The potential impact of bystander effects on radiation risks in a Mars mission

Mutational signatures in tumours induced by high and low energy radiation in Trp53 deficient mice

Ionising radiation (IR) is a recognised carcinogen responsible for cancer development in patients previously treated using radiotherapy, and in individuals exposed as a result of accidents at nuclear energy plants. However, the mutational signatures induced by distinct types and doses of radiation are unknown. Here, we analyse the genetic architecture of mammary tumours, lymphomas and sarcomas induced by high (56Fe-ions) or low (gamma) energy radiation in mice carrying Trp53 loss of function alleles. In mammary tumours, high-energy radiation is associated with induction of focal structural variants, leading to genomic instability and Met amplification. Gamma-radiation is linked to large-scale structural variants and a point mutation signature associated with oxidative stress. The genomic architecture of carcinomas, sarcomas and lymphomas arising in the same animals are significantly different. Our study illustrates the complex interactions between radiation quality, germline Trp53 deficiency and tissue/cell of origin in shaping the genomic landscape of IR-induced tumours.

Effects of spaceflight on cartilage: implications on spinal physiology

Spaceflight alters normal physiology of cells and tissues observed on Earth. The effects of spaceflight on the musculoskeletal system have been thoroughly studied, however, the effects on cartilage have not. This area is gaining more relevance as long duration missions, such as Mars, are planned. The impact on intervertebral discs and articular cartilage are of particular interest to astronauts and their physicians. This review surveys the literature and reports on the current body of knowledge regarding the effects of spaceflight on cartilage, and specifically changes to the spine and intervertebral disc integrity and physiology. A systematic literature review was conducted using PubMed, MEDLINE, and Google Scholar. Eighty-six unique papers were identified, 15 of which were included. The effect of spaceflight on cartilage is comprehensively presented due to limited research on the effect of microgravity on the spine/intervertebral discs. Cellular, animal, and human studies are discussed, focusing on human physiologic changes, cartilage histology, mineralization, biomechanics, chondrogenesis, and tissue engineering. Several common themes were found, such as decreased structural integrity of intervertebral disks and impaired osteogenesis/ossification. However, studies also presented conflicting results, rendering strong conclusions difficult. The paucity of human cartilage studies in spaceflight leaves extrapolation from other model systems the only current option for drawing conclusions despite known and unknown limitations in applicability to human physiology, especially spinal pathophysiology which is special interest. The aerospace and biomedical research communities would benefit from further human spaceflight articular cartilage and intervertebral disc studies. Further research may yield beneficial application for spaceflight, and crossover in understanding and treating terrestrial diseases like osteoarthritis and vertebral disc degeneration.

Effect of modeled microgravity on radiation-induced adaptive response of root growth in Arabidopsis thaliana

Space particles have an inevitable impact on organisms during space missions; radio-adaptive response (RAR) is a critical radiation effect due to both low-dose background and sudden high-dose radiation exposure during solar storms. Although it is relevant to consider RAR within the context of microgravity, another major space environmental factor, there is no existing evidence as to its effects on RAR. In the present study, we established an experimental method for detecting the effects of gamma-irradiation on the primary root growth of Arabidopsis thaliana, in which RAR of root growth was significantly induced by several dose combinations. Microgravity was simulated using a two-dimensional rotation clinostat. It was shown that RAR of root growth was significantly inhibited under the modeled microgravity condition, and was absent in pgm-1 plants that had impaired gravity sensing in root tips. These results suggest that RAR could be modulated in microgravity. Time course analysis showed that microgravity affected either the development of radio-resistance induced by priming irradiation, or the responses of plants to challenging irradiation. After treatment with the modeled microgravity, attenuation in priming irradiation-induced expressions of DNA repair genes (AtKu70 and AtRAD54), and reduced DNA repair efficiency in response to challenging irradiation were observed. In plant roots, the polar transportation of the phytohormone auxin is regulated by gravity, and treatment with an exogenous auxin (indole-3-acetic acid) prevented the induction of RAR of root growth, suggesting that auxin might play a regulatory role in the interaction between microgravity and RAR of root growth.

NEUDOSE: A CubeSat Mission for Dosimetry of Charged Particles and Neutrons in Low-Earth Orbit

During space missions, astronauts are exposed to a stream of energetic and highly ionizing radiation particles that can suppress immune system function, increase cancer risks and even induce acute radiation syndrome if the exposure is large enough. As human exploration goals shift from missions in low-Earth orbit (LEO) to long-duration interplanetary missions, radiation protection remains one of the key technological issues that must be resolved. In this work, we introduce the NEUtron DOSimetry & Exploration (NEUDOSE) CubeSat mission, which will provide new measurements of dose and space radiation quality factors to improve the accuracy of cancer risk projections for current and future space missions. The primary objective of the NEUDOSE CubeSat is to map the in situ lineal energy spectra produced by charged particles and neutrons in LEO where most of the preparatory activities for future interplanetary missions are currently taking place. To perform these measurements, the NEUDOSE CubeSat is equipped with the Charged & Neutral Particle Tissue Equivalent Proportional Counter (CNP-TEPC), an advanced radiation monitoring instrument that uses active coincidence techniques to separate the interactions of charged particles and neutrons in real time. The NEUDOSE CubeSat, currently under development at McMaster University, provides a modern approach to test the CNP-TEPC instrument directly in the unique environment of outer space while simultaneously collecting new georeferenced lineal energy spectra of the radiation environment in LEO.

Harderian Gland Tumorigenesis: Low-Dose and LET Response

Increased cancer risk remains a primary concern for travel into deep space and may preclude manned missions to Mars due to large uncertainties that currently exist in estimating cancer risk from the spectrum of radiations found in space with the very limited available human epidemiological radiation-induced cancer data. Existing data on human risk of cancer from X-ray and gamma-ray exposure must be scaled to the many types and fluences of radiations found in space using radiation quality factors and dose-rate modification factors, and assuming linearity of response since the shapes of the dose responses at low doses below 100 mSv are unknown. The goal of this work was to reduce uncertainties in the relative biological effect (RBE) and linear energy transfer (LET) relationship for space-relevant doses of charged-particle radiation-induced carcinogenesis. The historical data from the studies of Fry et al. and Alpen et al. for Harderian gland (HG) tumors in the female CB6F1 strain of mouse represent the most complete set of experimental observations, including dose dependence, available on a specific radiation-induced tumor in an experimental animal using heavy ion beams that are found in the cosmic radiation spectrum. However, these data lack complete information on low-dose responses below 0.1 Gy, and for chronic low-dose-rate exposures, and there are gaps in the LET region between 25 and 190 keV/μm. In this study, we used the historical HG tumorigenesis data as reference, and obtained HG tumor data for 260 MeV/u silicon (LET ∼70 keV/μm) and 1,000 MeV/u titanium (LET ∼100 keV/μm) to fill existing gaps of data in this LET range to improve our understanding of the dose-response curve at low doses, to test for deviations from linearity and to provide RBE estimates. Animals were also exposed to five daily fractions of 0.026 or 0.052 Gy of 1,000 MeV/u titanium ions to simulate chronic exposure, and HG tumorigenesis from this fractionated study were compared to the results from single 0.13 or 0.26 Gy acute titanium exposures. Theoretical modeling of the data show that a nontargeted effect model provides a better fit than the targeted effect model, providing important information at space-relevant doses of heavy ions.

Modulation of modeled microgravity on radiation-induced bystander effects in Arabidopsis thaliana

Both space radiation and microgravity have been demonstrated to have inevitable impact on living organisms during space flights and should be considered as important factors for estimating the potential health risk for astronauts. Therefore, the question whether radiation effects could be modulated by microgravity is an important aspect in such risk evaluation. Space particles at low dose and fluence rate, directly affect only a fraction of cells in the whole organism, which implement radiation-induced bystander effects (RIBE) in cellular response to space radiation exposure. The fact that all of the RIBE experiments are carried out in a normal gravity condition bring forward the need for evidence regarding the effect of microgravity on RIBE. In the present study, a two-dimensional rotation clinostat was adopted to demonstrate RIBE in microgravity conditions, in which the RIBE was assayed using an experimental system of root-localized irradiation of Arabidopsis thaliana (A. thaliana) plants. The results showed that the modeled microgravity inhibited significantly the RIBE-mediated up-regulation of expression of the AtRAD54 and AtRAD51 genes, generation of reactive oxygen species (ROS) and transcriptional activation of multicopy P35S:GUS, but made no difference to the induction of homologous recombination by RIBE, showing divergent responses of RIBE to the microgravity conditions. The time course of interaction between the modeled microgravity and RIBE was further investigated, and the results showed that the microgravity mainly modulated the processes of the generation or translocation of the bystander signal(s) in roots.

Health risks of space exploration: targeted and nontargeted oxidative injury by high-charge and high-energy particles

SIGNIFICANCE

During deep space travel, astronauts are often exposed to high atomic number (Z) and high-energy (E) (high charge and high energy [HZE]) particles. On interaction with cells, these particles cause severe oxidative injury and result in unique biological responses. When cell populations are exposed to low fluences of HZE particles, a significant fraction of the cells are not traversed by a primary radiation track, and yet, oxidative stress induced in the targeted cells may spread to nearby bystander cells. The long-term effects are more complex because the oxidative effects persist in progeny of the targeted and affected bystander cells, which promote genomic instability and may increase the risk of age-related cancer and degenerative diseases.

RECENT ADVANCES

Greater understanding of the spatial and temporal features of reactive oxygen species bursts along the tracks of HZE particles, and the availability of facilities that can simulate exposure to space radiations have supported the characterization of oxidative stress from targeted and nontargeted effects.

CRITICAL ISSUES

The significance of secondary radiations generated from the interaction of the primary HZE particles with biological material and the mitigating effects of antioxidants on various cellular injuries are central to understanding nontargeted effects and alleviating tissue injury.

FUTURE DIRECTIONS

Elucidation of the mechanisms underlying the cellular responses to HZE particles, particularly under reduced gravity and situations of exposure to additional radiations, such as protons, should be useful in reducing the uncertainty associated with current models for predicting long-term health risks of space radiation. These studies are also relevant to hadron therapy of cancer.

RAD51 plays a crucial role in halting cell death program induced by ionizing radiation in bovine oocytes

Reproductive health of humans and animals exposed to daily irradiants from solar/cosmic particles remains largely understudied. We evaluated the sensitivities of bovine and mouse oocytes to bombardment by krypton-78 (1 Gy) or ultraviolet B (UV-B; 100 microjoules). Mouse oocytes responded to irradiation by undergoing massive activation of caspases, rapid loss of energy without cytochrome-c release, and subsequent necrotic death. In contrast, bovine oocytes became positive for annexin-V, exhibited cytochrome-c release, and displayed mild activation of caspases and downstream DNAses but with the absence of a complete cell death program; therefore, cytoplasmic fragmentation was never observed. However, massive cytoplasmic fragmentation and increased DNA damage were induced experimentally by both inhibiting RAD51 and increasing caspase 3 activity before irradiation. Microinjection of recombinant human RAD51 prior to irradiation markedly decreased both cytoplasmic fragmentation and DNA damage in both bovine and mouse oocytes. RAD51 response to damaged DNA occurred faster in bovine oocytes than in mouse oocytes. Therefore, we conclude that upon exposure to irradiation, bovine oocytes create a physiologically indeterminate state of partial cell death, attributed to rapid induction of DNA repair and low activation of caspases. The persistence of these damaged cells may represent an adaptive mechanism with potential implications for livestock productivity and long-term health risks associated with human activity in space.

Ionizing radiation-induced metabolic oxidative stress and prolonged cell injury

Cellular exposure to ionizing radiation leads to oxidizing events that alter atomic structure through direct interactions of radiation with target macromolecules or via products of water radiolysis. Further, the oxidative damage may spread from the targeted to neighboring, non-targeted bystander cells through redox-modulated intercellular communication mechanisms. To cope with the induced stress and the changes in the redox environment, organisms elicit transient responses at the molecular, cellular and tissue levels to counteract toxic effects of radiation. Metabolic pathways are induced during and shortly after the exposure. Depending on radiation dose, dose-rate and quality, these protective mechanisms may or may not be sufficient to cope with the stress. When the harmful effects exceed those of homeostatic biochemical processes, induced biological changes persist and may be propagated to progeny cells. Physiological levels of reactive oxygen and nitrogen species play critical roles in many cellular functions. In irradiated cells, levels of these reactive species may be increased due to perturbations in oxidative metabolism and chronic inflammatory responses, thereby contributing to the long-term effects of exposure to ionizing radiation on genomic stability. Here, in addition to immediate biological effects of water radiolysis on DNA damage, we also discuss the role of mitochondria in the delayed outcomes of ionization radiation. Defects in mitochondrial functions lead to accelerated aging and numerous pathological conditions. Different types of radiation vary in their linear energy transfer (LET) properties, and we discuss their effects on various aspects of mitochondrial physiology. These include short and long-term in vitro and in vivo effects on mitochondrial DNA, mitochondrial protein import and metabolic and antioxidant enzymes.

Effects of ionizing radiation on three-dimensional human vessel models: differential effects according to radiation quality and cellular development

Little is known about the effects of space radiation on the human body. There are a number of potential chronic and acute effects, and one major target for noncarcinogenic effects is the human vasculature. Cellular stress, inflammatory response, and other radiation effects on endothelial cells may affect vascular function. This study was aimed at understanding the effects of space ionizing radiation on the formation and maintenance of capillary-like blood vessels. We used a 3D human vessel model created with human endothelial cells in a gel matrix to assess the effects of low-LET protons and high-LET iron ions. Iron ions were more damaging and caused significant reduction in the length of intact vessels in both developing and mature vessels at a dose of 80 cGy. Protons had no effect on mature vessels up to a dose of 3.2 Gy but did inhibit vessel formation at 80 cGy. Comparison with γ radiation showed that photons had even less effect, although, as with protons, developing vessels were more sensitive. Apoptosis assays showed that inhibition of vessel development or deterioration of mature vessels was not due to cell death by apoptosis even in the case of iron ions. These are the first data to show the effects of radiation with varying linear energy transfer on a human vessel model.

Medium-mediated DNA repair response after ionizing radiation is correlated with the increase of specific cytokines in human fibroblasts

Radiation induced bystander effects, either protective or adverse, have been identified in a variety of cells and for different endpoints. They are thought to arise from communication between cells through direct cell-cell contacts and via transmissible molecules secreted into the medium by targeted cells. We have investigated medium-mediated damage response in human dermal fibroblasts (HDF) after exposure to ionizing irradiation. We show that HDF experience an elevated level of double stranded DNA damage repair response when incubated with conditioned growth medium of irradiated cells. The magnitude of this response is much lower than observed for directly irradiated cells and is proportional to the radiation dose, as is its persistence across time. Since secretion of cytokines is one of the possible pathways linking targeted and non-targeted cells a multiplex analysis was performed. Four cytokines - IL6, IL8, MCP-1 and RANTES - were identified in the growth medium of irradiated cells after exposure to X-rays (2Gy). These cytokines were significantly upregulated and each cytokine showed differential upregulation kinetics. Finally we performed a functional analysis to see if IL6 and MCP-1 could induce gammaH2AX foci formation. IL6 caused a significant increase in spot occupancy compared to controls. Although only indicative MCP-1 appears to have the opposite effect as it caused a drop in spot occupancy. The combined addition of these two cytokines produced no significant response was observed. Both IL6 and MCP-1 have an effect on the gammaH2AX spot occupancy possibly linking these cytokines to the bystander response.

Adaptive response and human benefit: Part I. A microdosimetry dose-dependent model

PURPOSE

It is important to evaluate how adaptive response may be of human benefit from the risks of ionizing radiation. The purpose of this work is to develop and apply a microdosimetric dose response model capable of explicitly determining, for broad beam exposures, the threshold and progressive activation of natural spontaneous and radiation damage protective mechanisms associated with adaptive response and other cellular negative response behavior.

MATERIALS AND METHODS

A biophysical model was developed quantifying the accumulation of Poisson distributed microdose specific energy hits to cell critical nucleus volumes. The model was applied to the adaptive response data of Wiencke et al., Redpath et al., Azzam et al. and Pohl-Ruling et al. The model was also applied to non-adaptive response data showing dose response reductions below the zero dose natural spontaneous level and to data exhibiting mid-range non-monotonic dose response plateaus.

RESULTS

We find good fits of the model to all data. For adaptive response, a significant result is, that only one or two specific energy hits of low linear energy transfer (LET) radiation in the cell nucleus activates the protective mechanisms for both the natural spontaneous and radiation damage. Several data support a dose plateau for radon progeny alpha production of chromosome aberrations in human lymphocytes. Using the model, a bystander factor of about 30 is obtained with the model for high dose rate, in vitro alpha particle data. For low dose rate in vivo, the bystander effect is minimal suggesting for alphas that the bystander effect may be dose rate dependent. There is no evidence of bystander effects in the low LET adaptive response data analysis.

CONCLUSIONS

The microdosimetry model allows concise determinations of specific energy hits within the cell critical nucleus volume to activate both protective and damage mechanisms. One or two low LET hits can result in reduction of both zero dose natural spontaneous and radiation-induced, carcinogenic causing damage. The model should be useful in comparing in vitro and in vivo broad beam to single track microbeam exposure data. The model is capable of determining, to an accuracy of +/- one specific energy hit, the minimum threshold for induction of radioprotective mechanisms--crucial to assessing the potential human benefit of adaptive response and other negative dose response behavior.

Applications of particle microbeams in space radiation research

Galactic cosmic radiation is acknowledged as one of the major barriers to human space exploration. In space, astronauts are exposed to charged particles from Z = 1 (H) up to Z = 28 (Ni), but the probability of a hit to a specific single cell in the human body is low. Particle microbeams can deliver single charged particles of different charge and energy to single cells from different tissues, and microbeam studies are therefore very useful for improving current risk estimates for long-term space travel. 2D in vitro cell cultures can be very useful for establishing basic molecular mechanisms, but they are not sufficient to extrapolate risk, given the substantial evidence proving tissue effects are key in determining the response to radiation insult. 3D tissue or animal systems represent a more promising target for space radiobiology using microbeams.

"Protective bystander effects simulated with the state-vector model"--HeLa x skin exposure to Cs not protective bystander response but mammogram and diagnostic X-rays are

The recent Dose Response journal article "Protective Bystander Effects Simulated with the State-Vector Model" (Schollnberger and Eckl 2007) identified the suppressive (below natural occurring, zero primer dose, spontaneous level) dose response for HeLa x skin exposure to (137)Cs gamma rays (Redpath et al 2001) as a protective Bystander Effect (BE) behavior. I had previously analyzed the Redpath et al (2001) data with a Microdose Model and conclusively showed that the suppressive response was from Adaptive Response (AR) radio-protection (Leonard 2005, 2007a). The significance of my microdose analysis has been that low LET radiation induced single (i.e. only one) charged particle traversals through a cell can initiate a Poisson distributed activation of AR radio-protection. The purpose of this correspondence is to clarify the distinctions relative to the BE and the AR behaviors for the Redpath groups (137)Cs data, show conversely however that the Redpath group data for mammography (Ko et al 2004) and diagnostic (Redpath et al 2003) X-rays do conclusively reflect protective bystander behavior and also herein emphasize the need for radio-biologist to apply microdosimetry in planning and analyzing their experiments for BE and AR. Whether we are adamantly pro-LNT, adamantly anti-LNT or, like most of us, just simple scientists searching for the truth in radio-biology, it is important that we accurately identify our results, especially when related to the LNT hypothesis controversy.

A composite microdose Adaptive Response (AR) and Bystander Effect (BE) model-application to low LET and high LET AR and BE data

PURPOSE

It has been suggested that Adaptive Response (AR) may reduce risk of adverse health effects due to ionizing radiation. But very low dose Bystander Effects (BE) may impose dominant deleterious human risks. These conflicting behaviors have stimulated controversy regarding the Linear No-Threshold human risk model. A dose and dose rate-dependent microdose model, to examine AR behavior, was developed in prior work. In the prior work a number of in vitro and in vivo dose response data were examined with the model. Recent new data show AR behavior with some evidence of very low dose BE. The purpose of this work is to supplement the microdose model to encompass the Brenner and colleagues BaD (Bystander and Direct Damage) model and apply this composite model to obtain new knowledge regarding AR and BE and illustrate the use of the model to plan radio-biology experiments.

MATERIALS AND METHODS

The biophysical composite AR and BE Microdose Model quantifies the accumulation of hits (Poisson distributed, microdose specific energy depositions) to cell nucleus volumes. This new composite AR and BE model provides predictions of dose response at very low dose BE levels, higher dose AR levels and even higher dose Direct (linear-quadratic) Damage radiation levels.

RESULTS

We find good fits of the model to both BE data from the Columbia University microbeam facility and combined AR and BE data for low Linear Energy Transfer (LET) and high LET data. A Bystander Factor of about 27,000 and an AR protection factor of 0.61 are obtained for the low LET in vivo mouse spleen exposures. A Bystander Factor of 317 and an AR protection factor of 0.53 are obtained for high LET radon alpha particles in human lymphocytes. In both cases the AR is activated at most by one or two radiation induced charged particle traversals through the cell nucleus.

CONCLUSIONS

The results of the model analysis is consistent with a premise that both Bystander damage and Adaptive Response radioprotection can occur in the same cell type, derived from the same cell species. The model provides an analytical tool to biophysically study the combined effects of BE and AR.

A review: Development of a microdose model for analysis of adaptive response and bystander dose response behavior

Prior work has provided incremental phases to a microdosimetry modeling program to describe the dose response behavior of the radio-protective adaptive response effect. We have here consolidated these prior works (Leonard 2000, 2005, 2007a, 2007b, 2007c) to provide a composite, comprehensive Microdose Model that is also herein modified to include the bystander effect. The nomenclature for the model is also standardized for the benefit of the experimental cellular radio-biologist. It extends the prior work to explicitly encompass separately the analysis of experimental data that is 1.) only dose dependent and reflecting only adaptive response radio-protection, 2.) both dose and dose-rate dependent data and reflecting only adaptive response radio-protection for spontaneous and challenge dose damage, 3.) only dose dependent data and reflecting both bystander deleterious damage and adaptive response radio-protection (AR-BE model). The Appendix cites the various applications of the model. Here we have used the Microdose Model to analyze the, much more human risk significant, Elmore et al (2006) data for the dose and dose rate influence on the adaptive response radio-protective behavior of HeLa x Skin cells for naturally occurring, spontaneous chromosome damage from a Brachytherapy type (125)I photon radiation source. We have also applied the AR-BE Microdose Model to the Chromosome inversion data of Hooker et al (2004) reflecting both low LET bystander and adaptive response effects. The micro-beam facility data of Miller et al (1999), Nagasawa and Little (1999) and Zhou et al (2003) is also examined. For the Zhou et al (2003) data, we use the AR-BE model to estimate the threshold for adaptive response reduction of the bystander effect. The mammogram and diagnostic X-ray induction of AR and protective BE are observed. We show that bystander damage is reduced in the similar manner as spontaneous and challenge dose damage as shown by the Azzam et al (1996) data. We cite primary unresolved questions regarding adaptive response behavior and bystander behavior. The five features of major significance provided by the Microdose Model so far are 1. Single Specific Energy Hits initiate Adaptive Response. 2. Mammogram and diagnostic X-rays induce a protective Bystander Effect as well as Adaptive Response radio-protection. 3. For mammogram X-rays the Adaptive Response protection is retained at high primer dose levels. 4. The dose range of the AR protection depends on the value of the Specific Energy per Hit, 1 >. 5. Alpha particle induced deleterious Bystander damage is modulated by low LET radiation.

The radon inverse dose rate effect and high-LET galactic hazards

The lung dose rate per unit 222Rn concentration in enclosed spaces is shown to experience transitions at high radon concentrations. This has implications on the radon inverse dose rate effect. At an air change rate (ACH) of 0.194 h(-1) and relative humidity (RH) of 52.3% in a 0.283 m3 test chamber, the total human lung dose for an adult male in a residential setting (breathing rate 0.78 m3 h(-1)) would undergo a reduction of 2.5 using the ICRP 66 human respiratory tract model and the BEIR VI methodology. Using the same methodology of both Cross (Pacific Northwest Laboratory rat exposures) and Lubin et al. (miners dose rates), adjustments are necessary for effects of RH and ACHs. These adjustments, however, do not affect the reduction behaviour. It is thus shown that the enhanced deposition effect (EDE) must influence the magnitude of the purported inverse dose rate effect (IDRE). In the analysis of animal data, Cross rat exposures in a 2.0 m3 chamber, a reduction in lung dose is estimated to be over a factor of 3 the transition between the 50 and 500 WLM week(-1) dose rate range. For an estimation of the EDE, using a hypothetical 30 m3 enclosure for underground miners, we obtain a factor of approximately 4 in human lung dose reduction. Although the extensive analyses required make these results qualitative, the EDE behaviour is sufficiently conclusive that these estimates show that the radon IDRE for lung cancer must be an EDE dosimetric issue as well as a radiological lung cell dose response issue. The consequence of analysis of other animal data would achieve the same conclusion.

Role of shielding in modulating the effects of solar particle events: Monte Carlo calculation of absorbed dose and DNA complex lesions in different organs

Distributions of absorbed dose and DNA clustered damage yields in various organs and tissues following the October 1989 solar particle event (SPE) were calculated by coupling the FLUKA Monte Carlo transport code with two anthropomorphic phantoms (a mathematical model and a voxel model), with the main aim of quantifying the role of the shielding features in modulating organ doses. The phantoms, which were assumed to be in deep space, were inserted into a shielding box of variable thickness and material and were irradiated with the proton spectra of the October 1989 event. Average numbers of DNA lesions per cell in different organs were calculated by adopting a technique already tested in previous works, consisting of integrating into "condensed-history" Monte Carlo transport codes--such as FLUKA--yields of radiobiological damage, either calculated with "event-by-event" track structure simulations, or taken from experimental works available in the literature. More specifically, the yields of "Complex Lesions" (or "CL", defined and calculated as a clustered DNA damage in a previous work) per unit dose and DNA mass (CL Gy-1 Da-1) due to the various beam components, including those derived from nuclear interactions with the shielding and the human body, were integrated in FLUKA. This provided spatial distributions of CL/cell yields in different organs, as well as distributions of absorbed doses. The contributions of primary protons and secondary hadrons were calculated separately, and the simulations were repeated for values of Al shielding thickness ranging between 1 and 20 g/cm2. Slight differences were found between the two phantom types. Skin and eye lenses were found to receive larger doses with respect to internal organs; however, shielding was more effective for skin and lenses. Secondary particles arising from nuclear interactions were found to have a minor role, although their relative contribution was found to be larger for the Complex Lesions than for the absorbed dose, due to their higher LET and thus higher biological effectiveness.

Progeny enhanced deposition rates primarily from increased particle diffusivity at high radon concentrations

Prior work found that high 222Rn concentrations produced an enhanced radon progeny surface deposition effect (EDE) in a 0.283-m3 electrically grounded aluminum test chamber. To study a possible charge mobility effect, we report here additional measurements with the chamber lined with nonconducting, insulated materials. Specifically, with insulated, chemically clean glass we provide results of measurements at a number of 222Rn concentrations past the threshold and transition 222Rn levels (up to 60 kBq m(-3)). For EDE it was found at most a 14% charge mobility deposition effect. Without electrical grounding in the Al chamber with +/- 2,000 DC volts applied showed a 10% increase in 218Po surface deposition. Ambient and hot (37 degrees C) wetted filter paper to simulate lung tissue was measured and showed EDE of the same factor of > or = 2 as for all other materials. It is concluded that the enhanced progeny deposition from high radon concentration is primarily from a decrease in attached fraction, thus reducing progeny mean particle size, and increasing diffusivity and surface deposition.

Tumor control probability predictions for genetic radiotherapy

PURPOSE

Genetic radiotherapy, the combination of gene therapy and radiation therapy, for cancer treatment is evolving from laboratory studies to clinical trials. Genetic radiotherapy involves the viral infection of cells that change the sensitivity of transduced cells to radiation. Because there is no patient outcome data for genetic radiotherapy, prospective models are needed to determine the expected benefit of this new modality. Such a prospective model has been developed in this work.

METHODS AND MATERIALS

An existing tumor control probability (TCP) calculation model developed for external beam radiotherapy was modified for genetic radiotherapy. Specifically, the (1) transduced fraction and (2) enhancement factor of the transduced cells were included in the model. Parametric studies of the effects of these two variables on TCP for head-and-neck cancer were performed.

RESULTS

Using reasonable transduction fraction and enhancement factor values (0.8 and 1.4, respectively), the model predicts an increase in the TCP for genetic radiotherapy over radiotherapy alone by up to 15% for the same radiotherapy dose. The theoretical limit of TCP increase was calculated to be near 70%, which may occur with improved techniques that increase the transduced fraction or because of a strong bystander effect. To maintain existing TCP, dose reductions from 5 Gy (reasonable values) to >30 Gy (ideal case) are predicted for genetic radiotherapy over radiotherapy alone.

CONCLUSIONS

Our results indicate that genetic radiotherapy has the potential to significantly improve tumor control over radiotherapy alone.

Genetic damage induced by in vitro irradiation of human G0 lymphocytes with low-energy protons (28 keV/microm): HPRT mutations and chromosome aberrations

Cell survival, mutations and chromosomal effects were studied in primary human lymphocytes exposed in G0 phase to a proton beam with an incident energy of 0.88 MeV (incident LET of 28 keV/microm) in the dose range 0.125-2 Gy. The curves for survival and mutations at the hypoxanthine-guanine phosphoribosyl transferase locus were obtained by fitting the experimental data to linear and linear-quadratic equations, respectively. In the dose interval 0-1.5 Gy, the alpha parameters of the curves were 0.42/Gy and 3.6 x 10(-6) mutants/Gy, respectively. The mutation types at the HPRT locus were analyzed by multiplex-PCR in 94 irradiated and 41 nonirradiated clones derived from T lymphocytes from five healthy donors. All clones showed a normal multiplex-PCR pattern and were classified as point mutations. Chromosome aberration data were fitted as a linear function of dose (alpha = 0.62 aberrations per cell Gy(-1)). By irradiating G0 lymphocytes from a single subject with 28 keV/microm protons and gamma rays, an RBE of 6.07 was obtained for chromosome aberrations. An overinvolvement of chromosome 9 relative to chromosome 7 was found in chromosome breaks after chromosome painting analysis.

Hematologic consequences of exposure to ionizing radiation

From the early 1900s, it has been known that ionizing radiation (IR) impairs hematopoiesis through a variety of mechanisms. IR exposure directly damages hematopoietic stem cells and alters the capacity of bone marrow stromal elements to support and/or maintain hematopoiesis in vivo and in vitro. Exposure to IR induces dose-dependent declines in circulating hematopoietic cells not only through reduced bone marrow production, but also by redistribution and apoptosis of mature formed elements of the blood. Recently, the importance of using lymphocyte depletion kinetics to provide a "crude" dose estimate has been emphasized, particularly in rapid assessment of large numbers of individuals who may be exposed to IR through acts of terrorism or by accident. A practical strategy to estimate radiation dose and triage victims based upon clinical symptomatology is presented. An explosion of knowledge has occurred regarding molecular and cellular pathways that trigger and mediate hematologic responses to IR. In addition to damaging DNA, IR alters gene expression and transcription, and interferes with intracellular and intercellular signaling pathways. The clinical expression of these disturbances may be the development of leukemia, the most significant hematologic complication of IR exposure among survivors of the atomic bomb detonations over Japan. Those at greatest risk for leukemia are individuals exposed during childhood. The association of leukemia with chronic, low-dose-rate exposure from nuclear power plant accidents and/or nuclear device testing has been more difficult to establish, due in part to lack of precision and sensitivity of methods to assess doses that approach background radiation dose. Nevertheless, multiple myeloma may be associated with chronic exposure, particularly in those exposed at older ages.