Ebook: Rapid Diagnosis in Populations at Risk from Radiation and Chemicals

Populations can potentially be exposed to varying doses of ionising radiation or to hazardous chemicals as a result of an accident, act of terrorism, or war. This exposure could cause direct clinical effects within days or weeks or bring about late effects on human health in such as an increased cancer rate. A determination of the magnitude of the exposure to individuals is crucial so that those persons with a significant health risk can have appropriate procedures initiated immediately. It is extremely unlikely that the potential victims will wear adequate exposure indicators. Therefore, there is a critical need for a method to measure the dose from molecular and cellular effects that occur within the individual and that are associated with levels of chemical, biological, radiological and nuclear (CBRN) agents, and their possible short and long term effects on potentially exposed members of a population at risk. Many methods are now available for biological monitoring both of the environment and of humans, and measuring genetic damage and other changes in macromolecules and body tissues. Damage to DNA and other molecules, tissues, and organs after acute or chronic exposures are referred to as biomarkers. For the best kind of study, human monitoring needs close contact with many other disciplines like epidemiology, medical physics and others. As an experimental science, it also has to keep pace with the rapidly growing understanding of the language of the genome and mechanisms of deterministic and stochastic effects, particularly risk of cancer. An individual’s genetic constitution, lifestyle, age, diet and levels of physical activity, can affect the body’s response to exogenous agents. Gene and pathway complexity have to be considered when investigating gene–environment interaction and “phenotypic” response of individuals at risk. The efforts of research and applied science should ultimately contribute to an approval of regulated biodosimetry and diagnostic tests integrated into the national and international radioprotection and human monitoring programs. The main aim of the RADIPER NATO Advanced Training Course was to cover scientific aspects in this field by taking in advance the best action to protect the public against consequences of terrorism and other threats on the basis of state-of-the-art knowledge. On the 95th anniversary of the First World Scientific and Scientists Service for Protection Against War Injury, and on the 60th anniversary of NATO, we have welcomed to Poland scientists and researchers from 22 countries, with the support of the NATO Science for Peace and Security Programme. We have welcomed them to the country of Madame Curie (awarded twice with the Nobel Prize). Maria Skłodowska-Curie was the first, in 1914, to organize radiological and therapeutic services for the frontline of the First World War and to start training courses for medical doctors and nurses from two continents.

We hope that participants of the RADIPER course have gained knowledge of the deterministic and stochastic effects and biomarkers associated with humans’ early and late health risk, after exposure to physical and chemical agents. We hope they have also gained expanded practical skills in detecting biomarkers of exposure to genotoxicants using different well established biological assays, such as dicentrics, micronuclei, translocations, premature chromosome condensations, comet and more.

Finally, we hope that the participants of the RADIPER course will cherish and preserve the created network for use in the future, in order to protect the public from a pointless health risk.

The Editors

Exposure to environmental toxic substances is a significant cause of human health problems. However, the critical question to ask is, at what concentrations and exposure conditions are these substances hazardous to human health. One approach to address the question is to conduct population monitoring studies using reliable biomarkers that can be used to indicate excessive exposure, identify biological damage and functional deficit, and predict increased risk for health problems. In this review, the application of traditional and functional biomarkers in population monitoring studies will be presented. A major emphasis in the review is to show how to design rigorous study protocols so that results from these studies will be appropriate for use in disease prevention programs.

The WHO/International Programme of Chemical Safety Guidelines for Human Monitoring provide concise guidance on the planning, performance and interpretation of studies to monitor groups or individuals exposed to genotoxic agents. Most human carcinogens are genotoxic but not all genotoxic agents are carcinogenic in humans. Although the main interest in these studies is due to the association of genotoxicity with carcinogenicty, there is also an interest in monitoring human genotoxicity independently of cancer as an endpoint. The most often studied genotoxicity endpoints selected for inclusion in this document are described with the exception of germ cells [1]. It is clear from results of international collaborative studies that no single assay can detect all genotoxic substances. In 1996, WHO/IPCS published its Harmonised Scheme for Mutagenicity testing and this has now been updated [2] and focuses on the identification of mutagens and genotoxic carcinogens. It considers appropriate in vitro and in vivo assays and a strategy for germ cell testing. A combination of tests assesses effects of the three major end-points of genetic damage which are associated with human disease (gene mutation, clastogenicity and aneuploidy). Most assays in bacteria and mammalian cells are used for hazard identification. When choosing in vivo assays, either in somatic or germ cells, expert judgement is required for the appropriate test system and the unnecessary use of animals. Germ cell tests measure damage in germ cells per se and effects in the offspring or potential offspring of exposed animals.

Cancer cells display characteristics that are acquired at different times of cancer development and with various strategies. Genomic instability and/or epigenetic changes are thought to initiate the carcinogenic processes. Thus, molecular classification of cancer may include either gene-expression profiles, chromosomal aberrations and cancer genome sequencing, or epigenetic changes such as microRNA aberrations, DNA methylation, and telomere changes. The aim of this study is the detection of molecular signatures of cancer related to specific exposure. Biological monitoring of exposures to carcinogens includes a variety of biomarkers, but the entire chain of events from exposure to disease has never been demonstrated. Genome-wide studies indicating associations between disease and single nucleotide polymorphism could define a way of selecting individuals for prospective biomonitoring. Exploring and understanding the pathways controlled by genes thought to be involved in carcinogenesis will likely pave the way in improving the molecular classification of cancer and in choosing the most appropriate endpoints to be used in biological monitoring.

The issues of low dose and low dose rate exposure of human as well as the acquisition of knowledge on common genetic factors that might determine inter-individual differences in low dose cancer risk are of continuing importance in respect of social/economic policy relating to the industrial and medical uses of ionizing radiation. Furthermore, for risk assessment among people occupationally are being exposed to low and/or high LET radiation, such as astronauts, pilots, stewardess, nuclear power plant workers and patients under radiotherapy treatment, namely children.

In order to emphasis on the acquisition of fundamental knowledge and the development of low dose as well as high dose risk models for low LET radiation (i.e. gamma-rays and X-rays) and high LET radiation (i.e. heavy ions, alpha-particles, neutrons) several biological assays were developed and attempts were made to investigate formation of radiation induced chromosome aberrations and induction of genomic instability in human lymphocytes and fibroblasts: Mechanisms, Kinetics and Spectra were defined.

Fluorescence in situ hybridization (FISH) technique using chromosome, chromosome- arm, chromosome region, centromere and telomere specific DNA libraries has improved the resolution of detecting all classes of radiation induced chromosomal inter- and intra-changes. Newly obtained data indicate that (a) FISH-based translocation assay is a unique model to assess acute as well as chronic exposure in cases of accidental as well as occupational exposure to ionizing radiation, either immediately following exposure, or retrospectively. This assay is capable of defining accumulative effects to red bone marrows. (c) Studies on the spectra of radiation induced chromosome aberrations revealed two distinct hallmarks for high LET radiation in comparison to low LET radiation. (d) Premature chromosome condensation assay proved to be a unique method to be used for immediate dose assessment at low (5cGy) as well as high doses (≥3 Gy) and to discriminate between whole- and partial-body exposure in case of mass casualties and accidental over-exposure to high doses of ionizing radiation. Furthermore, this assay is combined with multi-colour FISH assay, and for the first time could reveal the nature of chromosomal instability in primary tumours at different stages.

The outcomes of these studies lead to improve international radiation protection standards for protracted exposures, which is the most frequent form of occupational and population exposure in society.

Chromosomal aberrations (CAs), micronuclei (MNi), and sister chromatid exchanges (SCEs) in peripheral lymphocytes have widely been used as biomarkers of genotoxic exposure and effects in humans. International collaborative studies have shown that a high level of CAs in lymphocytes is associated with an increased risk of overall cancer. The CA-cancer association is seen in subjects with known carcinogenic exposure but also in individuals with no known history of exposure to carcinogens. This suggests that a high CA level is an indicator of increased cancer risk regardless of the reason for the increase. Chromosome-type CAs appear to be more important than chromatid-type CAs in predicting cancer risk. As chromosomal fragments can form MNi, it is not surprising that an association has also been observed between lymphocyte MN level and overall cancer risk. It has not been possible to show an association between the frequency of SCEs in lymphocytes and cancer risk, possibly due to technical variation in SCE level and the fact that SCEs seem to represent homologous recombination repair of DNA. The reasons for the associations of CAs and MN with cancer risk are not clear, but undetected cancer does not explain them, because the time between the cytogenetic analysis and cancer diagnosis does not modify the risk. Carcinogenic exposure remains an explanation, as everybody is exposed through diet, general environment, and internal generation of genotoxic species. The results may also represent individual susceptibility to chromosomal damage. Cancer risk prediction by CA level has been observed in all studies published on the topic, despite the use of heterogeneous cytogenetic data, suggesting that the association is robust. There are suggestions that the association is stronger than hitherto assumed. If the accuracy of assessing the level of chromosome could be improved, individual cancer risk estimation might become possible.

The challenge for molecular epidemiology and environmental studies is to further improve the process for assessing risk to human health from exposure to genotoxic agents. It is well recognized that exposure to ionizing radiation is unavoidable. The issue of exposures from nuclear power and nuclear waste receives the greatest public attention, yet these types of exposures are only a minor source of total environmental chemical and radiation exposures. However, for the populations at risk or close to these sources, the issue is a real one. The concern here is the risk of unforeseen accidents as well as the public’s lack of understanding the acute and chronic exposures differences and their associated risks.

Characterization of the dose response process is often done by the use of biomarkers detected in assays predictive of potential toxic outcomes and pathological changes. This often provides the opportunity for more effective preventive measures against exposures that deemed hazardous to human health. Biological markers, associated with environmentally induced genotoxic change, may also be useful for improving the prediction of risk to human health posed by accidental or environmental exposure. The desirable features of biological markers of genotoxic action are that biomarker measure should indicate a quantitative reaction to the action and should be associated with a genotoxic outcome. In order to define major contamination, its genotoxic effectiveness and to realize genetic or carcinogenic hazards in vitro studies are necessary which yield more information on genotoxicity of major pollutants. It is also important to search for correlation between evidence of the induced genotoxic damage in exposed living organisms or occupationally exposed human with accurate measures of exposures. This creates necessity to extrapolate from high doses (experimental, occupational or accidental) to low dose region. There may be many confounding factors affecting the simple extrapolation. In this paper are discussed influence of the shape of dose effects relationship, and physical or life style related factors on health risk.

The public has become more aware that exposure of males to certain agents can adversely affect their offspring. The hazards associated with exposure to ionizing radiation have been recognised for nearly a century. There was a civil court case on behalf of two of the alleged victims of paternal irradiation at Seascale against British Nuclear Fuels. The case foundered on “the balance of probabilities”. Nevertheless, there was support for paternal exposure from Japanese experimental X-ray studies in mice. The tumours were clearly heritable as shown by F2 transmission in vivo. In addition in humans, smoking fathers appear to give rise to tumours in the F₁ generation. Using rodent models, developmental abnormalities/congenital malformations and tumours can be studied after exposure of males in an extended dominant lethal assay and congenital malformations can be determined which have similar manifestations in humans. The foetuses can also be investigated for skeletal malformations and litters can be allowed to develop to adulthood when tumours, if present, can be observed. Karyotype analysis can be performed on foetuses and adult offspring to determine if induced genetic damage can be transmitted. Using this study design, cyclophosphamide, 1,3-butadiene and urethane have been examined and each compound produced positive responses: cyclophosphamide in all endpoints examined, 1,3-butadiene in some and urethane only produced liver tumours in F₁ male offspring. Using reactive oxygen species which are known to be produced during ionizing radiation, we examined human sperm and lymphocytes in vitro after treatment with six oestrogenic compounds in the Comet assay, which measures DNA damage, and observed that all caused damage in both cell types. The damage was diminished in nearly all cases by catalase, and in some instances by superoxide dismutase (SOD) and Vitamin C (Vit C). This response pattern was also seen with hydrogen peroxide. This similarity suggests that the oestrogen-mediated effects could be acting via the production of hydrogen peroxide since catalase always markedly reduced the response.

Previously we have reported that the human population located near the Semipalatinsk nuclear test site exhibited significantly enhanced levels of minisatellite mutations (MM), indicating an increased germ-line mutation rates from the exposure in 3 generations. It was assumed that the MM may potentially be used as a biomarker for monitoring functional genetic alterations. In this study, the influence of polymorphisms in genes on the expression of MM in the same two populations, and their relationships with radiation exposure were studied. Three polymorphic DNA - repair genes (XRCC1 Arg194Trp, XRCC1 Arg399Glu, and XRCC3 Thr241Met) and two xenobiotic detoxification genes (GST and GSTM1) were selected for analyses. The functionally active XRCC1 Arg194Trp was significantly linked to low MM and over-represented in the exposed in comparison with the control populations. Both GSTT1 and GSTM1 nulls were significantly associated with increased MM.ere. The former was under-represented but the latter was significantly over-represented in the exposed in comparison with the control populations. In summary, the data indicates that the expected enzymatic functions of the polymorphic genes are consistent with the MM expression, except the XRCC1 Arg399Glu variant gene. Moreover, the variant genes were retained in the three generations in association with their useful function, except for the GSTM1 null.

Morphofunctional properties of hemopoietic progenitor cells in victims of Chernobyl accident were investigated by the method of cultivating in gel diffusion chambers inserted into peritoneal cavity of experimental animals. The oppressing of colony-forming activity of CFU-GMdc (granulocyte-monocytic colony-forming units in diffusion chamber) in patients with ARS (acute radiation sickness) and recovery of those indices later testifies to the preservation and restoration of stem cells department during some period of time. No significant difference in colony-forming activity of bone marrow cells between control group and groups of people irradiated at different dose rates (from 0.25 to 0.5 Gy, from 0.5 to 0.75 Gy and from 0.75 to 1 Gy) was determined. However, depressing of efficiency and disturbance in differentiation in the colonies in some patients, predominance of eosinophilic colonies in majority of patients illustrate the need for a high degree of suspicion in establishing an early diagnosis. Early identification of these patients is important as they are at risk of eventual evolution into more hazardous clinical state.

For the last few decades genome damage caused by exposure to ionizing radiation has been estimated with the use cytogenetical methods like chromosome aberration assay (CA), micronucleus assay (MN), or since recently fluorescent in situ hybridisation. However, all available studies on reliability of CA and MN to predict cancer risk are done on adult population while data concerning children are mostly related to those collected after the Chernobyl nuclear accident. After industrial nuclear accident exposures, several long-term follow-up studies have shown a higher level of genome damage and higher accumulation of radioisotopes in the skeleton of children than of adults. Similarly, rogue cells in a long-term follow-up study of exposure in industrial regions are more frequently detected in children than in adults. Additionally, is impossible to predict the transplacental impact of radioisotopes on miscarriage rate using CA in mothers. Due to its transmutation to He, tritium level in water may have a more severe impact on DNA integrity in the foetus than in adults, which results from higher mitotic rate during development. Recent studies in oncology patients using DNA microarrays identified a specific group of genes and SNPs whose expression is a signature of radiation response. These promising methods will enable researchers to detect age-related response to ionizing radiation and will give a better insight into individual cancer risk, as opposed to the group risk determined by CA and MN assays.

Cytogenetic analysis was performed in blood lymphocytes of 60 radiation workers employed in the Ignalina Nuclear Power Plant. The control group comprised 64 healthy male donors. Chromosome aberration analyses revealed no significant differences between the workers with an external gamma radiation exposure and the controls (1.60 vs. 1.65 CA/100 cells, P=0.85). An increase in the chromosome aberration frequency was observed for the workers with an additional internal and internal plus neutron exposure with significant differences for the total chromosome aberration, chromosome-type aberration and dicentric frequencies. The confounding effects of the internal and neutron exposure on the frequency of chromosome aberrations were determined. The impact of other factors (smoking, age, duration of employment, cumulative dose) was found to be non-significant.

The aim of the present study was to evaluate the immunological status among medical staff occupationally exposed to ionizing radiation at the cardiac catheterisation units of three different University hospitals (Ain Shams, Al Azhar and National Heart Centre at Embaba) in Cairo, Egypt. The individual annual collective dose information was estimated with thermoluminescence dosimetry (TLD) system. It was 0.02 – 0.3 mSv/y 0.42 – 0.94, mSv/y and 1.16 – 8.44 mSv/y for the three hospitals respectively. Venous blood samples were obtained from 60 hospital workes (20 from each hospital) exposed to x-ray during fluoroscopy procedure as well as from 20 people not exposed to ionizing radiation and not working at hospitals, and from 20 people not exposed to ionizing radiation and working at hospitals. Blood samples were assayed for apoptosis percentage in circulating lymphocytes and lymphocyte’s phenotypes CD4 and CD8 and CD4/CD8 ratio. Results revealed significant increase in the apoptosis percentage in circulating lymphocytes, CD4, CD8 and CD4/CD8 ratio. However, the Embaba group, of which the highest level deviated from the latter finding where CD4 and CD4/CD were significantly decreased in comparison with controls and the other two groups of cardiac catheterisation workers. It is concluded that apoptosis percentage in circulating lymphocytes and cell phenotype analysis for lymphocyte phenotypes including CD4, CD8, CD4/CD8 can offer a more precise indicators for radiation exposure to effective low dose levels of ionizing radiation.

Twenty years after Chernobyl Nuclear Power Plant (NPP) explosion, several million people (by various estimates, from 5 to 8 million) still reside in areas that will remain highly contaminated by its radioactive pollution for many years to come, since the half-life of the major radioactive element released is a little over 30 years. So, new researches should be made to investigate the influence of Chernobyl accident-related pollution on persons living on the contaminated territories. The aim of this study was to examine functional activity of hematopoietic progenitor cells of persons living on the radiation-polluted territories suffering from Chronic Myeloid Leukaemia (CML). The bone marrow of patients with different phases of CML was examined. Immobilised mononuclear cells were cultivated by applying two cultivation models: in vitro and in vivo. Our data suggested that there is no significant difference between results obtained from both cultivation systems used, and the numbers achieved are comparable and can be used to develop the unique scheme of hematopoiesis in health or disease. Functional activity of Hematopoietic Progenitor cells (HPCs) of persons with CML living on the territories polluted after explosion on Chernobyl NPP is significantly higher in comparison with control data. The shift in bone marrow cell’s count to increased quantity of blast and granulocytic progenitor cells was observed after morphological analysis. The data obtained can be suggested as a confirmation of monoclonal nature of CML.

A technique for revealing surface morphology of human cervical cancer cells has been developed to facilitate early diagnostics of pre-cancer and cancer cells under reflected light microscopy. The offered method was borrowed from optical microscopy of a solid state surface, in which the metallographic inverted microscopy (MIM) is usually used. Unlike commonly accepted transmitted light microscopy for biological applications, the MIM technique allows to reveal a morphology and topology of a biological cells surface without any treatment by chemicals (fixing, staining, drying, freezing et al). The MIM method was demonstrated by analysing fresh native smears from epithelium of uterine neck. MIM micrographs of 167 patients with diagnosed cervical cancer allow to visualise numerous the light reflective formations (LRF) on the cancer cells surface. It is supposed that LRF are connected with exocytosis on the cell membrane.

Due to an increasing risk of terroristic attacks or nuclear accidents potentially some of populations can be exposed to dangerous doses of ionizing radiation. In such cases it is needed to develop an effective method for triage so the individuals with significant risk can be treated immediately with proper medical procedures. The fact that EPR can measure absorbed doses was first reported by Brady et al. in 1968. Nowadays ongoing works on such system which can measure in vivo doses absorbed in human teeth give an opportunity for a quite good estimation. Also results of research on fingernails and teeth ‘biopsies’ using EPR system are forecasting well. Getting smaller detecting system size and solving many technical issues can be made in the next few years which can make EPR technique a great tool for a retrospective dosimetry in case of an emergency and health risk.

This paper describes the monitoring of occupational staff who worked in ionizing radiation field of the diagnostic centres in Albania between 2003 and 2007 as well as analyses and discusses the mean annual dose rate recorded for the above-mentioned period.

The monitoring was based in TLD-100 dosimetric cards and the control was performed all over the country on bimonthly basis covering major cities like: Tirana, Durresi, Shkodra, Fieri, Vlora, Korça etc. The Department of Human & Environment Protection, at the Centre of Applied Nuclear Physics, through the dosimetric service carried out the monitoring for approximately 350 radiation workers.

In the case of an nuclear or radiological incident with several patients exposed to ionising radiation, diagnostic and therapeutic decisions are time critical. Since results of a physical dose-reconstruction but also from methods for biodosimetry (e.g. dicentrics) will be available after several days or even weeks, clinical signs and symptoms of the radiation exposed patients may be the only available basis for estimation of radiation effects and prediction of the clinical course. The METREPOL-System is based on indicators of effect and repair to manage radiation accident victims considering multi-organ involvement and potential treatment options.

In this paper we describe an emergency response system in Taiwan funded by the Taiwan Environmental Protection Administration that consists of seven emergency response teams (ERT) held by the Chung Yuan Christian University, National Yunlin University of Science and Technology, and National Kaohsiung First University of Science and Technology in the north, central, and south parts of the island, respectively. The ERT system is set up for the effective and efficient response to incidents involving toxic chemicals and hazardous materials, and to reduce the potential impact on the environment from these incidents. During the past three years, more than two hundreds and eighty on-scene response services were provided. The incidents range from a leak in a tank truck to a large-scale fire in a chemical plant. The prompt and effective response has greatly reduced the impact from these incidents.

This paper will describe the system and functions of the ERT. The responded incidents are analysed and possible root causes are identified and classified. The information provided should be beneficial for areas with growing industrial development in developing strategies to prevent or reduce the potential impacts from chemical incidents.

In the present study, uranium mine-workers in the Stepnogorsk mining-milling complex in Northern Kazakhstan were investigated for the expression of chromosome aberrations and for genetic factors that can modify the exposure-related expression of chromosome damage. From our interview of volunteers, 100 qualified workers occupationally exposed to uranium and 56 control people who had not been exposed to radiation or other hazardous agents were selected. The workers were subdivided into 3 groups according to the duration of exposure: group I – 1-10 years, group II – 11-20 years, group III – 21-25 years. Our data show that workers in all three exposure groups had higher frequencies of chromosome aberrations than the control group. Uranium-exposed workers who had inherited the null GSTM1 and/or GSTT1 genotypes had a significant increase in the frequency of chromosome aberrations compared with those who had the intact GSTM1 and GSTT1 genes for the different group of workers. Our study suggests that uranium mine-workers in Northern Kazakhstan have excessive exposure that can cause an increased risk for health consequences such as cancer. In addition, GSTM1 and/or GSTT1 null genetically susceptible individuals may have higher health risk.

At present time the most rapid and cheap way for ionizing susceptibility estimation is to analyse the human cells sensitivity to test-irradiation ex vivo. The peripheral blood lymphocytes are most often used for these purposes. These cells can be very easily be collected for an analysis and could be a good reflection for whole human body sensitivity to IR due to their high radiosensitivity. Complex investigations can give an appropriate results in the ex vivo test-irradiated cells using a combination such parameters as the free radicals production rate, the activity of key antioxidant enzymes as the superoxide anion dismutase, the DNA damages and their repair, the cytogenetic disturbances, and the cell death frequency. The results of estimation of people with abnormal radiosensitivity among pilots and prostate cancer patients are presented in the paper as an example of practical application of such phenomenological biomarkers in medicine. As criteria for individual radiosensitivity estimation using test-irradiation of blood lymphocytes ex vivo we chose: DNA double-strand breaks (DSB) induction; capacity of DNA SSB repair system; apoptotic cell death frequency. Our results show that criteria used in the study allow to make judgment about the state of blood lymphocytes in individuals. An analysis of cellular response to test-irradiation ex vivo can be recommended for identification of people with an increased sensibility to ionizing irradiation.

Results of our studies, performed on more than 700 subjects, have demonstrated that occupational or environmental exposures to genotoxic agents (i.e. benzene related compounds, pesticides, mercury ions, cigarettes smoke) were associated with elevated levels of cytogenetic damage and were followed by increased risks of cancers. Results have also shown that genotoxic exposures significantly reduced efficiency of DNA repair. Reported results suggested that deficiency in DNA repair could be related to genotoxic exposures and life-styles (diet, vitamins, smoking). Groups of unexposed or exposed to PAHs subjects from Prague, Kosice and Sofia were monitored under the EXPAH EC project in many health, biological and polymorphic features (i.e. CYP1A1, GSTM1, NAT2, EPHX4, XRCC1). In those studies, results from application of the DNA repair competence assay in vitro have shown variability between subjects in responses of lymphocytes to the challenging X-ray dose and in repair efficiencies of the induced DNA damage. Significant decreases of repair efficiencies were observed when the whole investigated group was stratified first according to various genotypes and then to both exposures to PAHs (from cigarettes smoking and occupational). Application of the stochastic approach based on the molecular theory of radiation biology demonstrated that DNA repair competence was dependent on both combined exposure to PAHs and genetic predisposition (polymorphism). Other studies have shown that significant reduction of cellular repair efficacy of the DNA damage was also observed in groups of cancer patients. On the other hand, results of our studies have also shown a strong association between decrease in DNA repair efficiency and increase of aberration frequencies. In a conclusion, our results display that exposure to genotoxic agents, can be hazardous via directly induced cytogenetic damage or via alteration of the DNA repair processes, and can result in increased health risk, particularly in cancer risk. In both pathways, molecular and cytogenetic, results follow the stochastic theory of molecular actions. Results also show that application of the challenging X-rays dose and DNA repair competence assay combined with the detection of damage by SCGE, might be used as the fast, reliable and phenotype related biomarker of individuals vulnerability to the health risk. In epidemiology or pre-clinical studies, it might indicate subjects with elevated risk of health hazard and could also predict the cellular susceptibility to various treatments (environmental, occupational, accidental or therapeutic).