Low Doses Of low-LET Radiation Research Articles

Commentaries on Viewpoint: A call for research to assess and promote functional resilience in astronaut crews

Commentaries on Viewpoint: A call for research to assess and promote functional resilience in astronaut crews

Effect of low doses of low-let radiation on the innate anti-tumor reactions in radioresistant and radiosensitive mice.

BALB/c and C57BL/6 mice differ in their Th1/Th2 lymphocyte and M1/M2 macrophage phenotypes, radiosensitivity, and post-irradiation tumor incidence. In this study we evaluated the effects of repeated low-level exposures to X-rays on the development of artificial tumor colonies in the lungs of animals from the two strains and cytotoxic activities of natural killer (NK) cells and macrophages obtained from these mice. After ten daily irradiations of BALB/c or C57BL/6 mice with 0.01, 0.02, and 0.1 Gy X-rays NK cell-enriched splenocytes collected from the animals demonstrated significant and comparable up-regulation of their anti-tumor cytotoxic function. Likewise, peritoneal macrophages collected from the two irradiated strains of mice exhibited the similarly stimulated cytotoxicities against susceptible tumor cells and produced significantly more nitric oxide. These results were accompanied by the significantly reduced numbers of the neoplastic colonies induced in the lungs by intravenous injection of syngeneic tumor cells. The obtained results indicate that ten low-level irradiations with X-rays stimulate the generally similar anti-tumor reactions in BALB/c and C57BL/6 mice.

Low-dose radiation risk extrapolation fallacy associated with the linear-no-threshold model

Managing radiation risks typically involves establishing regulations that limit radiation exposure. The linear-no-threshold (LNT) dose-response model has been the traditional regulatory default assumption. According to the LNT model, for low a linear-energy-transfer (LET) radiation-induced stochastic effects (e.g., neoplastic transformation and cancer), the risk increases linearly without a threshold. Any radiation exposure is predicted to increase the number of cancer cases among a large population of people. Cancer risk extrapolation from high to low doses based on this model is widespread. Here, indirect evidence is provided that the excess cancer risk calculated at very low doses of low-LET radiation (e.g., around 1 mGy), based on extrapolating from high dose data for an irradiated human population using the LNT model, is likely a phantom excess risk. Indirect evidence is provided, suggesting that for brief exposures to low-LET radiation doses on the order of 1 mGy, that a decrease below the spontaneous level is many orders of magnitude more probable than for any increase in risk as would be predicted by extrapolating from high to low doses using the LNT model. Such a decrease is, however, not expected after exposure to high-LET alpha radiation. The risk reduction has been largely attributed to the induction of a protective apoptosis-mediated (PAM) process that selectively eliminates cells that contain genomic instability (e.g., mutant and neoplastically transformed cells). The PAM process appears to require a dose-rate-dependent stochastic threshold for activation whose minimum is estimated to possibly be as low as 0.01 mGy for X-rays and gamma rays. However, if the dose is too high (e.g., above 250 mGy for brief exposure at a high rate to X-rays or gamma rays), the PAM process is not expected to be activated. For protracted exposure to X-rays or gamma rays, doses as high as 400 mGy (and possibly higher) may activate the PAM process.

Radiation-Induced Neoplastic Transformation and Cancer Risk Assessment at Low Doses of Low-LET Radiation

Epidemiologic studies of cancer incidence are insensitive at low doses (<10 cGy). For this reason it is necessary to turn to relevant experimental systems in order to shed light on what may be happening regarding cancer risk at these low doses. In terms of in vitro studies, a cancer-relevant endpoint is neoplastic transformation. Over the past 10 years we have investigated the effects of a range of doses for a variety of sources of low-LET radiation with the goal of determining the shape of the dose-response curve, and underlying biologic responses, at low doses. The first conclusion we have arrived at is the dose-response for high dose-rate radiation is not linear but J-shaped. The transformation frequency only begins to rise above background at doses above 10 to 20 cGy. Secondly, we have evidence that the low dose suppression is most likely a consequence of multiple mechanisms (upregulation of DNA repair and antioxidant status as well as hyper-radio-sensitivity of cells destined to become neoplastically transformed). Thirdly, we have evidence that this suppression of transformation also occurs for radiation delivered at low (mGy/min) to very low (mGy/day) dose-rates. In addition, we have shown that exposure to very low dose-rate radiation (mGy/day) over 2 to 3 months confers a resistance to neoplastic transformation by a subsequent challenge dose, i.e. an adaptive response as classically defined. It would therefore appear that low doses of low-LET radiation mount responses that protect cell populations against neoplastic transformation in vitro. Clearly, in vitro systems have limitations compared the in vivo situation, nonetheless, they have historically given comparable conclusions at high radiation doses to those seen in vivo in terms of the effect of dose-fractionation, dose-rate, LET and chemical modifiers. A priori, there is therefore no reason to doubt that the same will be true at low doses. Indeed, there are several in vivo studies that would support this contention. Additionally, the relative risk for neoplastic transformation determined from our studies agrees well with those for cancer incidence from radiation epidemiology in the case of breast cancer and leukemia. Based on the above, and other extensive evidence in the literature, it is unlikely that cancer risk from low-LET radiation has a linear-no-threshold dose-response and that threshold, or even hormetic, responses are more probable. Acknowledgement. Supported by the U.S. Department of Energy Low Dose Radiation Research Program.

Detrimental and protective bystander effects: a model approach.

This work integrates two important cellular responses to low doses, detrimental bystander effects and apoptosis-mediated protective bystander effects, into a multistage model for chromosome aberrations and in vitro neoplastic transformation: the State-Vector Model. The new models were tested on representative data sets that show supralinear or U-shaped dose responses. The original model without the new low-dose features was also tested for consistency with LNT-shaped dose responses. Reductions of in vitro neoplastic transformation frequencies below the spontaneous level have been reported after exposure of cells to low doses of low-LET radiation. In the current study, this protective effect is explained with bystander-induced apoptosis. An important data set that shows a low-dose detrimental bystander effect for chromosome aberrations was successfully fitted by additional terms within the cell initiation stage. It was found that this approach is equivalent to bystander-induced clonal expansion of initiated cells. This study is an important step toward a comprehensive model that contains all essential biological mechanisms that can influence dose-response curves at low doses.

Protective Bystander Effects Simulated with the State-Vector Model

Apoptosis induced in non-hit bystander cells is an important biological mechanism which operates after exposure to low doses of low-LET radiation. This process was implemented into a deterministic multistage model for in vitro neoplastic transformation: the State-Vector Model (SVM). The new model is tested on two data sets that show a reduction of the transformation frequency below the spontaneous level after exposure of the human hybrid cell line CGL1 to low doses of gamma-radiation. Stronger protective effects are visible in the data for delayed plating while the data for immediate plating show more of an LNT-like dose-response curve. It is shown that the model can describe both data sets. The calculation of the time-dependent numerical solution of the model also allows to obtain information about the time-dependence of the protective apoptosis-mediated process after low dose exposures. These findings are compared with experimental observations after high dose exposures.

The Effect of Dose Rate on Radiation-Induced Neoplastic TransformationIn Vitroby Low Doses of Low-LET Radiation

The dependence of the incidence of radiation-induced cancer on the dose rate of the radiation exposure is a question of considerable importance to the estimation of risk of cancer induction by low-dose-rate radiation. Currently a dose and dose-rate effectiveness factor (DDREF) is used to convert high-dose-rate risk estimates to low dose rates. In this study, the end point of neoplastic transformation in vitro has been used to explore this question. It has been shown previously that for low doses of low-LET radiation delivered at high dose rates, there is a suppression of neoplastic transformation frequency at doses less than around 100 mGy. In the present study, dose-response curves up to a total dose of 1000 mGy have been generated for photons from (125)I decay (approximately 30 keV) delivered at doses rates of 0.19, 0.47, 0.91 and 1.9 mGy/min. The results indicate that at dose rates of 1.9 and 0.91 mGy/min the slope of the induction curve is about 1.5 times less than that measured at high dose rate in previous studies with a similar quality of radiation (28 kVp mammographic energy X rays). In the dose region of 0 to 100 mGy, the data were equally well fitted by a threshold or linear no-threshold model. At dose rates of 0.19 and 0.47 mGy/min there was no induction of transformation even at doses up to 1000 mGy, and there was evidence for a possible suppressive effect. These results show that for this in vitro end point the DDREF is very dependent on dose rate and at very low doses and dose rates approaches infinity. The relative risks for the in vitro data compare well with those from epidemiological studies of breast cancer induction by low- and high-dose-rate radiation.

Low doses of radiation are protective in vitro and in vivo: evolutionary origins.

Research reports using cells from bacteria, yeast, alga, nematodes, fish, plants, insects, amphibians, birds and mammals, including wild deer, rodents or humans show non-linear radio-adaptive processes in response to low doses of low LET radiation. Low doses increased cellular DNA double-strand break repair capacity, reduced the risk of cell death, reduced radiation or chemically-induced chromosomal aberrations and mutations, and reduced spontaneous or radiation-induced malignant transformation in vitro. In animals, a single low, whole body dose of low LET radiation, increased cancer latency and restored a portion of the life that would have been lost due to either spontaneous or radiation-induced cancer in the absence of the low dose. In genetically normal fetal mice, a prior low dose protected against radiation-induced birth defects. In genetically normal adult-male mice, a low dose prior to a high dose protected the offspring of the mice from heritable mutations produced by the large dose. The results show that low doses of low-LET radiation induce protective effects and that these induced responses have been tightly conserved throughout evolution, suggesting that they are basic responses critical to life. The results also argue strongly that the assumption of a linear increase in risk with increasing dose in humans is unlikely to be correct, and that low doses actually reduce risk.

Low-LET-induced radioprotective mechanisms within a stochastic two-stage cancer model.

A stochastic two-stage cancer model with clonal expansion was used to investigate the potential impact on human lung cancer incidence of some aspects of the hormesis mechanisms suggested by Feinendegen (Health Phys. 52 663-669, 1987). The model was applied to low doses of low-LET radiation delivered at low dose rates. Non-linear responses arise in the model because radiologically induced adaptations in radical scavenging and DNA repair may reduce the biological consequences of DNA damage formed by endogenous processes and ionizing radiation. Sensitivity studies were conducted to identify critical model inputs and to help define the changes in cellular defense mechanisms necessary to produce a lifetime probability for lung cancer that deviates from a linear no-threshold (LNT) type of response. Our studies suggest that lung cancer risk predictions may be very sensitive to the induction of DNA damage by endogenous processes. For doses comparable to background radiation levels, endogenous DNA damage may account for as much as 50 to 80% of the predicted lung cancers. For an additional lifetime dose of 1 Gy from low-LET radiation, endogenous processes may still account for as much as 20% of the predicted cancers (Fig. 2). When both repair and scavengers are considered as inducible, radiation must enhance DNA repair and radical scavenging in excess of 30 to 40% of the baseline values to produce lifetime probabilities for lung cancer outside the range expected for endogenous processes and background radiation.

Inhibition of the development of pulmonary tumour nodules and stimulation of the activity of NK cells and macrophages in mice by single low doses of low-LET radiation

As indicated by a few recent reports, exposures to low doses of ionising radiation can trigger the activity of natural anti-tumour defence mechanisms and impede the development of transplantable tumours in rodents. In our experimental system, BALB/c mice were irradiated with a single dose of 0.1, 0.2, 0.5, or 1.0 Gy X-rays and then i.v. injected with L1 sarcoma cells. As shown by the results of the lung-colony assay the number of tumour nodules was significantly reduced in the animals exposed to 0.2 Gy in comparison to those irradiated with 0.5 and 1.0 Gy X-rays. This effect was accompanied by the significant stimulation of the activity of NK cells obtained from the spleens of mice exposed to 0.2 and 1.0 Gy as compared to the control, non-irradiated counterparts. However, since the spleen cellularity was substantially reduced by irradiation with 1.0 but not 0.1 or 0.2 Gy and in view of the relatively low radiosensitvity of NK cells compared to B and T lymphocytes, it is possible that the stimulatory effect of 1.0 Gy X-rays resulted from the enrichment of the spleen cell population in natural killer effectors by this dose of X-rays. Moreover, i.p. injection of the anti-asialoGM1 antibody abrogated the differences between the number of tumour cell colonies in the lungs of mice exposed to 0.2 and 1.0 Gy of X-rays. Finally, peritoneal macrophages obtained from mice exposed to 0.1 or 0.2 Gy X-rays and incubated with IFN&#947;-synthesised significantly greater amounts of nitric oxide than macrophages collected from either the non-irradiated or the 1.0 Gy-exposed mice. However, neither the unstimulated nor the IFN&#947;-treated macrophages lysed the L1 sarcoma cells in the in vitro assay. The results collectively indicate that the inhibitory effect of 0.2 Gy of X-rays on the development of secondary tumour nodules in mice is associated with the stimulation by this small dose of radiation of the NK cell- and/or macrophage-mediated functions.

The shape of the dose-response curve for radiation-induced neoplastic transformation in vitro: evidence for an adaptive response against neoplastic transformation at low doses of low-LET radiation.

A dose-response curve for gamma-radiation-induced neoplastic transformation of HeLa x skin fibroblast human hybrid cells over the dose range 0.1 cGy to 1 Gy is presented. In the experimental protocol used, the spontaneous (background) frequency of neoplastic transformation of sham-irradiated cultures was compared to that of cultures which had been irradiated with (137)Cs gamma radiation and either plated immediately or held for 24 h at 37 degrees C prior to plating, for assay for neoplastic transformation. The pooled data from a minimum of three repeat large-scale experiments at each dose demonstrated a reduced transformation frequency for the irradiated compared to the sham-irradiated cells for doses of 0.1, 0.5, 1, 5 and 10 cGy for the delayed-plating arm. The probability of this happening by chance is given by 1/2(n), where n is the number of observations (5); i.e., 1/32 congruent with 0.031. This is indicative of an adaptive response against spontaneous neoplastic transformation at least up to a dose of 10 cGy of gamma radiation. The high-dose data obtained at 30 and 50 cGy and 1 Gy showed a good fit to a linear extrapolation through the sham-irradiated, zero-dose control. The delayed-plating data at 10 cGy and below showed a statistically significant divergence from this linear extrapolation.

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

Densely ionizing (high-LET) galactic cosmic rays (GCR) contribute a significant component of the radiation risk in free space. Over a period of a few months-sufficient for the early stages of radiation carcinogenesis to occur-a significant proportion of cell nuclei will not be traversed. There is convincing evidence, at least in vitro, that irradiated cells can send out signals that can result in damage to nearby unirradiated cells. This observation can hold even when the unirradiated cells have been exposed to low doses of low-LET radiation. We discuss here a quantitative model based on the a formalism, an approach that incorporates radiobiological damage both from a bystander response to signals emitted by irradiated cells, and also from direct traversal of high-LET radiations through cell nuclei. The model produces results that are consistent with those of a series of studies of the bystander phenomenon using a high-LET microbeam, with the end point of in vitro oncogenic transformation. According to this picture, for exposure to high-LET particles such as galactic cosmic rays other than protons, the bystander effect is significant primarily at low fluences, i.e., exposures where there are significant numbers of untraversed cells. If the mechanisms postulated here were applicable in vivo, using a linear extrapolation of risks derived from studies using intermediate doses of high-LET radiation (where the contribution of the bystander effect may be negligible) to estimate risks at very low doses (where the bystander effect may be dominant) could underestimate the true risk from low doses of high-LET radiation. It would be highly premature simply to abandon current risk projections for high-LET, low-dose radiation; however, these considerations would suggest caution in applying results derived from experiments using high-LET radiation at fluences above approximately 1 particle per nucleus to risk estimation for a Mars mission.

Application of Bayesian Inference to Characterize Risks Associated with Low Doses of Low-LET Radiation

Improved risk characterization for stochastic biological effects of low doses of low-LET radiation is important for protecting nuclear workers and the public from harm from radiation exposure. Here we present a Bayesian approach to characterize risks of stochastic effects from low doses of low-LET radiation. The stochastic effect considered is neoplastic transformation of cells because it relates closely to cancer induction. We have used a published model of neoplastic transformation called NEOTRANS1. It is based on two different classes of cellular sensitivity for asynchronous, exponentially growing populations (in vitro). One sensitivity class is the hypersensitive cell; the other is the resistant cell. NEOTRANS1includes the effects of genomic damage accumulation, DNA repair during cell cycle arrest, and DNA misrepair (non-lethal repair errors). The model-associated differential equations are solved for conditions of in vitro irradiation at a fixed rate. Previously published solutions apply only to high dose rates and were incorrectly assumed to apply to only high-LET radiation. Solutions provided here apply to any fixed dose rate and to both high- and low-LET radiations. Markov chain Monte Carlo methods are used to carry out the Bayesian inference of the low-dose risk for neoplastic transformation of aneuploid C3H 10T1/2 cells for X-ray doses from 0 to 1000 mGy. We have assumed that for this low-dose range only the hypersensitive fraction of the cells are affected. Our results indicate that the initial slope of the risk vs dose relationship for neoplastic transformation is as follows: (1) directly proportional to the fraction, f1, of hypersensitive cells; (2) directly proportional to the radiosensitivity of the genomic target; and (3) inversely proportional to the rate at which hypersensitive cells with radiation-induced damage are committed to undergo correct repair of genomic damage. Further, our results indicate that very fast molecular events are associated with the commitment of cells to the correct repair pathway. Results also indicate a relatively large probability for misrepair that leads to genomic instability. Our results are consistent with the view that for very low doses, dose rate is not an important variable for characterizing low-LET radiation risks so long as age-related changes in sensitivity do not occur during irradiation.

The risk of leukemia from low doses of low-LET radiation

In this communication, we examine the evidence (if any) for a nonlinear dose response in relation to leukemia mortality in the Japanese A-bomb population. Specifically, we seek an estimate of the probability that, at low doses of radiation, the relative risk (RR) is smaller than one. Using Bayesian bootstrap techniques, we find that there is a 90% probability that—for at least one dose group at or below 200 mSv—there is a reduction of effect relative to the control group. We take this as evidence for radiation hormesis (end point: leukemia mortality) at low doses of radiation.

Relative contribution of bystander and targeted cell killing to the low-dose region of the radiation dose-response curve.

Human keratinocytes show a bystander effect when exposed to low doses of low-LET radiation. In this paper, data are presented showing a method of correcting the overall survival curve to enable analysis of the relative contributions of the bystander effect and the effect attributable to direct interaction of the radiation with the target cell. The technique used is to obtain a standard clonogenic survival curve using the assay of Puck and Marcus and, with a different set of flasks containing cloning densities of unirradiated cells, to assay the cell killing caused by medium harvested from 2 x 10(5) cells irradiated with the same doses. The data show that for this human epithelial cell line, doses of 0.01-0. 5 Gy show clonogenic death by the bystander effect only, if maximum potential bystander killing is assumed. The magnitude of the effect is relatively constant, and it appears to saturate at doses in the range of 0.03-0.05 Gy. After doses greater than 0.5 Gy, the curves for clonogenic death are the result of a dose-dependent non-bystander effect and a dose-independent bystander effect. If these particular dose-response effects occur in epithelial cells in vivo, they may have important consequences for therapy and studies of low-dose risk.

Human Health Effects of Low Doses of Ionizing Radiation: The BEIR III Controversy

Controversy in the BEIR III Subcommittee on Somatic Effects concerning human health effects of low doses of low-LET radiation has centered on (a) the appropriate dose-response relationship by which extrapolation to low doses of data obtained at relatively high doses should be governed, and (b) the appropriate human evidence which should be the basis of estimation of lifetime cancer risk from radiation exposure. It is shown that the use of the linear no-threshold dose-response relationship for extrapolation purposes is an excellent approximation that is in agreement with widely accepted fundamental radiobiological principles. The appropriate human data for derivation of cancer risks are the composite age-specific risks derived from all epidemiologic studies of human cancer resulting from partial-body and whole-body radiation exposure; this composite is in good agreement with the currently available cancer incidence dose-response data obtained from the Nagasaki Tumor Registry. The current version of BEIR III significantly underestimates the radiationinduced cancer risk because it ignores the effect of high-dose-rate, low-LET radiation on cell survival in relation to cancer induction probability, and because it emphasizes cancer mortality rather than cancer incidence. Use of the appropriate basis of risk estimates gives a lifetime cancer risk per rad per million persons exposed of 260 to 880 excess incident cases for males, and 550 to 1620 excess cases for females. This result is about five times higher than the values given in the current version. Rossi's contention that the appropriate relationship is that the carcinogenic effect of radiation is proportional only to dose squared is contrary to the majority of current theoretical, experimental, and epidemiological evidence. The controversy and the way in which it was resolved raises important questions about how the public and its representatives can in the future obtain objective scientific evaluations of issues that may have significant economic, social, and political implications.