yield of aberrant cells as a function of the absorbed dose (3.2-63.4 mGy) and the fluence (4.8-44.2 particles/cm2) are linear for the exposure duration ranging from 40 to 457 days.     

Heavy ion induced mutations in genetic effective cells of a higher plant.

Arabidopsis thaliana offers different possibilities for investigating heavy ion induced early and late damage. Mutations in genetic effective cells can yield early damage, in the form of reduced vitality of the descending cell-lines and/or late damage, such as mutation induction visible in the following generations. Investigation is possible on different levels of ploidy (4n, 2n, n). Different genetic effective cells with equal genomes are available. Additionally, several different biological endpoints for each level of genome ploidy can be observed. Recent results of work in this field are presented.     

Results on Artemia cysts, lettuce and tobacco seeds in the Biobloc 4 experiment flown aboard the Soviet Biosatellite Cosmos 1129.

Artemia cysts, lettuce and tobacco seeds were flown aboard the Cosmos 1129 for 19 days. A correlative method was used in order to determine the passage of cosmic heavy ions (HZE particles) through the biological test objects. This space flight resulted in a decrease on hatchability, nucleic acid and protein synthesis in hydrated Artemia cysts. HZE particle effects on plant cellular chromosomes are confirmed. In tobacco seeds, a stimulating effect on germination rate and a higher frequency of abnormalities were observed. Dormant biological objects are a very suitable material to study cosmic ray effects: these objects can be arranged in monolayers and sandwiched between visual track detectors in order to determine the passage of the cosmic heavy ions (HZE particles). On the other hand this method allows us to study effects of microgravity and those of the protonic component of cosmic rays in the objects not hit by the HZE articles.     

Countermeasures for space radiation induced adverse biologic effects

Radiation exposure in space is expected to increase the risk of cancer and other adverse biological effects in astronauts. The types of space radiation of particular concern for astronaut health are protons and heavy ions known as high atomic number and high energy (HZE) particles. Recent studies have indicated that carcinogenesis induced by protons and HZE particles may be modifiable. We have been evaluating the effects of proton and HZE particle radiation in cultured human cells and animals for nearly a decade. Our results indicate that exposure to proton and HZE particle radiation increases oxidative stress, cytotoxicity, cataract development and malignant transformation in in vivo and/or in vitro experimental systems. We have also shown that these adverse biological effects can be prevented, at least partially, by treatment with antioxidants and some dietary supplements that are readily available and have favorable safety profiles. Some of the antioxidants and dietary supplements are effective in preventing radiation induced malignant transformation in vitro even when applied several days after the radiation exposure. Our recent progress is reviewed and discussed in the context of the relevant literature.     

Simulations of an accelerator-based shielding experiment using the particle and heavy-ion transport code system PHITS.

In order to estimate the biological effects of HZE particles, an accurate knowledge of the physics of interaction of HZE particles is necessary. Since the heavy ion transport problem is a complex one, there is a need for both experimental and theoretical studies to develop accurate transport models. RIST and JAERI (Japan), GSI (Germany) and Chalmers (Sweden) are therefore currently developing and bench marking the General-Purpose Particle and Heavy-Ion Transport code System (PHITS), which is based on the NMTC and MCNP for nucleon/meson and neutron transport respectively, and the JAM hadron cascade model. PHITS uses JAERI Quantum Molecular Dynamics (JQMD) and the Generalized Evaporation Model (GEM) for calculations of fission and evaporation processes, a model developed at NASA Langley for calculation of total reaction cross sections, and the SPAR model for stopping power calculations. The future development of PHITS includes better parameterization in the JQMD model used for the nucleus-nucleus reactions, and improvement of the models used for calculating total reaction cross sections, and addition of routines for calculating elastic scattering of heavy ions, and inclusion of radioactivity and burn up processes. As a part of an extensive bench marking of PHITS, we have compared energy spectra of secondary neutrons created by reactions of HZE particles with different targets, with thicknesses ranging from <1 to 200 cm. We have also compared simulated and measured spatial, fluence and depth-dose distributions from different high energy heavy ion reactions. In this paper, we report simulations of an accelerator-based shielding experiment, in which a beam of 1 GeV/n Fe-ions has passed through thin slabs of polyethylene, Al, and Pb at an acceptance angle up to 4 degrees.     

The effect of space radiation of the nervous system.

The long-term effects of irradiation by accelerated heavy ions on the structure and function of the nervous system have not been studied extensively. Although the adult brain is relatively resistant to low LET radiation, cellular studies indicate that individual heavy ions can produce serious membrane lesions and multiple chromatin breaks. Capillary hemorrhages may follow high LET particle irradiation of the developing brain as high RBE effects. Evidence has been accumulating that the glial system and blood-brain barrier (BBB) are relatively sensitive to injury by ionizing radiation. While DNA repair is active in neural systems, it may be assumed that a significant portion of this molecular process is misrepair. Since the expression of cell lethality usually requires cell division, and nerve cells have an extremely low rate of division, it is possible that some of the characteristic changes of premature aging may represent a delayed effect of chromatin misrepair in brain. Altered microcirculation, decreased local metabolism, entanglement and reduction in synaptic density, premature loss of neurons, myelin degeneration, and glial proliferation are late signs of such injuries. HZE particles are very efficient in producing carcinogenic cell transformation, reaching a peak for iron particles. The promotion of viral transformation is also efficient up to an energy transfer of approximately 300 keV/micron. The RBE for carcinogenesis in nerve tissues remains unknown. On the basis of available information concerning HZE particle flux in interplanetary space, only general estimates of the magnitude of the effects of long-term spaceflight on some nervous system parameters may be constructed.     

Microlesions: theory and reality.

Efforts to assess radiation risk in space have been complicated by the considerable unknowns regarding the biological effects of the heavy ion component (HZE particles) of the cosmic rays. The attention has focused primarily on the assignation of a quality factor (Q) which would take into account the greater effectiveness of heavy ions vis-a-vis other forms of ionizing radiation. If however, as the so-called "Microlesion Theory" allows, the passage of HZE particles through living tissue produces unique biological damage, the traditional use of Q becomes meaningless. Therefore, it is critical to determine if microlesions, in fact, do exist. While the concept does not necessarily require detectable morphological damage, "tunnel-lesions" or holes in ocular tissues have been cited as evidence of microlesions. These data, however, are open to reinterpretation. On-going light, scanning and transmission electron microscopic studies of the corneas, lenses and retinas of rat eyes exposed to 450 MeV/amu 56Fe ions thus far have not revealed tunnel-lesion damage. The morphological effects of the heavy ions have been found to be qualitatively similar to the changes following other kinds of ionizing radiation.     

In vitro neurotoxic effects of 1 GeV/n iron particles assessed in retinal explants.

The heavy ion component of the cosmic radiation remains problematic to the assessment of risk in manned space flight. The biological effectiveness of HZE particles has yet to be established, particularly with regard to nervous tissue. Using heavy ions accelerated at the AGS of Brookhaven National Laboratory, we study the neurotoxic effects of iron particles. We exposed retinal explants, taken from chick embryos, to determine the dose response relationships for neurite outgrowth. Morphometric techniques were used to evaluate the in vitro effects of 1 GeV/a iron particles (LET 148 keV/micrometer). Iron particles produced a dose-dependent reduction of neurite outgrowth with a maximal effect achieved with a dose of 100 cGy. Doses as low as 10-50 cGy were able to induce reductions of the neurite outgrowth as compared to the control group. Neurite generation is a more sensitive parameter than neurite elongation, suggesting different mechanism of radiation damage in our model. These results showed that low doses/fluences of iron particles could impair the retinal ganglion cells' capacity to generate neurites indicating the highly neurotoxic capability of this heavy charged particle.     

Estimates of HZE particle contributions to SPE radiation exposures on interplanetary missions.

Estimates of radiation doses resulting from possible HZE (high energy heavy ion) components of solar particle events (SPEs) are presented for crews of manned interplanetary missions. The calculations assume a model spectrum obtained by folding measured solar flare HZE particle abundances with the measured energy spectra of SPE alpha particles. These hypothetical spectra are then transported through aluminum spacecraft shielding. The results, presented as estimates of absorbed dose and dose equivalent, indicate that HZE components by themselves are not a major concern for crew protection but should be included in any overall risk assessment. The predictions are found to be sensitive to the assumed spectral hardness parameters.     

Mice heterozygous for the ATM gene are more sensitive to both X-ray and heavy ion exposure than are wildtypes

Previous studies have shown that the eyes of ATM heterozygous mice exposed to low-LET radiation (X-rays) are significantly more susceptible to the development of cataracts than are those of wildtype mice. The findings, as well as others, run counter to the assumption underpinning current radiation safety guidelines, that individuals are all equally sensitive to the biological effects of radiation. A question, highly relevant to human space activities is whether or not, in similar fashion there may exist a genetic predisposition to high-LET radiation damage.Mice haplodeficient for the ATM gene and wildtypes were exposed to 325 mGy of 1 GeV/amu ⁵⁶Fe ions at the AGS facility of Brookhaven National Laboratory. The fluence was equivalent to 1 ion per lens epithelial cell nuclear area. Controls consisted of irradiated wildtype as well as unirradiated wildtype and heterozygous mice. Prevalence analyses for stage 0.5–3.0 cataracts indicated that not only cataract onset but also progression were accelerated in the mice haplo-deficient for the ATM gene.The data show that heterozygosity for the ATM gene predisposes the eye to the cataractogenic influence of heavy ions and suggest that ATM heterozygotes in the human population may also be radiosensitive. This may have to be considered in the selection of individuals who will be exposed to both HZE particles and low-LET radiation as they may be predisposed to increased late normal tissue damage.     

The effects of proton exposure on neurochemistry and behavior.

Future space missions will involve long-term travel beyond the magnetic field of the Earth, where astronauts will be exposed to radiation hazards such as those that arise from galactic cosmic rays. Galactic cosmic rays are composed of protons, alpha particles, and particles of high energy and charge (HZE particles). Research by our group has shown that exposure to HZE particles, primarily 600 MeV/n and 1 GeV/n 56Fe, can produce significant alterations in brain neurochemistry and behavior. However, given that protons can make up a significant portion of the radiation spectrum, it is important to study their effects on neural functioning and on related performance. Therefore, these studies examined the effects of exposure to proton irradiation on neurochemical and behavioral endpoints, including dopaminergic functioning, amphetamine-induced conditioned taste aversion learning, and spatial learning and memory as measured by the Morris water maze. Male Sprague-Dawley rats received a dose of 0, 1.5, 3.0 or 4.0 Gy of 250 MeV protons at Loma Linda University and were tested in the different behavioral tests at various times following exposure. Results showed that there was no effect of proton irradiation at any dose on any of the endpoints measured. Therefore, there is a contrast between the insignificant effects of high dose proton exposure and the dramatic effectiveness of low dose (<0.1 Gy) exposures to 56Fe particles on both neurochemical and behavioral endpoints.     

Theoretical model of HZE particle fragmentation by hydrogen targets.

The fragmenting of high energy, heavy ions (HZE particles) by hydrogen targets is an important, physical process in several areas of space radiation research. In this work quantum mechanical optical model methods for estimating cross sections for HZE particle fragmentation by hydrogen targets are presented. The cross sections are calculated using a modified abrasion-ablation collision formalism adapted from a nucleus-nucleus collision model. Elemental and isotopic production cross sections are estimated and compared with report measurements for the breakup of neon, sulphur, and iron, nuclei at incident energies between 400 and 910 MeV/nucleon. Good agreement between theory and experiment is obtained.     

Microscopic track structure of equal-LET heavy ions.

The spatial distributions of ionization and energy deposition produced by high-velocity heavy ions are crucial to an understanding of their radiation quality as exhibited eg., in track segment experiments of cell survival and chromosome aberrations of mammalian cells. The stopping power (or LET) of a high velocity ion is proportional to the ratio z2/v2, apart from a slowly varying logarithmic factor. The maximum delta-ray energy that an ion can produce is proportional to v2 (non-relativistically). Therefore, two HZE ions having the same LET, but in general differing z and v will have different maximum delta-ray energies and consequently will produce different spatial patterns of energy deposition along their paths. To begin to explore the implications of this fact for the microscopic dosimetry of heavy ions, we have calculated radial distributions in energy imparted and ionization for iron and neon ions of approximately equal LET in order to make a direct comparison of their delta-ray track structure. Monte Carlo techniques are used for the charged particle radiation transport simulation.     

A model of the effects of heavy ion radiation on human tissue

In heavy ion radiotherapy and space travel humans are exposed to energetic heavy ions (C, Si, Fe and others). This type of irradiation often produces more severe biological effects per unit dose than more common X-rays. A new Monte Carlo model generates a physical space with the complex geometry of human tissue or a cell culture based model of tissue, which is affected by the passage of ionizing radiation. For irradiation, the model relies on a physical code for the ion track structure; for tissues, cellular maps are derived from two- or three-dimensional confocal microscopy images using image segmentation algorithm, which defines cells as pixilated volumes. The model is used to study tissue-specific statistics of direct ion hits and the remote ion action on cells. As an application of the technique, we considered the spatial pattern of apoptotic cells after heavy ion irradiation. The pattern of apoptosis is modeled as a stochastic process, which is defined by the action cross section taken from available experimental data. To characterize the degree of apoptosis, an autocorrelation function that describes the spatial correlation of apoptotic cells is introduced. The values of the autocorrelation function demonstrate the effect of the directionality of the radiation track on the spatial arrangements of inactivated cells in tissue. This effect is intrinsic only to high linear-energy-transfer radiation.     

Track structure in biological models.

High-energy heavy ions in the galactic cosmic radiation (HZE particles) may pose a special risk during long term manned space flights outside the sheltering confines of the earth's geomagnetic field. These particles are highly ionizing, and they and their nuclear secondaries can penetrate many centimeters of body tissue. The three dimensional patterns of ionizations they create as they lose energy are referred to as their track structure. Several models of biological action on mammalian cells attempt to treat track structure or related quantities in their formulation. The methods by which they do this are reviewed. The proximity function is introduced in connection with the theory of Dual Radiation Action (DRA). The ion-gamma kill (IGK) model introduces the radial energy-density distribution, which is a smooth function characterizing both the magnitude and extension of a charged particle track. The lethal, potentially lethal (LPL) model introduces lambda, the mean distance between relevant ion clusters or biochemical species along the track. Since very localized energy depositions (within approximately 10 nm) are emphasized, the proximity function as defined in the DRA model is not of utility in characterizing track structure in the LPL formulation.     

The microlesion concept in HZE particle dosimetry.

High energy, high-Z (HZE) particles are present in high-altitude and high-inclination satellite orbits. Most of the HZE dose above LET = 200 keV/micrometer is due to Fe nuclei. Individual HZE particles can damage several cells adjacent to one another along the particle track in tissue. The outcome has been described as a "microlesion" by D. Grahn. The present study attempts to define conditions for microlesions in specific tissues, to seek biological evidence that microlesions are produced, and to evaluate the microlesion as a potentially useful unit of dose in assessing hazards to spaceworkers. Microlesions in individuals cells and hair follicles have been described. Microbial studies have provided some evidence for independent secondary electron action. Whether or not a few hundred microlesions would be damaging to the whole organism depends upon the nature of damage to critical tissues. For example, cancer may occur if microlesions kill several cells in a straight line and mutate other cells alongside the particle track. Fe particle irradiation of the mouse Harderian gland (Fry et al., this issue) produces tumors efficiently. Microlesions in the lens, cornea, and retina need to be considered. Further dialogue is required before a final decision can be made concerning the most appropriate way to assess the HZE hazard.     

Cosmic ionizing radiation effects in plant seeds after short and long duration exposure flights.

Recently, comparison of biophysical data obtained from orbital flights of short and long duration led to results which will be significant for long and/or repeated stay of man in space. Under orbital conditions biological stress is induced in dry seeds of Arabidopsis thaliana by cosmic radiation especially its high energetic, densely ionizing component, the heavy ions (HZE). For comparison of radiation impact during different space flights a biological attempt at estimating the impact of single particles with high mass and energy (HZE-particles) on seeds was developed. Subdivision into LET-groups showed a remarkable contribution of an intermediate group (LET = 35 to 100 keV/micrometer) due to medium heavy ions (Z = 6 to 10). Efficiency factors for radiation damage experimentally determined and assigned to different LET-classes were compared to radiation quality factors discussed in literature.