A Strategy for Research in Space Biology and Medicine in the New Century (1998) / Chapter Skim
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11 Radiation Hazards
11 Radiation Hazards
Pages 171-193
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From page 171... ...
report, Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies, 2 on current knowledge of the types and levels of radiation to which crews will be exposed in space, and also discusses the range of possible human health effects that must be protected against. It suggests steps to be taken and the types of experiments needed to reduce significantly the level of uncertainty regarding health risks to human crews in space, and it recommends priorities for research from which NASA can obtain the information required to evaluate the biological risks faced by humans exposed to radiation in space and to mitigate such risks.
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From page 172... ...
Determination of the frequencies of chromosomal aberrations in lymphocytes irradiated in vitro versus gamma ray dose has been used to estimate the dose equivalent for chromosomal aberrations in the lymphocytes of two astronauts on the Mir-18 115-day spaceflight.7 The differences in postflight minus preflight frequencies are calibrated to give the dose equivalent from the results of acute exposures of preflight lymphocyte samples to gamma rays. The average dose equivalent was 0.15 Sv.
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From page 173... ...
This dose reduction, in going from acute to chronic exposure, also depends on the biological system and may range from a factor of 2 to 10.~2 The dose rate reduction factor for HZE particles is not well known but is probably closer to 1.~3 Two other factors that must be considered, but whose impacts are currently unknown, are the effects of biochemical or cellular repair reactions following exposure to HZE particles and the effects of microgravity on such reactions. Thus, in estimating the risks to humans exposed to radiation in space, the uncertain factors are the radiation fields behind the shielding and the extrapolalion, via cell culture and animal experiments, from the uncertain risks posed by acute low-LET exposure to risks posed by chronic high-LET exposure.
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From page 174... ...
It is important to note that radiation weighting factors or quality factors apply only to stochastic effects in the dose range pertinent to radiation protection. Based on studies of atomic bomb survivors of Hiroshima who were exposed to acute levels mainly of gamma rays but also of fission neutrons at very high dose rates, estimates for the risk of contracting leukemia have been refine, and there are also data on mortality and the incidence of solid cancers at more than 20 sites in the human body.~7 ~8 (The precise contribution of the neutrons from the fission reactions to the total dose at Hiroshima is poorly known but is not considered a major contributor to the risk of cancer in those exposed at Hiroshima.)
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From page 175... ...
The maximum RBE values for effects involving cell killing, such as shown in Figure 1 1.1, are in the range of 2.0 to 3.0.3~ However, for cataract induction in rats and mice, the RBE values obtained by Merriam are in the range of 40 to 50 at low doses.32 Although the work of Lett and co-workers and of Worgul and co-workers suggests that it may be possible, with further data, to extrapolate across species to obtain RBE values for cataract induction, current data do not allow reliable estimation of the risk of cataract induction occurring in humans as a result of exposure to radiation in deep space.33 34 Observations on radiotherapy patients indicate that very high doses of low-LET radiation give rise to deterministic-type damage. HZE particles produce high-dose ionization tracks and kill the cells they traverse.
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It is agreed that the RBE for carcinogenesis increases with the increasing LET of the radiation. The evidence comes largely from animal experiments with fission neutrons but also from data on induction of lung tumors in humans exposed to alpha particles from radon.35 There has been only one systematic study of the relationship between the LET of heavy ions and the RBE values of the ions for tumor induction, which was carried out on the Harderian gland of mice.36 Although this gland is a suitable epithelial system, it is the only tumor model that has been examined over the range of LET values encountered in space.
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Strategy This key recommendation requires that two related questions be addressed: (1) Can the risk due to irradiation by protons in the energy range of the space environment be predicted on the basis of the risk posed by exposure to low-LET radiation, such as gamma rays, and is there evidence for the repair of Although the recommendations of the Task Group on the Biological Effects of Space Radiation were published independently in the 1996 report, it was the intent of CSBM that the task group's report would also constitute the basis of the material on radiation hazards presented in this CSBM strategy for research in space biology and medicine into the new century.
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Chromosomal aberrations could also be studied in lymphocytes from animals irradiated in viva. By conducting such studies with both acute and fractionated exposure regimens, it would be possible to determine whether fractionation effects (sparing of radiation response by allowing for DNA repair between fractions)
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It is recommended that bone marrow cells and peripheral lymphocytes, which are easily analyzed cytogenetically, be examined for chromosomal aberrations. Based on the information obtained in these cytogenetic studies, it would then be feasible to design a study to assess the induction of leukemia and breast cancer in mice exposed, behind shielding, to acute doses of HZE-particle radiation incident on the shielding (if cellular studies indicate this is necessary)
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From page 180... ...
Therefore, a secondary measure of relative sensitivity pertinent to cancer risk assessment would be a comparison of the features of DNA repair in human and rodent cells in vitro following acute exposure to protons and HZE particles. Techniques based on pulsed-field gel electrophoresis have been developed that can measure DNA strand breaks at very low exposure levels (<10 cGy)
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From page 181... ...
6. Determine how the selection and design of the space vehicle affect the radiation environment in The capability for determining effects of space vehicle selection and design is based in part on having accurate knowledge of the incident radiation field, the reaction cross sections for the incident particles reacting with vehicle materials, and the fragmentation or recoil products that such reactions produce.
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Strategy The ability to predict the time of occurrence and/or the magnitude of solar particle events is currently an inexact science at best. Protecting a mission crew from SPE radiation requires improving the capability to accurately predict solar events.
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A better understanding is needed of the effects of protracted exposure at low dose rates for both low- and high-LET radiation, because the data currently available for humans are for high-dose-rate, low-LET radiation. Since research efforts on atomic bomb survivors (who were exposed to low-LET radiation)
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, it is estimated that such studies would require approximately 4 years to complete, assuming that cells for these studies could be "piggybacked" with those of other cellular studies. Under more ideal conditions, with 3 months of beam time available each year, these studies would require approximately 2 years to complete.
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From page 185... ...
To validate the assay, it is also of considerable importance that it be conducted in several different laboratories and that an extensive sample from the general population be assessed to obtain an estimate of the range of sensitivities. Since the assay can be validated with low-LET x rays or gamma rays, no beam time is required.
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From page 186... ...
NEED FOR ANIMAL USE There are no estimates for the risk of cancer induction in humans exposed to protons, the major component of galactic cosmic radiation and solar radiation, or to heavy ions such as iron. Therefore, risk estimates currently must be based on either (1)
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Limited data are available on cancer induction in rodents exposed to high-LET radiation; information on other biological effects is also sparse. It will be necessary to conduct additional cancer studies in rodents exposed to different types of high-LET radiation and to characterize the resulting tumors at the molecular level.
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189 so Cat au o o o c a' .~ Cat a' o C)
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Recent ground-based experiments, summarized by Kronenberg, on radiation-induced DNA fragmentation, neoplastic transformation of cells plated 24 hours after irradiation, and the effects of a chemical radioprotector on mutation induction showed that DNA repair and cell recovery take place readily after low-LET irradiation, but not after exposure to HZE particles.62 Since the only reported significant effect of m~crogravity may be on DNA repair and cell recovery following low-LET exposure, and there seems to be no DNA repair or cell recovery following high-LET exposure, m~crogravity should not be important for HZE particle effects. The above considerations indicate that HZE particles are a very important factor in the damage resulting from long space missions and that the effects of m~crogravity probably will not alter the cellular response to HZE particles but might actually increase the effect of low-LET radiation.
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1997. Uncertainties in the Fatal Cancer Risk Estimates Used in Radiation Protection.
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1987. Effects of heavy ions on rabbit tissues: Induction of DNA strand breaks in retinal photoreceptor cells by high doses of radiation.
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62. Kronenberg, A., NASA space radiation health program: Ground-based radiobiology research program, presentation to the Task Group on the Biological Effects of Space Radiation, Washington, D.C.
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