■        A Lower Dose Threshold for the in vivo Protective Adaptive Response to Radiation. Tumorigenesis in Chronically Exposed Normal and Trp53  Heterozygous C57BL/6 Mice. R. E. J. Mitchel, P. Burchart and H. Wyatt.  Submitted 2008. Radiat Res.

Low doses of ionizing radiation to cells and animals may induce adaptive responses that reduce the risk of cancer. However, there are upper dose thresholds above which these protective adaptive responses do not occur. We have now tested the hypothesis that there are similar lower dose thresholds that must be exceeded in order to induce protective effects in vivo. We examined the effects of low dose/low dose rate fractionated exposures on cancer formation in Trp53 normal or cancer-prone Trp53 heterozygous female C57BL/6 mice. Beginning at 6 weeks of age, mice were exposed 5 days/week to single daily doses (0.33 mGy, 0.7 mGy/h) totaling 48, 97 or 146 mGy over 30, 60 or 90 weeks. The exposures for shorter times (up to 60 weeks) appeared to be below the level necessary to induce overall protective adaptive responses in Trp53 normal mice, and detrimental effects (shortened lifespan, increased frequency) evident for only specific tumor types (B- and T-cell lymphomas), were produced. Only when the exposures were continued for 90 weeks did the dose become sufficient to induce protective adaptive responses, balancing the detrimental effects for these specific cancers, and reducing the risk level back to that of the unexposed animals. Detrimental effects were not seen for other tumor types, and a protective effect was seen for sarcomas after 60 weeks of exposure, which was then lost when the exposure continued for 90 weeks. As previously shown for the upper dose threshold for protection by low doses, the lower dose boundary between protection and harm was influenced by Trp53 functionality. Neither protection nor harm was observed in exposed Trp53 heterozygous mice, indicating that reduced Trp53 function raises the lower dose/dose rate threshold for both detrimental and protective tumorigenic effects.  

■        CT Scans May Reduce Rather than Increase the Risk of Cancer  Bobby R. Scott, Charles L. Sanders, Ron E. J. Mitchel and Douglas R. Boreham. . Journal of American Physicians and Surgeons, 13(1) 8-11, 2008.

Extrapolating from data on atomic bomb survivors on the basis of the linear no-threshold (LNT) model as applied to radiation exposure, a recent paper concludes that within a few decades 1.5–2 percent of all cancers in the U.S. population could be caused by current rates of use of computed tomography (CT). This paper ignores the other war-related exposures of the Japanese population, which would be expected to shift the dose-response relationship for cancer induction to the left. Moreover, the LNT model is shown to fail in four tests involving low-dose radiation exposures. Considering the available information, we conclude that CT scans may reduce rather than increase lifetime cancer risk. 

■        Scientific Issues and Emerging Challenges for Radiological Protection, Report of the Expert Group on the Implications of Radiological Protection Science R. E. J. Mitchel, and 17 other authors, . OECD Nuclear Energy Agency report No. 6167, OECD Publishing, Paris, 2007 [ISBN 978-92-64-99032-6].

The structure of the report is as follows:

          • Part 1: Possible Scientific Issues and their Implications

            − Challenges from Non-targeted and Delayed Effects

            − Individual Sensitivity

            − Epidemiology

            − Challenges to the Concept of Dose as a Surrogate for Risk

          • Part 2: Possible Emerging Challenges in the Application of Radiological Protection

            − Radiological Protection in Medical Exposure

            − Radiological Protection of the Environment

            − Health Impacts of Radiological Terrorist Attacks

          • Possible Areas of Collaborative Research

          • Policy Implications  

■        Low Doses of Radiation Reduce Risk in vivo. R. E. J. Mitchel.   Dose-Response, 5(1) 1-10, 2007.

The “Linear No Threshold” hypothesis, used in all radiation protection practices, assumes that all doses, no matter how low, increase the risk of cancer, birth defects and heritable mutations. In vitro cell based experiments show adaptive processes in response to low doses and dose rates of low LET radiation, and do not support the hypothesis. This talk will present cellular data and data from animal experiments that test the hypothesis in vivo for cancer risk. The data show that a single, low, whole body dose (less than about 100 mGy) of low LET radiation, given at low dose rate, increased cancer latency and consequently reduced both spontaneous and radiation-induced cancer risk in both genetically normal and cancer-prone mice. This adaptive response lasted for the entire lifespan of all the animals that developed these tumors, and effectively restored a portion of the life that would have been lost due to the cancer in the absence of the low dose. Overall, the results demonstrate that the assumption of a linear increase in risk with increasing dose in vivo is not warranted, and that low doses actually reduce risk. 

■        Detrimental and Protective Bystander Effects: A Model Approach H. Schöllnberger,  R. E. J. Mitchel, J. L. Redpath, D. J. Crawford-Brown and W. Hofmann. . Radiation Research 168: 614-626, 2007

 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. 

■        Cancer and Low Dose Responses in vivo: Implications for Radiation Protection. R. E. J. Mitchel.  Dose-Response 5: 284–291, 2007.

  The Linear No Threshold (LNT) hypothesis states that ionizing radiation risk is directly proportional to dose, without a threshold. This hypothesis, along with a number of additional derived or auxiliary concepts such as radiation and tissue type weighting factors, and dose rate reduction factors, are used to calculate radiation risk estimates for humans, and are therefore fundamental for radiation protection practices. This system is based mainly on epidemiological data of cancer risk in human populations exposed to relatively high doses (above 100 mSv), with the results linearly extrapolated back to the low doses typical of current exposures. The system therefore uses dose as a surrogate for risk. There

is now a large body of information indicating that, at low doses, the LNT hypothesis, along with most of the derived and auxiliary concepts, is incorrect. The use of dose as a predictor of risk needs to be re-examined and the use of dose limits, as a means of limiting risk needs to be re-evaluated. This re-evaluation could lead to large changes in radiation protection practices. 

■        Low-Dose-Rate Ionizing Radiation Exposure and Chronic Ulcerative Dermatitis in Normal and Trp53 Heterozygous C57BL/6 Mice  .R. E. J. Mitchel, P. Burchart and H. Wyatt. Fractionated,  Radiation Research 168, 716-724 (2007).

The influence of low-dose-rate chronic radiation exposure and adaptive responses on non-cancer diseases is largely unknown. We examined the effect of low-dose/low-dose-rate fractionated or single exposures on spontaneous chronic ulcerative dermatitis in Trp53 normal or heterozygous female C57BL/6 mice. From 6 weeks of age, mice were exposed 5 days/week to single daily doses (0.33 mGy, 0.7 mGy/h) totalling 48, 97 or 146 mGy over 30, 60 or 90 weeks, and other Trp53-/- mice were exposed to a single dose of 10 mGy (0.5 mGy/min) at 20 weeks of age. The 90-week exposure produced an adaptive response, decreasing both disease frequency and severity in Trp53-/- mice and extending the life span of older animals euthanized due to severe disease. The 30- or 60-week exposures had no significant protective or detrimental effect. In contrast, the chronic, fractionated exposure for 30 or 60 weeks significantly increased the frequency and severity of the disease in older Trp53-/- mice, significantly decreasing the life span of the animals required to be euthanized for disease. Similarly, the single 10-mGy exposure also increased disease frequency in older animals. However, the chronic, fractionated exposure for 90 weeks prevented these detrimental effects, with disease frequency and severity not different from unexposed controls. We conclude that very low-dose fractionated exposures can induce a protective adaptive response in both Trp53 normal and heterozygous mice, but that a lower threshold level of exposure, similar in both cases, must first be passed. In mice with reduced Trp53 functionality, doses below the threshold can produce detrimental effects. 

■         Adaptive Response in vitro and in vivo and its Impact on Low-Dose Radiation Risk, R. E. J. Mitchel, in Chapter 10. Biological Effects of Low doses of Ionizing Radiation, in Advances in Medical Physics 2006, (A. B. Wolbarst, R. G. Zamenhof, W. R. Hendee, eds.) Medical Physics Publishing, Madison, WI, USA, 2006.

Current national and international radiation risk estimates and all radiation-protection standards and practices are based on the so-called “Linear No-Threshold Hypothesis”. Risk is assumed to be directly proportional to dose, and without a threshold. This LNT hypothesis is based mainly on epidemiological data of humans exposed to high doses and dose rates of radiation. The hypothesis is applied to low doses and dose rates by introducing a dose/dose-rate effectiveness factor (DDREF), allowing for a two-fold reduction in risk at low doses and dose rate. The LNT hypothesis is attractive because it allows radiation dose to be used as a surrogate for radiation risk, even down into a dose range where risk itself cannot be assessed statistically with confidence. At low doses, the LNT hypothesis is generally acknowledged to be only an assumption, and other dose responses may also be possible, including the supralinear, sublinear, and threshold/hormetic.  

■        , Cancer and Low Dose Responses in vivo: Implications for Radiation Protection. R. E. J. MitchelCanadian Nuclear Society Bulletin, 27(4) 23-26, 2006.

Radiation protection practices assume that cancer risk is linearly proportional to total dose, without a threshold, both for people with normal cancer risk and for people who may be genetically cancer prone.  Mice heterozygous for the Tp53 gene are cancer prone, and their increased risk from high doses was not different from Tp53 normal mice. However, in either Tp53 normal or heterozygous mice, a single low dose of low LET radiation given at low dose rate protected against both spontaneous and radiation-induced cancer by increasing tumor latency. Increased tumor latency without a cancer frequency change implies that low doses in vivo primarily slow the process of genomic instability, consistent with the elevated capacity for correct DSB rejoining seen in low dose exposed cells.

The in vivo animal data indicates that, for low doses and low dose rates in both normal and cancer prone adult mice, risk does not increase linearly with dose, and dose thresholds for increased risk exist. Below those dose thresholds (which are influenced by Tp53 function) overall risk is reduced below that of unexposed control mice, indicating that Dose Rate Effectiveness Factors (DREF) may approach infinity, rather than the current assumption of 2. However, as dose decreases, different tissues appear to have different thresholds at which detriment turns to protection, indicating that individual tissue weighting factors (W_t) are also not constant, but vary from positive values to zero with decreasing dose. Measurements of Relative Biological Effect between high and low LET radiations are used to establish radiation weighting factors (W_r) used in radiation protection, and these are also assumed to be constant with dose. However, since the risk from an exposure to low LET radiation is not constant with dose, it would seem unlikely that radiation-weighting factors for high LET radiation are actually constant at low dose and dose rate. 

■         Enhanced biological effectiveness of low energy X-rays and implications for the UK breast screening programme J. L. Redpath and R. E. J. Mitchel. . Br. J. Radiol. Correspondence. 79: 854-855, 2006.

 A recent paper by Heyes et al published in the British Journal of Radiology discusses the potential carcinogenic side effects of screening mammography, a topic of great public interest, concern and importance. Unfortunately, the authors’ risk estimation is based on high dose data, with a linear extrapolation to low doses. Even more unfortunately, the authors fail to cite and discuss several papers using the same endpoint as that of the authors’, showing that the dose–response curves for both high and low energy low LET radiations do not adhere to a linear extrapolation at low doses (<100 mGy) and demonstrating that extrapolation from higher doses is likely to significantly overestimate the risk. In particular, the authors fail to discuss a paper, using the same cell assay system and the same mammographic energy X-rays, that reported the effect of doses as low as 0.54 mGy (i.e. in the range of screening mammography examinations) and up to 220 mGy, and which also showed a clear J-shaped dose–response curve. 

 ■         A model for the induction of chromosome aberrations through direct and bystander mechanisms, H. Schöllnberger, R. E. J. Mitchel, D.J. Crawford-Brown, W. Hofmann .  Radiation Protection Dosimetry.. 122 (1-4) 2006

 A state vector model (SVM) for chromosome aberrations and neoplastic transformation has been adapted to describe  detrimental bystander effects. The model describes initiation (formation of translocations) and promotion (clonal expansion and loss of contact inhibition of initiated cells). Additional terms either in the initiation model or in the rate of clonal expansion of initiated cells, describe detrimental bystander effects for chromosome aberrations as reported in the scientific literature. In the present study, the SVM with bystander effects is tested on a suitable dataset. In addition to the simulation of non-linear effects, a classical dataset for neoplastic transformation in C3H 10T1/2 cells after alpha particle irradiation is used to show that the model without bystander features can also describe LNT-like dose responses. A published model for bystander induced neoplastic transformation was adapted for chromosome aberration induction and used to compare the results obtained with the different models. 

■          The adaptive response and protection against heritable mutations and fetal malformation. D. R. Boreham, J.-A. Dolling, C. Somers, J. Quinn and R. E. J. Mitchel.  Dose-Response, 4(4):317–326, 2006

  There are a number of studies that show radiation can cause heritable mutations in the offspring of irradiated organisms. These “germ-line mutations” have been shown to occur in unique sequences of DNA called “minisatellite loci”. The high frequencies of spontaneous and induced mutations at minisatellite loci allow mutation induction to be measured at low doses of exposure in a small population, making minisatellite mutation a powerful tool to investigate radiation-induced heritable mutations. However, the biological significance of these mutations is uncertain, and their relationship to health risk or population fitness is unknown. We have adopted this mutation assay to study the role of adaptive response in protecting mice against radiation-induced heritable defects. We have shown that male mice, adapted to radiation with a low dose priming exposure, do not pass on mutations to their offspring caused by a subsequent large radiation exposure to the adapted males. This presentation and paper provide a general overview of radiationinduced mutations in offspring and explain the effect of low dose exposures and the adaptive response on these mutations. It is also known that exposure of pregnant females to high doses of radiation can cause death or malformation (teratogenesis) in developing fetuses. Malformation can only occur during a specialized stage of organ formation known as organogenesis. Studies in rodents show that radiation-induced fetal death and malformation can be significantly reduced when a pregnant female is exposed to a prior low dose of ionizing radiation. The mechanism of this protective effect, through an adaptive response, depends on the stage of organogenesis when the low dose exposures are delivered. To better understand this process, we have investigated the role of an important gene known as p53. Therefore, this report will also discuss fetal effects of ionizing radiation and explain the critical stages of development when fetuses are at risk. Research will be explained that investigates the biological and genetic systems (p53) that protect the developing fetus and discuss the role of low dose radiation adaptive response in these processes.

 ■        Low Doses of Radiation are Protective in vitro and in vivo: Evolutionary Origins. R. E. J. Mitchel Dose Response 4(2): 75-90, 2006 

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 Linear Energy Transfer (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 for low LET radiation, and that low doses of low LET radiation can actually reduce risk.  

■        Low-LET-induced radioprotective mechanisms within a stochastic two-stage cancer model,  H. Schöllnberger, R. D. Stewart, and R. E. J. Mitchel  Dose-Response, 3(4) 508-518, 2005  

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). 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. The model was applied to low doses of low-LET radiation delivered at low dose rates. 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.

 ■        The Bystander Effect:  Recent Developments and Implications for Understanding the Dose-Response.  R. E. J. Mitchel.  Nonlinearity in Biology, Toxicology and medicine, 2(3), 173-183, 2004.  

The bystander effect refers to the biological response of a cell resulting from an event in an adjacent or nearby cell. Such effects depend upon intercellular communication, and amplify the consequences of the original event. These responses are of particular interest in the assessment of ionizing radiation risk since at public or occupational exposure levels not every cell receives a radiation track. Current radiation protection regulations and practices are based on the assumption of a linear increase in risk with dose, including low doses where not all cells are hit.  Mechanisms that amplify biological effects are inconsistent with these assumptions.  Evidence suggests that there are two different bystander effects in mammalian cells. In one type, a radiation track in one cell leads to damaging, mutagenic and sometimes lethal events in adjacent, unhit cells. In the other type, a radiation track in one cell leads to an adaptive response in bystander cells, increasing resistance to spontaneous or radiation-induced events. This paper describes some of the data for radiation induced bystander effects in vitro and correlates that data with in vitro and in vivo observations of risk at low doses. The data suggest that beneficial bystander effects outweigh detrimental effects at doses below about 100 mGy, but that the reverse is true above this threshold.

■       Upper Dose Thresholds for Radiation-Induced Adaptive Response Against Cancer in High-Dose-Exposed, Cancer-Prone, Radiation-Sensitive Trp53 Heterozygous Mice.  R. E. J. Mitchel, J. S. Jackson and S. M. Carlisle. Radiation Research, 162(1): 20-30 2004. 

Groups of about 170 female mice heterozygous for Trp53 (Trp53 +/-) and their normal female littermates (Trp53 +/+) were exposed at 7-8 weeks of age to ⁶⁰Co-γ radiation doses of 0, 1, 2, 3 or 4 Gy at high dose rate (0.5 Gy/min) or 4 Gy at low dose rate (0.5 mGy/min). Unexposed heterozygous mice were at greater risk of early death than normal mice exposed to 4 Gy at either dose rate. In the absence of radiation exposure, heterozygosity for Trp53 reduced lifespan by death from both cancer and non-cancer reasons. Per unit dose, the absolute magnitude of life shortening for all radiation-exposed animals with malignant tumors and all animals without malignant tumors was the same when normal mice were compared to Trp53 heterozygous animals, indicating that Trp53 heterozygosity, did not confer increased life shortening, per unit dose, from high radiation doses.  In Trp53+/- mice, malignant tumor frequency increased and latency decreased with increasing acute doses up to 4 Gy, indicating that life shortening was related to both malignant tumor formation and decreased tumor latency. A similar malignant tumor response was observed in normal mice, but only up to 2 Gy. Above 2 Gy the fraction of Trp53 normal mice without tumors remained constant with increasing dose, indicating that above 2 Gy, normal Trp53 function protected against tumor initiation, and further life shortening reflected only decreased tumor latency. At high doses (4 Gy) delivered at low dose rate, dose rate reduction factors were in the range 1.7-4.6 for both animal genotypes and all end-points except one. The dose rate reduction factor for normal mice dying without malignant tumors was 36; about 20 fold greater than for Trp53 +/- mice, indicating a major role for Trp53 in the suppression of radiation-induced non-cancer mortality at low dose rates. We conclude that Trp53 heterozygous mice are not radiation sensitive on an absolute basis, that the Trp53 gene influences both cancer and non-cancer mortality in unexposed mice, and that the reduced effectiveness for both cancer induction and non-cancer mortality at increasingly high doses in genetically normal mice is governed by the functional activity of the Trp53 gene.

 ■        An Examination of Radiation Hormesis Mechanisms using a Multi-stage Carcinogenesis Model. H. Schöllnberger, R.D. Stewart, R. E. J. Mitchel, and W. Hofmann.  Nonlinearity in Biology, Toxicology and medicine, 2(4) 317-352 (2004).  

A multi-stage cancer model that describes the putative rate-limiting steps in carcinogenesis is developed and used to investigate the potential impact on cumulative lung cancer incidence of the hormesis mechanisms suggested by Feinendegen and Pollycove. In the model, radiation and endogenous processes damage the DNA of target cells in the lung. Some fraction of the misrepaired or unrepaired DNA damage induces genomic instability and, ultimately, leads to the accumulation of malignant cells. The model explicitly accounts for cell birth and death processes, the clonal expansion of initiated cells, malignant conversion, and a lag period for tumour formation. Radioprotective mechanisms are incorporated into the model by postulating dose and dose rate dependent radical scavenging. The accuracy of DNA damage repair also depends on dose and dose rate. As currently formulated, the model is most applicable to low-LET radiation delivered at low dose-rates.

Sensitivity studies are conducted to identify critical model inputs and to help define the shapes of the cumulative lung cancer incidence curves that may arise when dose and dose rate dependent cellular defense mechanisms are incorporated into a multi-stage cancer model. For lung cancer, both linear no-threshold (LNT) and non-LNT shaped responses can be obtained. If experiments demonstrate that the effects of DNA damage repair and radical scavenging are enhanced at least 3-fold under low dose conditions, our studies would support the existence of U-shaped responses. The overall fidelity of the DNA damage repair process may have a large impact on the cumulative incidence of lung cancer. The reported studies also highlight the need to know whether or not (or to what extent) multiply damaged DNA sites are formed by endogenous processes. Model inputs that give rise to U-shaped responses are consistent with an effective cumulative lung cancer incidence threshold that may be as high as 300 mGy (4 mGy per year for 75 years) for low-LET radiation.

■        Depleted Uranium Dust From Fired Munitions: Physical, Chemical And Biological Properties.  R. E. J. Mitchel and S. Sunder, Health Physics,  87(1): 57-67, 2004

This paper reports physical, chemical and biological analyses of samples of dust resulting from munitions containing depleted uranium (DU) that had been live-fired and had impacted an armoured target.  Mass spectroscopic analysis indicated that the average atom% of ²³⁵U was 0.198 +/- 0.10, consistent with depleted uranium. Other major elements present were Fe, Al and Si. About 47% of the total mass was particles with diameters <300 μm, of which about 14% was <10 μm.  X-ray diffraction analysis indicated that the uranium was present in the sample as uranium oxides - mainly U₃O₇ (47%), U₃O₈  (44%) and UO₂ (9%).  DU dust, instilled into the lungs or implanted into the muscle of rats, contained a rapidly soluble uranium component and a more slowly soluble uranium component. The fraction that underwent dissolution in 7 days declined exponentially with increasing initial burden.  At the lower lung burdens tested (<15 μg DU dust/lung) about 14% of the uranium appeared in urine within 7 days. At the higher lung burdens tested (~80-200 μg DU dust/lung) about 5% of the DU appeared in urine within 7 days. In both cases about 50% of that total appeared in urine within the first day. DU implanted in muscle similarly showed that about half of the total excreted within 7 days appeared in the first day. At the lower muscle burdens tested (<15 μg DU dust/injection site) about 9% was solubilized within 7 days. At muscle burdens >35 μg DU dust/injection site about 2% appeared in urine within 7 days.  Natural uranium (NU) ore dust was instilled into rat lungs for comparison. The fraction dissolving in lung showed a pattern of exponential decline with increasing initial burden similar to DU. However, the decline was less steep, with about 14% appearing in urine for lung burdens up to about 200 μg NU dust/lung and 5% at lung burdens >1100 µg NU dust/lung. NU also showed both a fast and a more slowly dissolving component. At the higher lung burdens of both DU and NU that showed lowered urine excretion rates, histological evidence of kidney damage was seen. Kidney damage was not seen with the muscle burdens tested. DU dust produced kidney damage at lower lung burdens and lower urine uranium levels than NU dust, suggesting that other toxic metals in DU dust may contribute to the damage.

■       Cytogenetic Dose-Responses in the Cells of Three Ungulate Species Exposed to High and Low Doses of Ionizing Radiation    B. Ulsh, S. Miller, D. Boreham, F. Mallory, R. E. J. Mitchel, and D. Morrison, , Journal of Environmental Radioactivity, 74: 73-81 2004

            In the studies reported here, the micronucleus assay, a common cytogenetic technique, was used to examine the dose-responses in fibroblast cells from three ungulate species (white-tailed deer, woodland caribou, and Indian muntjac) exposed to high doses of ionizing radiation (1-4 Gy of ⁶⁰Co gamma radiation). The same assay was also used to examine the effects of exposure to low doses (1-100 mGy) more typical of what these species might experience in a year from natural and anthropogenic environmental sources. An adaptive response, defined as the induction of resistance to a stressor by a prior exposure to a small “adapting” stress, was observed after exposure to low doses. The same level of protection was seen at all adapting doses, including 1 radiation track per cell, the lowest possible cellular dose. This result implies that environmental regulations predicated on the idea that even the smallest dose of radiation carries a quantifiable risk of direct adverse consequences to the exposed organism may not be suitable. This work indicates that very small doses are protective in the exposed organisms. Cytogenetic assays provide affordable and feasible biological effects-based alternatives that are more biologically relevant than traditional contaminant concentration-based radioecological risk assessment.

■       Skin Tumor Promotion by Vitamin E In Mice:  Amplification by Ionizing Radiation and Vitamin C.     R. E. J. Mitchel, and R. A. McCann,  Cancer Detection and Prevention, 27:102-108, 2003.

            We have shown previously that vitamin E acts as a tumor promoter in dimethylbenz(a)anthracene initiated mouse skin. We now show that high concentrations (80μm) of vitamin E are required for promotion, and that ten fold lower concentrations do not promote tumor formation. The same high concentration of the water-soluble antioxidant vitamin C did not act as a tumor promoter, but did amplify the promoting effect of high, but not low, concentrations of vitamin E. Oxidizing free radicals generated by β-radiation exposure of the skin at the time of vitamin E treatment also enhanced promotion by high (but not low) concentrations of vitamin E. The results are consistent with a process whereby tumor promotion by the lipid-soluble vitamin E occurs as a result of α-tocopherol acting as a free radical scavenger, with the formation and subsequent transfer of the α-tocopherol free radical center to the surrounding lipids, resulting in lipid oxidations.  

■        Low Doses of Radiation Increase the Latency of Spontaneous Lymphomas and Spinal Osteosarcomas in Cancer Prone, Radiation Sensitive Trp53 Heterozygous Mice  R. E. J. Mitchel, J. S. Jackson, D. P. Morrison and S. M. Carlisle,   Radiation Research, 159:320-7 (2003).

Mice heterozygous for Trp53 are radiation sensitive and cancer prone, spontaneously developing a variety of cancer types. Osteosarcomas in the spine lead to paralysis, while lymphomas lead rapidly to death, distinct events that provide objective measures of latency. The effects of a single low dose (10 or 100 mGy) low dose rate (0.5 mGy/min) ⁶⁰Co-g  irradiation on lymphoma or spinal osteosarcoma frequency and latency, defined as time of death or of onset of paralysis, respectively, were examined. Compared to spontaneous lymphomas or to spinal osteosarcomas leading to paralysis in unexposed mice, a 10 or 100 mGy exposure of 7-8 week old Trp53 +/- mice had no significant effect on tumor frequency, indicating no effect on tumor initiation. All tumors are therefore assumed to be of spontaneous origin. However, a 10 mGy exposure reduced the risk of both lymphomas and spinal osteosarcomas by significantly increasing tumor latency, indicating that the main in vivo effect of a low dose exposure is a reduction in the rate at which spontaneously initiated cells progress to malignancy. The effect of this adaptive response persisted for the entire lifespan of all the animals that developed these tumors.  Exposure to100 mGy delayed lymphoma latency longer than the 10 mGy exposure. However the 100 mGy dose increased spinal osteosarcoma risk by decreasing over-all latency compared to unexposed control mice. That result suggested that this higher dose was in a transition zone between reduced and increased risk, but that the dose at which the transition occurs varies with the tumor type.

■      Explanation of Protective Effects of Low Doses of g-Radiation With A Mechanistic Radiobiological Model    H. Schöllnberger, R. E. J. Mitchel, D. J. Crawford-Brown, and W. Hofmann,    International Journal of Radiation Biology, 78:1159-1173 (2002).

Purpose: To test whether data that show protective effects of low doses against spontaneous neoplastic transformation of C3H 10T1/2 cells can be explained with a biomathematical model that includes radioprotective mechanisms. Important features of the model will be identified with known biological processes and supporting evidence from the literature will be presented.

Materials and Methods: A model was used that simulates DSB formation in transcriptionally active and in bulk DNA, translocation of DNA segments, and the fixation of damage via mitosis. Promotion was also included. The model equations were solved numerically using a stiff solver.

Results: The data were successfully simulated by the model: cell transformation-reducing effects of low doses of g-radiation delivered at low dose-rates were explained by radiation-inducible DNA repair and enzymatic scavenging.

Conclusions: The data can be simulated with the model. The highly nonlinear features of the data point to a nonlinear dose-effect relationship at low doses and indicate that linear extrapolation from moderate (or high) to low doses and dose-rates may not be justified for in vitro studies of the cell line under consideration.  

■       Nonlinear dose–response relationships and inducible cellular defence mechanisms   H. Schollnberger, R. E. Mitchel, D. J. Crawford-Brown and W. Hofmann,   Journal of Radiological Protection 22:A21–A25 (2002).

With the inclusion of inducible radioprotective mechanisms in a radiobiological state-vector model it was possible to explain plateaus in dose–response relationships for neoplastic transformation produced by in vitro irradiation of different cell lines with low-LET irradiation at high dose rates. The current study repeated the simulation of one data set that contains a plateau at mid doses. In contrast to earlier studies, the new one did not model the repair of double-strand breaks (DSBs) located in bulk DNA (likely via non-homologous end joining) as being inducible. Repair of specific DSBs located in actively transcribed genes was assumed to occur via homologous recombination and was considered to be inducible. This reduced the number of parameters that have to be determined by fitting the model to data. In addition, all types of radical scavengers were formerly considered to be inducible by radiation. This was redefined in the current work and the effectiveness of scavengers was implemented in a refined way. The current work investigated whether these and other model adjustments lead to an improved fit of the data set. 

■           Teratogenic Effects of Mild Heat Stress During Mouse Embryogenesis: Effect of Trp53

         D. R. Boreham, J-A. Dolling, J. Misonoh, and R.E.J. Mitchel.  Radiation Research 158:443-448 (2002).

         Hyperthermia can be teratogenic in fetal mice exposed during organogenesis, an effect considered to be due to heat-induced apoptosis of cells in the developing organs. We exposed pregnant mice carrying Trp53 +/+, Trp53 +/- and Trp53 -/- fetuses to mild whole-body hyperthermia that raised their core temperature to 40.5ºC for 60 min on either day 10 or 11 of gestation. On day 18 of gestation, the fetuses were removed from control and hyperthermia treated mice, genotyped, and tail length measured. Limb digits were examined for abnormalities.  Tail length in unheated control fetuses was influenced by Trp53 status. A complete lack of functional Trp53 ( Trp53 -/-) but not partial lack of function (Trp53 +/-) resulted in shorter tails, as compared to Trp53 +/+ fetuses, indicating a role for Trp53 in the regulation of tail lengthening in mouse fetuses.  In all three genotypes, hyperthermia on gestational day 10 resulted in tails shorter than unheated controls, and hyperthermia on day 11 resulted in tails longer than controls. There was no effect on limb digit abnormalities. The data suggest that Trp53 dependent or independent apoptosis may not be directly involved in heat induced teratogenesis, but that the primary teratogenic effect of heat results from the disruption of another tail length regulating process that is independent of Trp53. However, the nature of the teratogenic outcome of that disruption depends on the gestation time.  The ability of Trp53 to additionally regulate the tail lengthening process was also sensitive to the effects of heat, but that sensitivity again depended on the gestational time of the heat stress.

■            Radiation Induced Teratogenic Effects in Fetal Mice with Varying Trp53 Function: Influence of Prior Heat Stress

         D. R. Boreham, J-A. Dolling, J. Misonoh, and R.E.J. Mitchel  Radiation Research 158:449-457 (2002).

         Teratogenesis induced by radiation in fetal mice has been closely linked to Trp53 dependent apoptosis. This study examined teratogenesis in tails and limb digits of fetal mice with varying Trp53 status, after a 4 Gy radiation exposure, with and without a prior 40.5ºC, 60 min heat stress. Irradiation earlier in gestation (day 11) produced greater effects than later (day 12) exposure, but in both cases the maximum teratogenic effect of radiation occurred in Trp53 normal fetuses, the minimum in Trp53 null fetuses and intermediate effects in Trp53 heterozygotes, indicating dominance of Trp53 dependent apoptosis. Heat stress 24 h prior to radiation on day 11 did not alter the teratogenic effects in Trp53 normal or heterozygous fetuses, but reduced effects in the Trp53 null fetuses. Conversely, heat stress immediately before day 11 irradiation, amplified teratogenesis in Trp53 null fetuses, still with little or no effect on fetuses with full or partial Trp53 function. These results indicate no effect of mild heat on Trp53 dependent apoptotic responses to radiation, but also suggest heat-induced amplification of Trp53 independent apoptotic processes, when heat is delivered near the time of radiation exposure, and heat-induced protection of that process when sufficient expression time was allowed. However, Trp53 dependent apoptosis, when functional, acted as the ultimate determinant of radiation-induced teratogenic effects during early organogenesis. On gestational day 12, radiation effects were diminished, but heat stress 24 h prior to radiation exposure had a large amplifying effect in Trp53 normal or heterozygous fetuses. In the absence of functional Trp53, the sensitizing effect of the heat was diminished. The results may suggest that at later times in organ development, DNA repair is more active, allowing some cells to escape radiation induced Trp53 dependent apoptosis. However, heat may be able to significantly inhibit this active repair and increase the teratogenic effect of radiation. A diminished effect in the absence of functional Trp53 is consistent with an influence of heat on inhibiting DNA repair, but with a diminished probability of apoptosis.

■    Influence of Prior Exposure to Low Dose Adapting Radiation on Radiation-Induced Teratogenic Effects in Fetal Mice With Varying Trp53 Function

         R.E.J. Mitchel, J-A. Dolling, J. Misonoh, and D. R. Boreham   Radiation Research 158:458-463 (2002).

         Teratogenesis in tails and limb digits of fetal mice with varying Trp53 status was examined after a 4 Gy radiation exposure of pregnant females, with and without a prior 30 cGy exposure. Prior low dose exposure modified the teratogenic effects of radiation in a manner dependent upon Trp53 status and gestational time. A 4 Gy exposure on gestational day 11 resulted in tail shortening and digit abnormalities. A 30 cGy exposure 24 h prior to a 4 Gy radiation exposure on day 11 reduced the extent of both digit abnormalities and the tail shortening effects in Trp53 +/+ fetuses, and also reduced tail shortening in Trp53 +/- fetuses, but to a lesser extent. However, the pre-exposure enhanced the tail shortening effects of 4 Gy in Trp53 -/- fetuses. In contrast, a 30 cGy exposure given 24 h prior to a 4 Gy exposure on gestational day 12 had no effect on the reduced tail length resulting from the 4 Gy exposure of Trp53 +/+ or Trp53 +/- fetuses, but partly protected Trp53 -/- fetuses against reduced tail length. A 4 Gy exposure alone on day 12 did not result in any increase in the frequency of digit abnormalities in Trp53 -/- fetuses so any protective effect of the pre-irradiation could not be detected. However, the pre-irradiation did result in protection against in digit abnormalities in Trp53 +/- fetuses. We conclude that radiation-induced teratogenesis reflects both Trp53 dependent and independent apoptotic processes, and these respond differently to prior adapting doses.

■           Dose Responses for Adaption to Low Doses of (60)Co gamma Rays and (3)H beta Particles in Normal Human Fibroblasts.

E. J. Broome, D. L. Brown and R. E. J. Mitchel  Radiation Research. 158:181-186 (2002)

The dose response for adaption to radiation at low doses was compared in normal human fibroblasts (AG1522) exposed to either (60)Co gamma rays or (3)H beta particles. Cells were grown in culture to confluence and exposed at either 37 degrees C or 0 degrees C to (3)H beta-particle or (60)Co gamma-ray adapting doses ranging from 0.1 mGy to 500 mGy. These cells, and unexposed control cells, were allowed to adapt during a fixed 3-h, 37 degrees C incubation prior to a 4-Gy challenge dose of (60)Co gamma rays. Adaption was assessed by measuring micronucleus frequency in cytokinesis-blocked, binucleate cells. No adaption was detected in cells exposed to (60)Co gamma radiation at 37 degrees C after a dose of 0.1 mGy given at a low dose rate or to 500 mGy given at a high dose rate. However, low-dose-rate exposure (1-3 mGy/min) to any dose between 1 and 500 mGy from either radiation, delivered at either temperature, caused cells to adapt and reduced the micronucleus frequency that resulted from the subsequent 4-Gy exposure. Within this dose range, the magnitude of the reduction was the same, regardless of the dose or radiation type. These results demonstrate that doses as low as (on average) about one track per cell (1 mGy) produce the same maximum adaptive response as do doses that deposit many tracks per cell, and that the two radiations were not different in this regard. Exposure at a temperature where metabolic processes, including DNA repair, were inactive (0 degrees C) did not alter the result, indicating that the adaptive response is not sensitive to changes in the accumulation of DNA damage within this range. The results also show that the RBE for low doses of tritium beta-particle radiation is 1, using adaption as the end point.

■           Low-Dose Radiation Risk: A Biological Reality Check,

R. E. J. Mitchel.  Radwaste Solutions (published by the American Nuclear Society) 9: 30-35, 2002. 

All current radiation risk estimates and all radiation-protection standards and practices are based on the so-called “Linear No-Threshold Hypothesis”. This paper summarizes results from some of our low dose and/or low dose rate experiments with low LET radiation in human and rodent cells, and in animals, and determines if the results support or reject the LNT hypothesis.

When DNA damage is created  by radiation in a cell, there are three possible outcomes, an error-free repair which restores the cell to normal, cell death by apoptosis, or error prone repair that creates a mutation and cancer risk.  The LNT hypothesis predicts that risk is influenced only by dose, and therefore that the relative proportions of these three biological possibilities must be constant.  If they were not constant, then risk would vary with their relative proportions, i.e., not only as a function of dose. We have tested the influence of prior low doses and low dose rate exposures on these processes. As a measure of the overall effect of these processes, we measured the frequency at which rodent cells in tissue culture are transformed into cancer cells after a low dose exposure.

The results show that low dose radiation induces an increase in error-free DNA repair competence. That repair system increases the probability of correctly repairing either radiation-induced or spontaneous DNA damage, or of triggering cell death if the repair is incorrect. This response therefore reduces the overall risk of either radiation-induced or spontaneous transformation to malignancy. It is apparent from these experiments that biological variables are important in determining the consequences of radiation exposures and that the risk of DNA damage is neither constant nor additive nor increasing with dose.

We have reported the results of similar investigations in mice. In one experiment, low doses of in vivo b-irradiation of mouse skin 24 h prior to treatment with a DNA damaging chemical carcinogen reduced tumor frequency by about 5-fold.  This result is consistent with the cell-based studies described above. It implies that the radiation exposure stimulated an error-free DNA repair system that was able to recognize and remove much of the chemically produced DNA damage. In another experiment, a prior low dose exposure delivered at low dose rate delayed the onset of myeloid leukemia induced in genetically normal mice by a subsequent exposure to a large dose. A similar result was seen in mice that were cancer prone due to a genetic defect, showing that low doses also protect mice predisposed to cancer.

The results indicate that low doses, or  doses delivered at low dose rate, reduce rather than increase cancer risk in cells and in animals. The results contradict the LNT hypothesis. 

■           Radiation Biology of Low Doses

R. E. J. Mitchel,  ATW. International Zeitschrift fur Kernenergie, 47(1): 28-30, 2002

          All current radiation risk estimates and all radiation-protection standards and practices are based on the so-called “Linear No-Threshold Hypothesis”  which states that risk is linearly proportional to dose, without a threshold.  This hypothesis therefore predicts that: 

·        every dose, no matter how low, carries with it some risk

·        risk per unit dose is constant, additive, and can only increase with dose

·        biological variables are insignificant compared to dose 

This talk summarizes results from some of our low dose and/or low dose rate experiments with low LET radiation in human and rodent cells, and in animals, and determines if the results support or reject the LNT hypothesis as it affects the risk of most concern, cancer. It is important to recognize that cancer arises from changes in a single cell and, therefore, this defines the limits of the meaning of “low dose”.  Unlike the concept of whole body dose, where dose is averaged over all cells in the body, a single cell is the smallest volume that is relevant for carcinogenic risk.  The lowest possible dose is, therefore, that dose which can be deposited in a single cell.  

         It is also important to recognize some physical characteristics of radiation: 

·        radiation deposits energy, and damage, in tracks

·        the smallest dose a cell can receive is that deposited by a single track

·        at total doses which are less than one track/cell, not all cells are hit, i.e., some cells receive no dose; however, those that are hit still receive the dose deposited by one track.

 While the lowest possible dose to a cell is that deposited by one track, the actual dose depends on the nature of the radiation.  For example, a single alpha particle track can deposit tens of cGy while a single ⁶⁰Co-g ray will deposit, on average, about 1 mGy.

          When DNA damage is created  by radiation in a cell, there are three possible outcomes, an error-free repair which restores the cell to normal, cell death by apoptosis, or error prone repair that creates a mutation and cancer risk.  The LNT hypothesis predicts that risk is influenced only by dose, and therefore that the relative proportions of these three biological possibilities must be constant.  If they were not constant, then risk would vary with their relative proportions, i.e., not only as a function of dose. 

We have tested the influence of prior low doses and low dose rate exposures on the ability of normal human skin cells to repair subsequent acute radiation damage to DNA which results in breaks in chromosomes. The combined exposure resulted in less broken chromosomes than the single acute exposure alone. The low dose rate exposure stimulated the cells to increase their ability to repair broken chromosomes, such that the consequences of a second large exposure were reduced. The same result occurred if the initial exposure was 100 mGy, or was 1 mGy, the lowest g dose possible in a single cell. This adaptive response to low doses of radiation can be seen in many other situations. For example, the influence of a low dose on cell death by apoptosis has also been tested. Those results show that low doses amplify the probability of apoptotic cell death resulting from a second exposure.  This sensitization of cells to radiation-induced cell death increases the probability that a cell will die rather than survive with a mutation, another type of adaptive response that is believed to reduce cancer risk in the whole organism.   

 As a measure of the overall effect of these processes, we used an assay that measures the frequency at which rodent cells in tissue culture are transformed into cancer cells.  We showed that a low dose rate exposure immediately before a large acute exposure did not further increase risk, as predicted by the LNT hypothesis, but actually decreased cancer risk by 2-3 fold.  In the absence of the second large exposure, an average of one track per cell (1 mGy) reduced the risk of cancer formation below that which occurred spontaneously in the absence of any radiation exposure.  Higher doses, up to 100 mGy delivered at a low dose rate, produced the same 2-3 fold reduction in spontaneous transformation risk. Since at 1 mGy not all cells actually receive a track of radiation, these results also indicate that some cells are protected in response to signals received from other cells that did receive a radiation track, an example of the bystander effect for adaption to radiation. 

The results show that low dose radiation induces an increase in error-free DNA repair competence. That repair system increases the probability of correctly repairing either radiation-induced or spontaneous DNA damage, or of triggering cell death if the repair is incorrect. This response therefore reduces the overall risk of either radiation-induced or spontaneous transformation to malignancy. It is apparent from these experiments that biological variables are important in determining the consequences of radiation exposures and that the risk of DNA damage is neither constant nor additive nor increasing with dose. Low doses or  doses delivered at low dose rate reduce rather than increase risk in normal cells. The results contradict  the LNT hypothesis. 

We have reported the results of similar investigations in mice. In one experiment, low doses of in vivo b-irradiation of mouse skin 24 h prior to treatment with a DNA damaging chemical carcinogen reduced tumor frequency by about 5-fold.  This result is consistent with the cell-based studies described above. It implies that the radiation exposure stimulated an error-free DNA repair system that was able to recognize and remove much of the chemically produced DNA damage. In another experiment, a prior low dose exposure delivered at low dose rate delayed the onset of myeloid leukemia induced in genetically normal mice by a subsequent exposure to a large dose. The protective responses observed in mammalian cells and in animals are consistent with those seen in lower eukaryotes, including yeast, indicating that they are evolutionarily conserved and lending credence to the idea that such responses are the normal and expected consequences of low dose exposures. 

It seems clear that in normal cells and normal adult animals, low doses and low dose rate exposures to low LET radiation decrease rather than increase cancer risk. However, the effects of low doses in two other important situations, exposure of cancer prone individuals and exposure of a fetus, are less clear and we are investigating those problems.  We have recently examined cancer risk after low dose exposure in mice that were cancer prone due to a genetic defect (heterozygosity for the gene p53), and showed that a low dose (10 mGy) also protects these cancer prone mice.  Another study of malformations in fetal mice showed that low doses can also induce an adaptive response that protects against radiation induced teratogenic effects, although this can occur only at certain times of gestation, and defects in the p53 gene can alter that protection. 

Since, at low doses and dose rates, there are no data in the literature that support the LNT as a general hypothesis for cancer risk, and considerable evidence contradicting it, including the evidence given above, then this hypothesis must be rejected. Some of the basic principals used in radiation protection, such as ALARA, as low as reasonably achievable, and the precautionary principle are not consistent with the biology of low doses. It is time for a new risk based approach to radiation protection, firmly linked to the actual biological responses.

■        Uranium and uranium decay series radionuclide dynamics in bone of rats following chronic uranium ore dust inhalation.
Dewit T, Clulow V, Jackson JS, Mitchel REJ  Health Physics. 81:502-13 (2001)
 
The accumulation and release of uranium and some uranium decay chain radionuclides were measured in the bones of rats that had been chronically exposed to inhaled uranium ore dust during the first half (approximately) of their natural adult lifespan. Endochondral bone (femur, tibia, humerus, radius, and ulna), membrane bone (skull roofing bones) and muscle of Sprague-Dawley rats (n = 55) that died at various times up to 65 weeks after the end of chronic inhalation of uranium ore dust aerosol (4.2 h d(-1) for 65 wk) and from age matched controls (n = 10), were analyzed for uranium, 230Th, 226Ra, 210Pb, and 210Po. Overall, during the period of dust inhalation, the nuclides accumulated in the above order of decreasing concentration in dry bone. However, the results demonstrate that there was some differential accumulation of uranium and uranium decay series radionuclides in muscle and two bone types of rats during the chronic inhalation period. The data also show that the bone levels of some, but not all, radionuclides decreased significantly with time after inhalation ceased. Lung uranium concentration at the time of death was a highly significant covariant for temporal changes in the levels of some radionuclides in both endochondral bone and membrane bone, indicating that lung remained a major source of these isotopes for accumulation in these bone types after ore dust inhalation had ceased. For some isotopes, the two bone types behaved differently during the dust inhalation period, and differently again after the dust inhalation ceased. The relative behavior of one bone type compared to the other for a particular isotope during the dust inhalation period did not predict the relative behavior after dust inhalation ceased. However, a faster accumulation of one bone type compared to the other for a particular isotope during the dust inhalation period predicted a faster decrease after dust inhalation ended.

■         Radiation Protection in the World of Modern Radiobiology: Time for A New Approach

R. E. J. Mitchel and D. R Boreham  Proceedings of 10th International Congress of the International Radiation Protection Association, Plenary Session 1-2 p. 140, Hiroshima, Japan, 2000.

 Current radiation protection practices utilize the concept of ALARA, based on the assumptions of the Linear No-Threshold Hypothesis (LNT). Operationally, the LNT hypothesis is attractive since it is viewed as conservative, uses dose as a surrogate for risk and assumes linearity. Risk control therefore translates into protection against exposure. Since every dose, no matter how low, is assumed to produce some risk, these assumptions, unfortunately, also translate into very large costs, as well as the logical public perception that there is no safe dose. Large amounts of money are being spent to reduce dose, and to protect workers and the public against exposure levels which are a small fraction of natural background, but this effort has only amplified public concern. 

Unfortunately, no actual scientific data support this approach at occupational and public exposure levels. If the nuclear industry is to regain public acceptance, it is crucial that risk estimates and radiation protection be based on sound science. Since the risk is biological, risk management must be based firmly on the actual biological responses of cells and whole organisms, rather than assumptions. Modern techniques in cellular and molecular radiobiology have recently allowed tests of the actual effects of such doses.  

Cancer is the risk of most concern, and since cancer ultimately arises from a series of genetic changes in a single cell, it is necessary to understand the effects of radiation on single cells. There are three potential outcomes in a cell of radiation generated DNA damage; cell death, correct repair, or incorrect repair creating a mutation which generates the risk of carcinogenesis.  Our experiments show that cells respond to low doses by altering the relative probabilities of these three possible outcomes, and therefore that risk is controlled by biology and not dose. Human and rodent cells, exposed to the lowest dose a cell can receive, an average of about one track per cell, or to many tracks per cell, responded by increasing their ability to correctly repair broken chromosomes. Cells unable to adequately repair their chromosomes were sensitized to die by apoptosis.  These “adaptive responses” of cells reduced the risk of being transformed into cancer cells by a subsequent exposure, and also protected them against their own inherent, spontaneous risk of transforming into cancer cells without further exposure. Mouse in vivo experiments showed that prior low doses reduced tumor formation from exposure to chemical carcinogens and delayed the onset of radiation induced leukemia, further indicating that low doses reduce rather than increase risk. 

These experiments show that at low dose, the assumptions of the LNT hypothesis were not supported by the human cell or animal data. The data indicate that the use of the LNT hypothesis and ALARA is not conservative, but may actually increase the overall risk of cancer. These biological realities call for a new risk-based approach to radiation protection, where the real biological effects of low doses are utilized to reduce the effects of a large accidental exposure, and reduce the incidence and severity of cancers arising from other causes.

■         Dose-rate effects for apoptosis and micronucleus formation in gamma-irradiated human lymphocytes.
Boreham DR, Dolling JA, Maves SR, Siwarungsun N, Mitchel REJ  Radiation  Research. 153: 579-586, (2000)
 
We have compared dose-rate effects for gamma-radiation-induced apoptosis and micronucleus formation in human lymphocytes. Long-term assessment of individual radiation-induced apoptosis showed little intraindividual variation but significant interindividual variation. The effectiveness of radiation exposure to cause apoptosis or micronucleus formation was reduced by low-dose-rate exposures, but the reduction was apparent at different dose rates for these two end points. Micronucleus formation showed a dose-rate effect when the dose rate was lowered to 0.29 cGy/min, but there was no accompanying cell cycle delay. A further increase in the dose-rate effect was seen at 0.15 cGy/min, but was now accompanied by cell cycle delay. There was no dose-rate effect for the induction of apoptosis until the dose rate was reduced to 0.15 cGy/min, indicating that the mechanisms or signals for processing radiation-induced lesions for these two end points must be different at least in part. There appear to be two mechanisms that contribute to the dose-rate effect for micronucleus formation. One of these does not affect binucleate cell frequency and occurs at dose rates higher than that required to produce a dose-rate effect for apoptosis, and one affects binucleate cell frequency, induced only at the very low dose rate which coincidentally produces a dose-rate effect for apoptosis. Since the dose rate at which cells showed reduced apoptosis as well as a further reduction in micronucleus formation was very low, we conclude that the processing of the radiation-induced lesions that induce apoptosis, and some micronuclei, is very slow in quiescent and PHA-stimulated lymphocytes, respectively.

■         Role of RAD9-dependent cell-cycle checkpoints in the adaptive response to ionizing radiation in yeast, Saccharomyces cerevisiae.
Dolling JA, Boreham DR, Bahen ME, Mitchel REJ  International Journal of Radiation Biology. 76: 1273-1280, (2000)
 
PURPOSE: To determine whether yeast cells (Saccharomyces cerevisiae) defective in damage-inducible cell-cycle arrest can invoke an adaptive response and become resistant to normally lethal doses of ionizing radiation. MATERIALS AND METHODS: Wild-type yeast cells, cells defective for DNA-damage-responsive G1 and G2 cell-cycle arrest (rad9delta), and cells defective for recombinational repair of DNA damage (rad50, 51, 52) were subjected to adapting treatments of heat or radiation and subsequently exposed to normally lethal doses of radiation. Survival, as measured by colony-forming ability, was compared with non-adapted, control cells. RESULTS: Wild-type and rad9delta cells became more resistant to potentially lethal doses of radiation after exposure to conditions that are known to elicit the adaptive response. Further, the relative magnitude of resistance developed by the normal, wild-type and rad9delta yeast cells was similar, with a dose modifying factor (at D1) for radiation-induced radiation resistance of 1.3 for both strains. Dose modifying factors (at D1) for heat-induced radiation resistance were 1.7 and 1.6 for wild-type and rad9delta cells, respectively. In contrast, none of the recombinational repair-defective cells exhibited radiation resistance after an adapting treatment. CONCLUSIONS: The ability of yeast cells to arrest in cell-cycle gap phases did not appear to contribute significantly to radiation resistance induced by radiation or heat. Instead, it is suggested that the adaptive response was due mainly to the existence and enhancement of cellular recombinational repair capacity, which was sufficient to repair any DNA damage without the requirement of a detectable cell-cycle delay.

■         Apoptosis And The Adaptive Response In Human Lymphocytes.
Cregan SP, Brown DL, Mitchel REJ   International Journal of Radiation Biology. 75:1087-1094, (1999)

PURPOSE: To determine whether the sensitivity of human lymphocytes for apoptosis induced by either a membrane oxidizing agent or a DNA damaging agent is modified by an adaptive response. MATERIALS AND METHODS: Peripheral blood lymphocytes from normal human donors were exposed to low doses of the DNA damaging agent gamma-radiation, or the membrane oxidizing agent t-butyl hydroperoxide (t-BuOOH), incubated for various times and then tested for their sensitivity to induction of apoptosis by a subsequent exposure to a high dose of either agent. Apoptosis was measured using a fluorescent assay of DNA unwinding or a terminal deoxynucleotide transferase assay. RESULTS: The results show that Go lymphocytes pre-exposed to an adapting dose of radiation or DNA strand breaking agent are not protected but can become sensitized to subsequent apoptosis induced by radiation (a kinetically slow process). Inter- and intraindividual variations were observed. However, neither pre-exposure to radiation nor to a membrane oxidizing agent sensitized lymphocytes from any donor to apoptosis induced by a membrane oxidizing agent (a kinetically fast process). CONCLUSIONS: Since an increase in the elimination of genetically damaged cells by apoptosis could reduce the risk of cancer from exposure to radiation or other DNA damaging agents, this cellular sensitization for apoptosis may represent a novel adaptive response mechanism.

■        Two pathways for the induction of apoptosis in human lymphocytes.
Cregan SP, Smith BP, Brown DL, Mitchel REJ  International Journal of Radiation Biology. 75:1069-1086, (1999)
 
PURPOSE: To assess the roles of cell membranes and DNA as targets in radiation-induced apoptosis. MATERIALS AND METHODS: Peripheral blood lymphocytes from normal human donors were exposed to different types of apoptosis-inducing agents. Several measures of apoptosis were used to compare the kinetics of the processes induced, as well as to correlate the processes with DNA damage and membrane oxidation. RESULTS: Two kinetically distinct processes were observed. DNA-damaging agents, such as ionizing radiation, bleomycin, cisplatin and the topoisomerase inhibitor m-amsacrine, induced apoptosis by a kinetically slow process initiated by DNA damage and dependent on protein synthesis, but which did not correlate with membrane oxidation. Conversely, the agents t-butyl hydroperoxide and cumene hydroperoxide induced apoptosis by a kinetically fast process independent of protein synthesis and which did correlate with membrane oxidation. CONCLUSIONS: Slowly repaired or unrepairable DNA lesions, such as some of those produced by ionizing radiation exposure, trigger apoptosis by a kinetically slow process. This slow apoptotic pathway is distinct from a fast process not induced by radiation but which is induced by membrane-oxidizing agents.

■        The adaptive response modifies latency for radiation-induced myeloid leukemia in CBA/H mice.
Mitchel REJ, Jackson JS, McCann RA, Boreham DR.  Radiation Research. 152:273-279, (1999)

We have investigated the effect of the adaptive response on acute myeloid leukemia (AML) induced in CBA/Harwell mice by a chronic radiation exposure. Groups of mice irradiated with a total dose of 1. 0 Gy at two different chronic dose rates (0.5, 0.004 Gy/h) had similar frequencies of AML. Compared to control animals that did not develop AML, irradiation at either of these dose rates did not change the longevity of the mice that did not die of leukemia. The survival rates of irradiated mice that did develop leukemia in the two groups were not different from each other, indicating that the dose rates produced similar responses and therefore were both chronic exposures. We then tested the ability of a chronic 10-cGy (0. 5 Gy/h) exposure to ionizing radiation, mild hyperthermia (40.5 degrees C whole-body, 60 min) or treatment with interleukin-1 (1500 U i.p.) to induce an adaptive response and modify the frequency or latency of AML which resulted from a subsequent (24 h later) 1.0-Gy (0.5 Gy/h) chronic radiation exposure. The frequency of radiation-induced leukemia was not changed in mice given any of the three adapting treatments 24 h prior to the chronic 1.0-Gy dose that induced leukemia. However, the latent period for development of AML was significantly increased by both the prior low radiation dose and mild hyperthermia treatment. Injection of interleukin-1, in contrast, may have reduced the latent period. Similar to the single 1.0-Gy chronic exposure alone, none of the adapting treatments prior to that exposure influenced the survival of animals that did not develop AML. These results indicate that an earlier exposure to a small adapting dose of radiation or to a mild heat stress can influence secondary steps in radiation-induced carcinogenesis.

■        Cisplatin-modification of DNA repair and ionizing radiation lethality in yeast, Saccharomyces cerevisiae.
Dolling JA, Boreham DR, Brown DL, Raaphorst GP, Mitchel REJ  Mutation Research. 433:127-136, (1999)

 Cis-diamminedichloroplatinum II (cisplatin) is a DNA inter- and intrastrand crosslinking agent which can sensitize prokaryotic and eukaryotic cells to killing by ionizing radiation. The mechanism of radiosensitization is unknown but may involve cisplatin inhibition of repair of DNA damage caused by radiation. Repair proficient wild type and repair deficient (rad52, recombinational repair or rad3, excision repair) strains of the yeast Saccharomyces cerevisiae were used to determine whether defects in DNA repair mechanisms would modify the radiosensitizing effect of cisplatin. We report that cisplatin exposure could sensitize yeast cells with a competent recombinational repair mechanism (wild type or rad3), but could not sensitize cells defective in recombinational repair (rad52), indicating that the radiosensitizing effect of cisplatin was due to inhibition of DNA repair processes involving error free RAD52-dependent recombinational repair. The presence or absence of oxygen during irradiation did not alter this radiosensitization. Consistent with this result, cisplatin did not sensitize cells to mutation that results from lesion processing by an error prone DNA repair system. However, under certain circumstances, cisplatin exposure did not cause radiosensitization to killing by radiation in repair competent wild type cells. Within 2 h after a sublethal cisplatin treatment, wild type yeast cells became both thermally tolerant and radiation resistant. Cisplatin pretreatment also suppressed mutations caused by exposure to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), a response previously shown in wild type yeast cells following radiation pretreatment. Like radiation, the cisplatin-induced stress response did not confer radiation resistance or suppress MNNG mutations in a recombinational repair deficient mutant (rad52), although thermal tolerance was still induced. These results support the idea that cisplatin adducts in DNA interfere with RAD52-dependent recombinational repair and thereby sensitize cells to killing by radiation. However, the lesions can subsequently induce a general stress response, part of which is induction of RAD52-dependent error free recombinational repair. This stress response confers radiation resistance, thermal tolerance, and mutation resistance in yeast.

■        Adaptation of human fibroblasts to radiation alters biases in DNA repair at the chromosomal level.
Broome EJ, Brown DL, Mitchel REJ  International Journal of Radiation Biology. 75:681-690 (1999)

 PURPOSE: To determine whether adaptation to ionizing radiation biases repair of radiation-induced chromosomal breaks. MATERIALS AND METHODS: Normal human fibroblasts were radiation-adapted by exposure to 10 cGy of gamma-radiation. FISH probes for chromosomes 2, 4, 7, 18 and 19 were used to determine the chromosomal origin of the DNA in micronuclei resulting from a subsequent 4Gy exposure of these cells, and corresponding non-adapted cells. RESULTS: Compared with 4 Gy exposed but non-adapted cells, the radiation-adapted cells subsequently exposed to 4 Gy showed an overall decrease in the frequency of micronuclei. However, the micronuclei that did form in the adapted cells had a decreased frequency of DNA originating from chromosomes 2 and 18, an increased frequency of DNA from chromosome 19 and no change in frequency of DNA from chromosomes 4 and 7. CONCLUSIONS: Adaptation to radiation increased the overall cellular repair of radiation-induced chromosomal breaks, but also created a repair bias such that some chromosomes were preferentially repaired or discriminated against, while the repair of others was unbiased.

■         Fluorescence in situ hybridization of micronuclei in binucleate fibroblasts: a protocol for cytoplasm preservation.
Broome EJ, Brown DL, Mitchel REJ  Biotechniques.  26:610-612, 614 (1999)

Previous studies using fluorescence in situ hybridization (FISH) in combination with the cytokinesis-block micronucleus technique have had limited success in preserving the cytoplasm of binucleated cells.  Some reports have discounted the problem of loss of cytoplasm, basing the identification of a binucleate cell on the close proximity of two nuclei each having roughly the same size and shape.  However without cytoplasm to distinguish one cell from another this scoring criteria is highly subjective.  The uncertainty in association between nuclei can be reduced by using low cell density, but this greatly reduces scoring efficiency and increases costs.  These concerns prompted a search for a protocol which would allow in situ hybridization while maintaining binucleate cell cytoplasm.  The development of such a FISH protocol using directly labeled whole chromosome paints in human fibroblasts is outlined in this report.

■         Low Doses Of Ionizing Radiation Incurred At Low Dose Rates
Prepared by the Task Group on Low Doses of the INTERNATIONAL NUCLEAR SOCIETIES COUNCIL
John Graham (USA), Donald J Higson (Australia), Chairman, Jae-Shik Jun (Korea), Sadayoshi Kobayashi (Japan), Ronald E J Mitchel (Canada)
Radiation Protection in Australasia; 16:32-47 (1999)

This paper is a draft report by a Task Group of the International Nuclear Societies Council. It addresses the scientific information available on the biological effects of low radiation doses and dose rates, defined for the purpose of the report as -

* total doses less than 10 mSv, received at high rates in single events, or
* dose rates less than 20 mSv per year, received continuously.

lt is concluded that there is no scientific evidence which supports the hypothesis that radiation causes an increase in the incidences of cancers or hereditary effects in humans at low doses.

For radiation protection purposes, the International Commission on Radiological Protection recommends the assumption that the risk of radiation induced cancer is proportional to the dose without a threshold. However, at low doses and low dose rates, the available evidence indicates either that there is no significant risk or that there may be benefits from exposure. For all purposes other than scientific research, the Task Group therefore recommends the assumption (on the current basis of information) that there is no significant biological effect from low doses of radiation.

There is a range of views amongst members of the Task Group on several matters, particularly the biopositive effects of low radiation doses. However, there is complete agreement that the possibility and significance of big-positive effects from radiation exposure of humans need to be accepted and investigated without prejudice.

■         Inhaled uranium ore dust and lung cancer risk in rats.
Mitchel REJ, Jackson JS, Heinmiller B.  Health Physics. 76:145-155, (1999)
 
Using a nose-only inhalation system, male Sprague-Dawley rats were exposed 4.2 h d(-1), 5 days per week for 65 weeks to one of two concentrations of natural uranium ore dust aerosol (44% U, 50 mg m(-3) and 19 mg m(-3)) without significant radon content. After inhalation exposure ceased, the rats were allowed to live for their natural lifetime. Lung uranium burdens, measured at the time of death of each animal, declined exponentially after dust inhalation ceased, and the rate of decline was independent of the initial lung burden. Lymph node specific burdens ranged from 1 to 60 fold greater than the specific lung burden in the same animal. No lymph node tumors were observed. The frequency of primary malignant lung tumors was 0.016, 0.175 and 0.328 and primary non-malignant lung tumors 0.016, 0.135 and 0.131 in the control, low and high aerosol exposed groups, respectively. There was no difference in tumor latency between the groups. Absorbed dose to the lung was calculated for each animal in the study. The average doses for all the animals exposed to the low and high dust aerosol concentrations were 0.87 Gy and 1.64 Gy respectively, resulting in an average risk of malignant lung tumors of about 0.20 tumors per animal per Gy in both groups. The frequency of primary lung tumors was also calculated as a function of dose increment for both exposed groups individually and combined. The data indicate that, in spite of the above result, lung tumor frequency was not directly proportional to dose. However, when malignant lung tumor frequency was calculated as a function of dose rate (as measured by the lung burden at the end of dust inhalation) a direct linear relationship was seen (p < 0.01) suggesting dose rate may be a more important determinant of lung cancer risk than dose. Conversely, non-malignant lung tumors were significantly correlated with low lung burdens (p = 0.01). We conclude that chronic inhalation of natural uranium ore dust alone in rats creates a risk of primary malignant and non-malignant lung tumor formation and that malignant tumor risk was not directly proportional to dose, but was directly proportional to dose rate.

■         Low Doses Of Ionising Radiation Incurred At Low Dose Rates

J. Graham, D.J. Higson, J-S. Jun, S. Kobayashi and R.E.J. Mitchel.

Chapter 7 In Worldwide Integrated View On Main Nuclear Energy Issues, Published by the European Nuclear Society, Belpstrasse, 23 (P.O. Box 5032), CH-3001 Berne (Switzerland) 1999

This paper addresses the scientific information available on the biological effects of radiation at low doses and low dose rates. It is concluded that there is no scientific evidence to support the hypothesis that radiation causes increases in the incidences of cancers or hereditary effects in humans, for total doses less than 10 mSv, received at high rates in single events, ordose rates less than 20 mSv per year, received continuously.

Except for the purpose of scientific research, it should therefore be assumed that there is no significant biological effect from such low levels of radiation.

■         Modulation Of Radiation-Induced Strand Break Repair By Cisplatin In Mammalian Cells.
Dolling JA, Boreham DR, Brown DL, Mitchel REJ, Raaphorst GP.  International Journal of Radiation Biology. 74:61-69, (1998)
 
PURPOSE: To investigate the repair of ionizing radiation-induced DNA lesions in human skin fibroblasts in the presence of cisplatin-DNA adducts and to determine the persistence of DNA repair inhibition by cisplatin. MATERIALS AND METHODS: Normal human fibroblasts (AG 1522) treated with cisplatin were exposed to 4 Gy 60Co gamma-radiation and assayed for repair of radiation-induced damage under growth-permissive conditions. DNA damage was measured by the fluorescence analysis of DNA unwinding (FADU) and cytokinesis-blocked micronucleus assays. RESULTS: Rejoining of strand breaks caused by 4 Gy radiation in cells without cisplatin pre-treatment appeared to be biphasic with an initial fast component (up to 15 min of repair time) followed by a slower component, and was completed by 90 min. Cisplatin treatment (10 microg/ml, 30 min) immediately before irradiation had no effect on the fast rejoining component, but inhibited the slow component (p<0.01). The same cisplatin treatment 24 h prior to irradiation inhibited both slow and fast components (p<0.01). In contrast, decreasing the cisplatin exposure to 1.0 microg/ml for 30 min, 24h prior to irradiation, resulted in an increased amount of strand break repair at each time point measured compared with irradiated control cells. This mild cisplatin treatment (95% survival) also resulted in a reduction of radiation-generated micronuclei indicating an adaptive response. CONCLUSIONS: Cisplatin used in combination with ionizing radiation can produce differential cellular responses depending upon the severity of the cisplatin treatment and the time interval between cisplatin and radiation exposures.

■        Heat-Induced Thermal Tolerance And Radiation Resistance To Apoptosis In Human Lymphocytes.
Boreham DR, Dolling JA, Maves SR, Miller S, Morrison DP, Mitchel REJ.  Biochemistry Cell Biology. 75:393-397 (1997)

We have investigated heat- and radiation-induced apoptosis in human lymphocytes in vitro. We have previously shown that apoptosis was induced by radiation at doses as low as 0.05 Gy. Here we report that heat induced apoptosis in human lymphocytes in a temperature- and time-dependent manner. Temperatures at or below 42 degrees C, for up to 90 min, did not cause lymphocytes to undergo apoptosis, whereas temperatures at or above 43 degrees C, for 30 min and longer, did induce apoptosis. Lymphocytes were protected against apoptosis induced by 44 degrees C heat by a prior heat shock of 42 degrees C for 30 min. Heat-induced thermal tolerance developed immediately following the inducing heat shock, was greater after 4 h, and persisted for at least 24 h. While heat also induced radiation resistance, this change was minor and not apparent until about 24 h after the heat shock. Prior to the development of radiation resistance, heat shock sensitized lymphocytes to radiation-induced apoptosis. We have previously shown that radiation-induced apoptosis in lymphocytes varies between donors and therefore may be useful in assessing individual radiosensitivity. We report here that heat also induced variable levels of apoptosis in lymphocytes from different donors, although the range of responses was not as large as those observed with radiation-induced apoptosis. In summary, heat shock induces tolerance to heat-induced apoptosis and results first in sensitization and then protection of lymphocytes against radiation-induced apoptosis.

■         Rearrangement Of Human Cell Homologous Chromosome Domains In Response To Ionizing Radiation.
Dolling JA, Boreham DR, Brown DL, Raaphorst GP, Mitchel REJ  International Journal of Radiation Biology. 72:303-311, (1997)
 
Chromosomes are located within the interphase nucleus in regions called domains. Using fluorescence in situ hybridization with whole chromosome paints, a pain of homologous chromosomes can be visualized as two discrete domains and their relative spatial location determined. This study examines the effects of an ionizing radiation exposure on the relative spatial location of chromosome 7 and 21 domains in human skin fibroblasts and lung endothelial cells. The distance between homologous chromosome domains was assessed for each nucleus, before and after exposure to ionizing radiation, using conventional epifluorescence and confocal laser scanning microscopy. Results from conventional microscopy indicated that homologous chromosome domains were re-positioned closer to each other within interphase nuclei after exposure to radiation. Analysis of three-dimensional data obtained from confocal microscopy confirmed these results. In control cells, and in cells examined immediately after irradiation, 66.2% +/- 2.1% of the homologous chromosome 21 domains within endothelial cell nuclei were located greater than 4.0 microns apart (33.8% +/- 1.9% were less than 4.0 microns apart). However, when cells were examined 2 h after a 4.0 Gy gamma-ray exposure, only 30.5% +/- 2.1% of the homologous chromosome domains were greater than 4.0 microns apart (69.5% +/- 2.1% were less than 4.0 microns apart). Similar results were obtained for chromosomes 7 and 21 in skin fibroblast nuclei. The results indicate that homologous chromosome domains rearranged and became closer together within the interphase nuclei in response to ionizing radiation. The exact mechanism of this response is unknown, but it may be related to DNA repair processes. It is speculated that chromosome domains are re-positioned to permit repair of radiation-induced DNA damage.

■         Adaption To Ionizing Radiation In Mammalian Cells

R.E.J. Mitchel, E.I. Azzam, and S.M. de Toledo  Stress-Inducible Processes in Higher Eukaryotes, T. Koval (editor), Plenum Press, New York, 1997 pp. 221-243, 1997.

Normal human skin fibroblasts (AG1522) unstimulated human lymphocytes and mouse embryo fibroblasts (C3H 10T½) have been used to study adaption to ionizing radiation in mammalian cells.  We have shown that both normal human and mouse embryo fibroblasts can adapt and become resistant to the effects of radiation.  In human fibroblasts this can be detected both as increased survival and a decreased rate of micronucleus formation.  In the mouse embryo cells, adapted cells show both a reduced rate of micronucleus formation and a reduced rate of transformation when exposed to a second radiation dose.  We have also shown that a dose which results in approximately one track per cell (0.1 cGy) is sufficient to protect mouse embryo cells against spontaneous transformation.  In an investigation of the mechanisms responsible for the increase in radio-resistance of adapted cells, we have demonstrated an increased rate of repair of those DNA double strand breaks (dsb) which lead to micronuclei but no change in the ability to repair otherwise unrepairable dsb.  Our data also suggests an increased division delay allowing more time for repair.  Gene expression studies at the level of mRNA are consistent with a cyclin modulated delayed progression into the cell cycle.  In unstimulated human lymphocytes, adapting doses of radiation resulted in an increase rather than a decrease in apoptotic cell death resulting from a subsequent exposure, an adaptive response which presumably benefits the whole organism, rather than the ind    Stimulation of the adaptive process by radiation was most effective at low dose rates and the dose itself was not critical if delivered at a low dose rate.  Large total adapting doses (over 4 Gy) delivered slowly prior to a large acute exposure, still resulted in less total deleterious effects than the acute dose alone.  However, a dose equivalent to only one track per cell also produced the same maximum increase in resistance to transformation.  While pre-exposure to radiation produced adaption to radiation in human cells, a mild heat stress also adapted cells to radiation.  These results raise questions about the validity of the linear no threshold model used as the basis for predicting cancer risk from radiation.

■        Low-Dose Ionizing Radiation Decreases The Frequency Of Neoplastic Transformation To A Level Below The Spontaneous Rate In C3H 10T1/2 Cells.
Azzam EI, de Toledo SM, Raaphorst GP, Mitchel REJ  Radiation Research. 146:369-73 (1996)

We have previously shown that chronic exposure of plateau-phase C3H 10 T1/2 cells to (60)Co gamma radiation at doses as low as 10 cGy protected the cells against neoplastic transformation by a subsequent large acute radiation exposure. We have also shown that this induced resistance to neoplastic transformation correlated with an increased ability to repair radiation-induced chromosome breaks. We now show that a single exposure of quiescent cells to doses as low as 0.1 cGy also reduces the risk of neoplastic transformation, from the spontaneous level to a rate three- to fourfold below that level. Higher doses, up to 10 cGy at the same dose rate (0.24 cGy/min), did not reduce the neoplastic transformation frequency further. This protective effect was seen only in irradiated cells that were allowed to incubate at 37 degrees C before release from contact inhibition. Cells released into low-density subcultures immediately after irradiation had unchanged neoplastic transformation frequencies. These results demonstrate that low or chronic exposure to radiation can induce processes which protect the cell against naturally occurring as well as radiation-induced alterations that lead to cell transformation. If similar processes are induced in human cells, the results also suggest that a single low dose, at background or occupational exposure levels, may in some circumstances reduce rather than increase cancer risk, a conclusion inconsistent with the linear no-threshold model of cancer risk from radiation.