Biological effects of high altitude as related to increased solar radiation, temperature fluctuations and reduced partial pressure of oxygen.
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The relative biological effectiveness of densely ionizing heavy-ion radiation for inducing ocular cataracts in wild type versus mice heterozygous for the ATM gene.
The accelerated appearance of ocular cataracts at younger ages has been recorded in both astronauts and airline pilots, and is usually attributed to high-energy heavy ions in galactic cosmic ray radiation. We have previously shown that high-LET 1-GeV/nucleon (56)Fe ions are significantly more effective than X-rays in producing cataracts in mice. We have also shown that mice haploinsufficient for ATM develop cataracts earlier than wild-type animals, when exposed to either low-LET X-rays or high-LET (56)Fe ions. In this paper we derive quantitative estimates for the relative biological effectiveness (RBE) of high energy (56)Fe ions compared with X-rays, both for wild type and for mice haploinsufficient for ATM. There is a clear trend toward higher RBE's in haploinsufficient animals, both for low- and high-grade cataracts. Haploinsufficiency for ATM results in an enhanced sensitivity to X-rays compared with the wild type, and this enhancement appears even larger after exposure to high-LET heavy ions.
Animals↗
Radiation safety standards: space hazards vs. terrestrial hazards.
The standards currently recommended for use in space travel were perhaps the first risk derived recommendations for dose limitations developed for quasi-occupational circumstances. They were based on data, considerations, and philosophy existing prior to 1970 and considered carcinogenesis primarily. In the intervening twelve years, not only has radiation risk information improved markedly but considerations relating to risk in general have become better known. The earlier recommendations have been examined with respect to changes in risk estimation and it is noted that the same philosophy used today, would probably lead to different dose limitations. However, other philosophies might be used; in particular a comparison of risks between terrestrial occupational radiation circumstances and also with fatal accident rates in a range of industries can be made and might be used in a modified philosophy with respect to risks from carcinogenesis. Developments have also taken place with respect to the knowledge of the biological effects of HZE particles but whether these effects are limiting as compared with radiation induced carcinogenesis is not yet clear. More studies on the effects of HZE particles, now becoming available, are needed. It is recommended that an in depth reexamination be undertaken of the biological effectiveness of space radiations and the philosophy of dose limitations in comparison with other risks.
Accidents, Occupational↗
Effects of heavy ions on cycling stem cells.
Murine marrow stem cells assayed with the spleen colony assay have been shown to be largely in a noncycling state, Go. In the unirradiated animal where these spleen-colony forming units (CFUs) transit normally between a non-proliferative state and active proliferation, exposure to a sufficient dose of ionizing radiation increases the frequency (probability) of this transition. For low-LET irradiation, marrow stem cells are not induced into cycle until a threshold dose is achieved. This dose appears to be in the range 50 to 100 cGy, inducing proliferation in an all-or-nothing manner. For irradiation with heavy charged-particles, however, the threshold dose is dependent on mass and energy. Irradiation with particles of sufficient mass and energy stimulates active proliferation even at the smallest doses tested, 5 cGy. Further, this response does not appear to result from an all-or-nothing effect. Rather, individual animals with intermediate levels of stem cell cycling have been observed. These data support the notion that locally controlled hemopoiesis can be affected by local deposition of radiation damage.
Animals↗
Radiation quality and risk estimation in relation to space missions.
While Q is specified as a function of linear energy transfer (LET) in practice the Q for neutrons has been selected by a judgment decision based on the relative biological effectiveness (RBE) to induce stochastic effects. There are no RBE values for tumor induction by heavy ions or protons in humans. Thus, selection of Q values has been based either on LET (or lineal energy) or RBEs from animal experiments. Estimates of Q for heavy ions in low earth orbit (LEO) range from about 5 to 14. The average Q value of all radiation in LEO has been estimated to be about 1.3. There is a lack of experimental data for RBEs for heavy ions but RBE increases as a function of LET. In the case of the Harderian gland the RBE reaches a maximum of 25-30 between about 100-200 keV/micrometer but does not appear to decrease at higher LETs. The International Commission of Radiological Protection have proposed the use of radiation weighting factors in lieu of quality factors. The weighting factors will range from 1 to 20.
Animals↗
Development of human epithelial cell systems for radiation risk assessment.
The most important health effect of space radiation for astronauts is cancer induction. For radiation risk assessment, an understanding of carcinogenic effect of heavy ions in human cells is most essential. In our laboratory, we have successfully developed a human mammary epithelial cell system for studying the neoplastic transformation in vitro. Growth variants were obtained from heavy ion irradiated immortal mammary cell line. These cloned growth variants can grow in regular tissue culture media and maintain anchorage dependent growth and density inhibition property. Upon further irradiation with high-LET radiation, transformed foci were found. Experimental results from these studies suggest that multiexposure of radiation is required to induce neoplastic transformation of human epithelial cells. This multihits requirement may be due to high genomic stability of human cells. These growth variants can be useful model systems for space flight experiments to determine the carcinogenic effect of space radiation in human epithelial cells.
Argon↗
The use of cell-oriented factors and the hit size effectiveness function in radiation protection.
It has long been argued that ionizing radiation can be considered to interact with matter in discrete, randomly occurring energy transferring events ("hits") and that the resulting microscopically nonuniform pattern of energy deposition strongly influences the biological effect of a given exposure. Microdosimetric measurements combined with cellular biological response data in the form of a "hit size effectiveness function" (HSEF) suggest a possible cell-oriented alternative method of correlating exposure with effect at low levels of any radiation or mixture of radiations. The instrumentation required, the validity of the approach, and its practical usefulness in radiation protection are examined, and its application to space radiation exposure is proposed as a test case.
Aerospace Medicine↗
Simulation of physical and biological aspects of GCR in ground-based experiments.
Many aspects of the interplanetary radiation environment can be simulated using high-energy charged-particle beams at ground based accelerator facilities. Some of the criteria and parameters used to define science requirements for such simulations are discussed. Some results from a ground-based radiation transport experiment are used to illustrate these considerations.
Acceleration↗
Cataractogenic potential of ionizing radiations in animal models that simulate man.
Aspects of experiments on radiation-induced lenticular opacification during the life spans of two animal models, the New Zealand white rabbit and the rhesus monkey, are compared and contrasted with published results from a life span study of another animal model, the beagle dog, and the most recent data from the ongoing study of the survivors from radiation exposure at Hiroshima and Nagasaki. An important connection among the three animal studies is that all the measurements of cataract indices were made by one of the authors (A.C.L.), so variation form personal subjectivity was reduced to a minimum. The primary objective of the rabbit experiments (radiations involved: 56Fe, 40Ar and 20Ne ions and 60Co gamma photons) is an evaluation of hazards to astronauts from galactic particulate radiations. An analogous evaluation of hazards from solar flares during space flight is being made with monkeys exposed to 32, 55, 138 and 400 MeV protons. Conclusions are drawn about the proper use of animal models to simulate radiation responses in man and the levels of radiation-induced lenticular opacification that pose risks to man in space.
Animals↗
Issues and problems for radiobiological research in space.
The uniqueness of the space radiation field creates specific problems in the evaluation of hazards to men and materials. Comprehensive measurements of all physical parameters are necessary but not sufficient. Particular attention has to be paid to variables like solar flares by applying fast-responding active dosimetry. The assessment of biological consequences poses even more problems. There are no human data for the kinds of particles seen in space and they will presumably never be available. The only reasonable approach is therefore to use the information obtained for other radiations and check their applicability for the space situation. This involves both the study of fundamental processes in ground experiments as well as their verification in space missions. Special emphasis has to be laid on the modification of radiation effects by flight-dynamic factors and microgravity. Radiation protection guidelines for space flights cannot simply be transformed from existent regulations designed for radiation workers on earth but have to be tailored to the specific situation in space.
Aerospace Medicine↗
Differential neutron energy spectra measured on spacecraft in low Earth orbit.
Two methods for measuring neutrons in the range from thermal energies to dozens of MeV were used. In the first method, alpha-particles emitted from the 6Li(n,alpha)T reaction are detected with the help of plastic nuclear track detectors, yielding results on thermal and resonance neutrons. Also, fission foils are used to detect fast neutrons. In the second method, fast neutrons are recorded by nuclear photographic emulsions (NPE). The results of measurements on board various satellites are presented. The neutron flux density does not appear to correlate clearly with orbital parameters. Up to 50% of neutrons are due to albedo neutrons from the atmosphere while the fluxes inside the satellites are 15-20% higher than those on the outside. Estimates show that the neutron contribution to the total equivalent radiation dose reaches 20-30%.
Cosmic Radiation↗
Nuclear model calculations and their role in space radiation research.
Proper assessments of spacecraft shielding requirements and concomitant estimates of risk to spacecraft crews from energetic space radiation requires accurate, quantitative methods of characterizing the compositional changes in these radiation fields as they pass through thick absorbers. These quantitative methods are also needed for characterizing accelerator beams used in space radiobiology studies. Because of the impracticality/impossibility of measuring these altered radiation fields inside critical internal body organs of biological test specimens and humans, computational methods rather than direct measurements must be used. Since composition changes in the fields arise from nuclear interaction processes (elastic, inelastic and breakup), knowledge of the appropriate cross sections and spectra must be available. Experiments alone cannot provide the necessary cross section and secondary particle (neutron and charged particle) spectral data because of the large number of nuclear species and wide range of energies involved in space radiation research. Hence, nuclear models are needed. In this paper current methods of predicting total and absorption cross sections and secondary particle (neutrons and ions) yields and spectra for space radiation protection analyses are reviewed. Model shortcomings are discussed and future needs presented.
Cosmic Radiation↗
Radiation measurements on the Mir Orbital Station.
Radiation measurements made onboard the MIR Orbital Station have spanned nearly a decade and covered two solar cycles, including one of the largest solar particle events, one of the largest magnetic storms, and a mean solar radio flux level reaching 250 x 10(4) Jansky that has been observed in the last 40 years. The cosmonaut absorbed dose rates varied from about 450 microGy day-1 during solar minimum to approximately half this value during the last solar maximum. There is a factor of about two in dose rate within a given module, and a similar variation from module to module. The average radiation quality factor during solar minimum, using the ICRP-26 definition, was about 2.4. The drift of the South Atlantic Anomaly was measured to be 6.0 +/- 0.5 degrees W, and 1.6 +/- 0.5 degrees N. These measurements are of direct applicability to the International Space Station. This paper represents a comprehensive review of Mir Space Station radiation data available from a variety of sources.
Astronauts↗
New measurements for hadrontherapy and space radiation: biology.
The dual goals of optimizing clinical efficacy of hadrontherapy and determining radiation risk estimates for space research have intersected to a common focus for investigation of the biological effects of charged particles. This paper briefly highlights recent international progress at accelerator facilities engaged in both biological and clinical studies of the effects of particle beams, primarily protons, carbon and iron ions. Basic mechanisms of molecular, cellular and tissue responses continue under investigation for radiations with a range of ionization densities. Late normal tissue effects, including the risk of cancer in particular, are of importance for both research fields. International cooperation has enhanced the rate of progress as evidenced by recent publications. Specific areas of biomedical research related to the biological radiotoxicity of critical organs (especially the central nervous system), individual radiosensitivities to radiation carcinogenesis, and the analysis of effects in mixed radiation fields still require more research. Recommendations for addressing these issues are made.
Animals↗
[Method of calculating self-shielding of the human critical organs in the anthropomorphic phantom].
Proposed is a procedure of calculating the self-screening function of a point on the human body model. The procedure has been developed for the anthropomorphic phantom approved in Russian standard GOST 25645.203-83. The procedure allows extrapolation of the self-shielding calculations to a different body model and a partially protected human body (intra- and extravehicular spacesuit, etc). Results can be useful in precise determination of radiation risks with consideration of the organs self-shielding factor.
Algorithms↗
Near-Earth radiation environment including time variations and secondary radiation. Proceedings of the Meetings F2.6 and F2.7 of COSPAR Scientific Commission F which was held during the Thirtieth COSPAR Scientific Assembly, Hamburg, Germany, 11-21 July, 1994.
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Cosmic Radiation↗
Haemopoietic cell renewal in radiation fields.
Space flight activities are inevitably associated with a chronic exposure of astronauts to a complex mixture of ionising radiation. Although no acute radiation consequences are to be expected as a rule, the possibility of Solar Particle Events (SPE) associated with relatively high doses of radiation (1 or more Gray) cannot be excluded. It is the responsibility of physicians in charge of the health of astronauts to evaluate before, during and after space flight activities the functional status of haemopoietic cell renewal. Chronic low level exposure of dogs indicate that daily gamma-exposure doses below about 2 cGy are tolerated for several years as far as blood cell concentrations are concerned. However, the stem cell pool may be severely affected. The maintenance of sufficient blood cell counts is possible only through increased cell production to compensate for the radiation inflicted excess cell loss. This behaviour of haemopoietic cell renewal during chronic low level exposure can be simulated by bioengineering models of granulocytopoiesis. It is possible to define a "turbulence region" for cell loss rates, below which an prolonged adaptation to increased radiation fields can be expected to be tolerated. On the basis of these experimental results, it is recommended to develop new biological indicators to monitor haemopoietic cell renewal at the level of the stem cell pool using blood stem cells in addition to the determination of cytokine concentrations in the serum (and other novel approaches). To prepare for unexpected haemopoietic effects during prolonged space missions, research should be increased to modify the radiation sensitivity of haemopoietic stem cells (for instance by the application of certain regulatory molecules). In addition, a "blood stem cell bank" might be established for the autologous storage of stem cells and for use in space activities keeping them in a radiation protected container.
Aerospace Medicine↗
Characterisation of bubble detectors for aircrew and space radiation exposure.
The Earth's atmosphere acts as a natural radiation shield which protects terrestrial dwellers from the radiation environment encountered in space. In general, the intensity of this radiation field increases with distance from the ground owing to a decrease in the amount of atmospheric shielding. Neutrons form an important component of the radiation field to which the aircrew and spacecrew are exposed. In light of this, the neutron-sensitive bubble detector may be ideal as a portable personal dosemeter at jet altitudes and in space. This paper describes the ground-based characterisation of the bubble detector and the application of the bubble detector for the measurement of aircrew and spacecrew radiation exposure.
Astronauts↗