Nontargeted effects include radiation-induced genomic instability (RIGI) which is observed in the progeny of cells exposed to ionizing radiation and can be manifested in different ways, including chromosomal instability and micronucleus (MN) formation. Since genomic instability is commonly observed in tumors and has a role in tumor progression, RIGI has the potential of being an important mechanism for radiation-induced cancer. The work presented explores the role of dose and dose rate on RIGI, determined using a MN assay, in normal primary human fibroblast (HF19) cells exposed to either 0.1 Gy or 1 Gy of X-rays delivered either as an acute (0.42 Gy/min) or protracted (0.0031 Gy/min) exposure. While the expected increase in MN was observed following the first mitosis of the irradiated cells compared to unirradiated controls, the results also demonstrate a significant increase in MN yields in the progeny of these cells at 10 and 20 population doublings following irradiation. Minimal difference was observed between the two doses used (0.1 and 1 Gy) and the dose rates (acute and protracted). Therefore, these nontargeted effects have the potential to be important for the low-dose and dose-rate exposure. The results also show an enhancement of the cellular levels of reactive oxygen species after 20 population doublings, which suggests that ionising radiation (IR) could potentially perturb the homeostasis of oxidative stress and so modify the background rate of endogenous DNA damage induction. In conclusion, the investigations have demonstrated that normal primary human fibroblast (HF19) cells are susceptible to the induction of early DNA damage and RIGI, not only after a high dose and high dose rate exposure to low linear energy transfer, but also following low dose, low dose rate exposures. The results suggest that the mechanism of radiation induced RIGI in HF19 cells can be correlated with the induction of reactive oxygen species levels following exposure to 0.1 and 1 Gy low-dose rate and high-dose rate x-ray irradiation.

Naturally occurring radiation can be found all around us and account for most of the radiation received by human beings each year. Indonesia has a region with high-dose natural radiation located in the suburb of West Sulawesi province with a dose rate up to 2800 nSv/h; however, its impact was not fully understood. The aim of this study was to evaluate the radiosensitivity of 12 peripheral blood lymphocytes of inhabitant from high background radiation area (HBRA) and 10 from normal background radiation area (NBRA) based on cytokinesis-block micronucleus (CBMN) assay after challenged with 1.5 Gy of gamma ray. The analysis of CBMN was done according to standard procedure as per IAEA guidelines, and frequency of binucleate (mitotic) cells with micronuclei (MN) was scored in around 2000 binucleate lymphocytes cells per culture in microscopic analysis. Mean MN frequency for HBRA was lower than that of NBRA (0.121 vs. 0.189) after irradiation, indicating an adaptive response in HBRA group that resulted in less radiosensitivity; however, there was no statistically significant different (P > 0.05) between these two groups. The MN number was higher in women compared to men for both HBRA (0.15 vs. 0.09) and NBRA (0.216 vs. 0.147) groups. Besides, there was no statistically significant difference (P > 0.05) in Nuclear Division Index (NDI), as measured in 500 metaphase cells with published formula, between HBRA and NBRA samples (1.24 vs. 1.21). The lower MN frequency prompts us to conclude that there is an adaptive response in the lymphocytes of inhabitants as an indicator of lower radiosensitivity to the high natural radiation exposure. Further studies using large number of samples are required to obtain more comprehensive conclusion along with the assessment of other types of radiosensitivity-related biomarkers.

Nuclear anomalies of different types appear in cells in response to the action of ionizing radiation after the passage of the first mitotic division. In this article, we present the results of the study of the frequency of occurrence of three types of nuclear anomalies (“tailed” nuclei, nucleoplasmic bridges, and dumbbell-shaped nuclei) in vitro in human lymphocytes cultured with cytochalasin B when exposed to X-rays at doses of 0.0, 0.1, 0.2, 0.4, 0.5, 0.7, 1.0, 1.5, and 2.0 Gy. To stop the cell cycle of cultured lymphocytes after the first mitotic division, a cytokinesis block was performed using cytochalasin B. Dose-dependent curves of the occurrence of lymphocytes containing “tailed” nuclei, nucleoplasmic bridges, or dumbbell-shaped nuclei after irradiation have been constructed. At the same time, frequencies of occurrence of chromosomal aberrations (dicentric and ring chromosomes) in the culture of lymphocytes exposed to the same radiation doses were studied. Comparison of the frequencies of occurrence of dicentric and ring chromosomes with frequencies of occurrence of nuclear anomalies allows us to conclude that these nuclear anomalies are formed as a result of chromosomal aberrations arising in lymphocytes under the action of ionizing radiation. More than that, most of the chromosomal aberrations are converted into dumbbell-shaped nuclei in vitro in the culture of lymphocytes in the cytochalasin block.

Humans are exposed to ionizing radiation not only through background radiation but also through the ubiquitous presence of devices and sources that generate radiation. With the expanded use of radiation in day.to.day life, the chances of accidents or misuse only increase. Therefore, a thorough understanding of the dynamic effects of radiation exposure on biological entities is necessary. The biological effects of radiation exposure on human cells depend on much variability such as level of exposure, dose rate, and the physiological state of the cells. During potential scenarios of a large.scale radiological event which results in mass casualties, dose estimates are essential to assign medical attention according to individual needs. Many attempts have been made to identify biomarkers which can be used for high throughput biodosimetry screening. In this study, we compare the results of different biodosimetry methods on the same irradiated cells to assess the suitability of current biomarkers and push forward the idea of employing a multiparametric approach to achieve an accurate dose and risk estimation.

The absence of a rapid and high-throughput technology for radiation biodosimetry has been a great obstacle in our full preparedness to cope with large-scale radiological incidents. The existing cytogenetic technologies have limitations, primarily due to their time-consuming methodologies, which include a tissue culture step, and the time required for scoring. This has seriously undermined its application in a mass casualty scenario under radiological emergencies for timely triage and medical interventions. Recent advances in genomics and proteomics in the postgenomic era have opened up new platforms and avenues to discover molecular biomarkers for biodosimetry in the future. Using a genomic-to-proteomic approach, we have identified a basket of twenty “candidate” radiation response genes.(RRGs) using DNA microarray and tools of bioinformatics immediately after ex vivo irradiation of freshly drawn whole blood of consenting and healthy human volunteers. The candidate RRGs have partially been validated using real-time quantitative polymerase chain reaction.(RT-qPCR or qPCR) to identify potential “candidate” RRGs at mRNA level. Two potential RRGs, CDNK1A and ZNF440, have so far been identified as genes with potentials to form radiation response proteins in liquid biopsy of blood, which shall eventually form the basis of fluorescence- or ELISA-based quantitative immunoprobe assay for a high-throughput technology of molecular biodosimetry in the future. More work is continuing.

One of the most widely employed histone deacetylases inhibitors in the clinic is the valproic acid (VA), proving to have a good tolerance and low side effects on human health. VA induces changes in chromatin structure making DNA more susceptible to damage induction and influence DNA repair efficiency. VA is also proposed as a radiosensitizing agent. To know if VA is suitable to sensitize human lymphocytes γ-irradiation in vitro, different types of chromosomal aberrations in the lymphocytes, either in the absence or presence of VA, were analyzed. For this purpose, blood samples from four healthy donors were exposed to γ-rays at a dose of 1.5 Gy and then treated with two different doses of VA (0.35 or 0.70 mM). Unstable and stable chromosomal aberrations were analyzed by means of fluorescence in situ hybridization. Human lymphocytes treated with VA alone did not show any increase in the frequency of chromosomal aberrations. However, a moderate degree of sensitization was observed, through the increase of chromosomal aberrations, when 0.35 mM VA was employed after γ-irradiation, whereas 0.70 mM VA did not modify chromosomal aberration frequencies. The lower number of chromosomal aberrations obtained when VA was employed at higher dose after γ-irradiation, could be related to the induction of a cell cycle arrest, a fact that should be taken into consideration when VA is employed in combination with physical or chemical agents.

The methodology of cytogenetic triage can be improved by optimizing a schedule of microscopy for different exposure scenarios. Chromosome aberrations were quantified by microscopy in human blood lymphocytes irradiated in vitro to ~2, 4, and 12 Gy acute 60Co γ-rays mixed with the unirradiated blood simulating 10%, 50%, 90%, and 100% exposure and in along with a sample from a homogeneous exposure to ~20 Gy. Biodosimetry workload was statistically modeled assuming that 0.5, 1, 5, or 25 h was available for scoring one case or for analysis of up to 1000 cells or 100 dicentrics plus centric rings by one operator. A strong negative correlation was established between the rates of aberration acquisition and cell recording. Calculations showed that the workload of 1 case per operator per·day (5 h of scoring by microscopy) allows dose estimates with high accuracy for either 90%–100% irradiations of 2 Gy or 50%–90% irradiations of 4–12 Gy; lethal homogeneous (100%) exposures of 12 and 20 Gy can be evaluated with just 1 h of microscopy. Triage analysis of 0.5 h scoring per case results in the minimum tolerable accuracy only for partial. and total.body exposure of 4–20 Gy. Time.related efficacy of conventional biodosimetry depends primarily on the aberration yield in the sample, which is dependent on the radiation dose and its distribution in the patient's body. An optimized schedule of microscopy scoring should be developed for different exposure scenarios in each laboratory to increase their preparedness to radiological emergencies.

Iodine-131 (I-131) is often used in thyroid diagnostics and therapy. External and internal exposure to radioiodine can lead to molecular and cellular damage in peripheral blood lymphocytes. The aim of this study was to explore the influence of low and high doses of I-131 on susceptibility to ionizing radiation. Study groups consisted of 30 individuals free of thyroid diseases, 41 patients exposed diagnostically to low doses of I-131, and 37 hyperthyroidism patients exposed therapeutically to high doses. The standardized DNA repair competence assay was used to test the efficacy of the fast DNA repair process in G₀ cells. Cytogenetic preparations were made in fresh blood samples before and after challenging cells in vitro with X-ray dose. The frequency of sister chromatid exchanges (SCE) and percentage of cells with significantly elevated numbers of SCE were used as cytogenetic biomarkers associated to homologous recombination and compared to reported earlier cytogenetic biomarkers of cancer risk. Strong individual variation in the biomarkers is observed in all investigated groups before and after challenging. Nevertheless, the efficiency of post challenging fast repair is significantly high in the patients exposed to diagnostic I-131 doses than in unexposed control group and linked to decreased cytogenetic damage. However, 5 weeks after administration of therapeutic doses, significant increases of unrepaired post challenging DNA and cytogenetic damages were observed indicating a health risk. Results also suggest that the appearance of cancers in immediate families might influence DNA repair differently in patients exposed to low than to high doses.

Patients receiving identical radiation treatments experience different effects, from undetectable to severe, on normal tissues. A crucial factor of radiotherapy related side effects is individual radiosensitivity. It is difficult to spare surrounding normal tissues delivering radiation to cancer cells during radiotherapy. Therefore, it may be useful to develop a simple routine cytogenetic assay which would allow the screening of a large number of individuals for radiosensitivity optimizing tumor control rates and minimizing severe radiotherapy effects with possibility to predict risk level for developing more severe early normal tissue adverse events after irradiation. This study was conducted to assess the correlation between in vitro radiosensitivity of peripheral blood lymphocytes from cancer patients who are undergoing radiotherapy using the cytokinesis-block micronucleus (CBMN), G2 chromosomal radiosensitivity assays, and normal tissue acute side effects. The CBMN and G2 chromosomal radiosensitivity assays were performed on blood samples taken from cancer patients before radiotherapy, after first fractionation, and after radiotherapy. Acute normal tissue reactions were graded according to the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer. This study suggests that there is a correlation between higher frequency of micronuclei after in vitro irradiation of blood samples and higher degree of normal tissue reactions. In addition, higher number of chromatid breaks was observed in patients with more severe normal tissue reactions. This pilot study included only 5 cancer patients, and therefore, further studies with a bigger cohort are required to identify radiosensitive patients.

Dynamic changes of the chromosomal aberrations and the DNA damage were analyzed in individuals exposed to low and therapeutic doses of radiation. The investigation included 37 persons living in areas where the radioactive sources were discovered 10–12 years ago. It was established by biodosimetry methods that the examined persons had absorbed dose of 0.2–0.7 Gy or had increased number of chromosomal aberrations, though insufficient to determine a dose. Clinical examination, chromosomal analysis, and assay of DNA damage by the comet (single-cell gel electrophoresis) assay were carried out. There was no correlation between the doses received 10 years ago and the cytogenetic changes with clinical outcome. The effect of the local fractionated gamma-irradiation with doses of 40–70 Gy was studied in cancer patients with localized head and neck tumors. The study of chromosomal abnormalities, the DNA damages by the comet assay, and the micronuclei detection of the buccal cells revealed a statistically significant correlation between the initial cytogenetic indices in cancer patients and their dynamic changes during and after the radiation exposure. In addition, the correlation was detected between the initial cytogenetic parameters and the functional stage of red blood system. Our results allow us to conclude that there is a need for further research to estimate the individual radiation risk to optimize and individualize the subsequent medical management of radiotherapy.

Three in vitro dose calibration curves for biodosimetry such as dicentric chromosome assay, fluorescence in situ hybridization (FISH) assay for translocation, and micronuclei (MNs) in binucleated cell assay were established after exposure to ionizing radiation. Peripheral blood lymphocyte samples obtained from healthy donors were irradiated with ⁶⁰Co source at a dose rate of 0.5 Gy/min to doses of 0.1–6 Gy. The results from three in vitro dose calibration curves for biodosimetry were analyzed to understand the relationship among biodosimetry assay techniques. Our comparison demonstrates that there is a very strong positive correlation among the dicentric assay, FISH, and MNs analysis, and these three biodosimetry assays strongly support the in vitro dose reconstruction and the emergency preparedness of public or occupational radiation overexposure.

Flow cytometry analysis was used to detect the changes of γH2AX protein expression in human peripheral blood lymphocytes. In the dose-effect study, the expression of γH2AX was detected 1 h after irradiation with 60Co γ-rays at doses of 0, 0.5, 1, 2, 4, and 6 Gy. Blood was cultivated for 0, 1, 2, 4, 6, 12, and 24 h after 4 Gy ⁶⁰Co γ-rays irradiation for the time-effect study. At the same time, the blood was divided into four treatment groups (ultraviolet [UV] irradiation, ⁶⁰Co γ-rays irradiation, UV plus ⁶⁰Co γ-rays irradiation, and control group) to detect the changes of protein expression of γH2AX. The results showed that the γH2AX protein expression was in dose-effect and time-effect relationship with ⁶⁰Co γ-rays. The peak expression of γH2AX was at 1 h after ⁶⁰Co γ-ray irradiation and began to decrease quickly. Compared to irradiation with ⁶⁰Co γ-rays alone, the expression of γH2AX was not significantly changed after irradiation with ⁶⁰Co γ-rays plus UV. Dose rate did not significantly change the expression of γH2AX. The expression of γH2AX induced by ⁶⁰Co γ-rays was basically consistent with the mice in vivo and in vitro. The results revealed that the detection of γH2AX protein expression changes in peripheral blood lymphocyte by flow cytometry analysis is reasonable and may be useful for biodosimetry.

Cytogenetic dosimetry plays an important role in the triage and medical management of affected people in radiological incidents/accidents. Cytogenetic biodosimetry uses different methods to estimate the absorbed dose in the exposed individuals, and each approach has its advantages and disadvantages. Premature chromosome condensation (PCC) assay presents several advantages that hopefully fulfill the gaps identified in the other cytogenetic methods. To introduce this technique into the panel of other cytogenetic methods, a calibration curve for PCC after γ-irradiation was generated for our laboratory.

The in vitro dose calibration curves using conventional biological dosimetry – dicentric chromosome assay (DCA) and premature chromosome condensation (PCC) assay – were performed for the first time in Thailand for reconstruction of radiation dose in the exposed individuals. The peripheral blood lymphocyte samples from healthy donors were irradiated with 137Cs source at a dose rate of 0.652 Gy/min to doses of 0.1, 0.25, 0.5, 0.75, 1, 2, 3, 4, and 5 Gy for DCA technique, and 5, 10, 15, 20, and 25 Gy for PCC technique. The blood samples were cultured and processed following the standard procedure as prescribed in the International Atomic Energy Agency report with slight modifications. The yield of dicentrics with dose from at least 1000 metaphases or 100 dicentrics was fitted to a linear quadratic model using Chromosome Aberration Calculation Software (CABAS, version 2.0) whereas those of PCC rings with dose from 100 rings was fitted to a linear quadratic equation at doses from 0 to 15 Gy. These curves will be useful for in vitro dose reconstruction and can support the preparedness for overexposure to radiation among public or occupational workers and eventual radiological accident in Thailand.

The cytokinesis-block micronucleus assay in peripheral blood lymphocytes is an established technique for biodosimetry. The aim of this project was to generate a X-ray induced micronuclei (MN) curve for peripheral blood lymphocytes taken from five healthy donors. The blood samples were irradiated with X-rays of 122 KeV at a dose rate of 0.652 Gy/min to doses of 0.5, 1, 2, 3, and 4 Gy. The blood samples were then cultured for 72 h at 37°C and processed following the International Atomic Energy Agency standard procedure with slight modifications. The result showed that the yields of MN frequencies were increased with the increase of radiation dose. Reconstruction of the relationship of MN with dose was fitted to a linear-quadratic model using Chromosome Aberration Calculation Software version 2.0. Due to their advantages, mainly, the dependence on radiation dose and dose rate, despite their limitation, these curves will be useful as alternative method for in vitro dose reconstruction and can support the preparedness for public or occupational radiation overexposure and protection. The results reported here also give us confidence to apply the obtained calibration curve of MN for future biological dosimetry requirements in Indonesia.

As part of a regional International Atomic Energy Agency-coordinated research project with the support from the National Health and Family Planning Commission of China, 22 laboratories participated in the intercomparison in cytogenetic dosimetry in China. Slides for chromosomal aberrations were prepared by the Department of Radiation Epidemiology, National Institute for Radiological Protection, which organized the exercise. Slides were sent to the other participating laboratories through Express Mail Service. For estimates of dose, each laboratory scored the frequency of dicentrics plus centric rings chromosomes. The whole blood samples were irradiated with 60Co γ-rays (1.3 Gy, 2.4 Gy and 1.5 Gy, 2.6 Gy). Each laboratory got one group of the slides. Ten of the 44 estimates of dose fell within ±5% of the true physical dose, 12 fell within ±5–10%, 9 fell within ±10–15%, 12 fell within ±15–20%, while only one sample fell ± >20%. The evaluation of the respective dose was achieved by 21 laboratories.

Biological dosimetry using chromosome aberration analyses in human peripheral blood lymphocytes is suitable and useful tool for estimating the dose when a nuclear or radiological emergency is investigated. Blood samples from five healthy donors were obtained to establish dose-response calibration curves for chromosomal aberrations after exposure to ionizing radiation. In this work, dicentric assay and CBMN assay were compared considering the sensitivity and accuracy of dose estimation. In a total of 21,688 analyzed metaphase spreads, 10,969 dicentric chromosomes, 563 centric rings and 11,364 acentric chromosomes were found. The number of metaphase cells decreased with increasing radiation dose. The centric rings were not found in the non-irradiated control. There was no relationship between radiation dose and acentric ring induction. The frequency of total MN increased in a dose-dependent manner. In comparison with the control value, MN increased about 9, 32, 75, 87, and 52 fold higher after treatment with 1, 2, 3, 4, and 5 Gy, respectively. The results revealed that the mean frequency of chromosomal aberrations, both in dicentric and in micronuclei analyses increased with increasing radiation dose.

The utility of a biological dosimeter based on the analysis of dicentrics is invaluable in the event of a radiological emergency wherein the estimated absorbed dose of an exposed individual is crucial in the proper medical management of patients. The technique is also used for routine monitoring of occupationally exposed workers to determine radiation exposure. An in vitro irradiation study of human peripheral blood lymphocytes was conducted to establish a dose-response curve for radiation-induced dicentric aberrations. Blood samples were collected from volunteer donors and together with optically stimulated luminescence (OSL) dosimeters and were irradiated at 0, 0.1, 0.25, 0.5, 0.75, 1, 2, 4, and 6 Gy using a cobalt-60 radiotherapy unit. Blood samples were cultured for 48 h, and the metaphase chromosomes were prepared following the procedure of the International Atomic Energy Agency's Emergency Preparedness and Response – Biodosimetry 2011 manual. At least 100 metaphases were scored for dicentric aberrations at each dose point. The data were analyzed using R language program. The results indicated that the distribution of dicentric cells followed a Poisson distribution and the dose-response curve was established using the estimated model, Y_dic = 0.0003 (±0.0003) +0.0336 (±0.0115) × D + 0.0236 (±0.0054) × D². In this study, the reliability of the dose-response curve in estimating the absorbed dose was also validated for 2 and 4 Gy using OSL dosimeters. The data were fitted into the constructed curve. The result of the validation study showed that the obtained estimate for the absorbed exposure doses was close to the true exposure doses.

The aim of this study was not only to obtain basic technical prerequisites for the establishment of capacity of biological dosimetry at the Ghana Atomic Energy Commission (GAEC) but also to stimulate interest in biological dosimetry research in Ghana and Sub-Saharan Africa. Peripheral blood from four healthy donors was exposed to different doses (0–6 Gy) of gamma rays from a radiotherapy machine and lymphocytes were subsequently stimulated, cultured, and processed according to standard protocols for 48–50 h. Processed cells were analyzed for the frequencies of dicentric and centric ring chromosomes. Radiation dose delivered to the experimental model was verified using GafChromic® EBT films in parallel experiments. Basic technical prerequisites for the establishment of capacity of biological dosimetry in the GAEC have been realized and expertise in the dicentric chromosome assay consolidated. We successfully obtained preliminary cytogenetic data for a dose-response relationship of the irradiated blood lymphocytes. The data strongly indicate the existence of significant linear (α) and quadratic (β) components and are consistent with those published for the production of chromosome aberrations in comparable absorbed dose ranges.

To develop a calibration curve for induction of dicentric chromosomes by radiation, we have used a 60Co gamma-ray source with dose rate of 12.5 mGy/s. Whole blood from 15 healthy donors was collected. Whole blood from each donor was divided equally into 8 parts for exposing to supposedly physical doses 0, 0.30, 0.50, 1.00, 1.50, 2.00, 3.00 and 4.00 Gy for a independent calibration curve. Whole blood from 15 donors was used to calibrate dose – effect and statistical for general calibration curve. Using Poisson test (U-test) for the distribution of dicentric chromosomes in the metaphases to determine the uniformity of the radiation field. The average from 15 independent calibration curves of linear correlated coefficient was determined to be r (y, d) = 0.5136 ± 0.0038. The model equation derived is y = aD + bD2 + C. The calibration equation of dose-effect was y = 1.01D + 4.43D2 + 0.56.

Background: Stress is a term used to define factors involved in changes in the physiological balances resulting in disease conditions. Chronic exposure to stress conditions in modern lifestyles has resulted in a group of disorders called lifestyle disorders. Genetic background and environmental factors are interrelated to lifestyle in determining the health status of individuals. Hence, identification of disease-associated genes is the primary step toward explanations of pathogenesis of these diseases. In functional genomics, large-scale molecular and physiological data are used for the identification of causative genes associated with a disease. Aim: The objective of our study was to find a common set of genes involved in chronic stress-related lifestyle diseases such as cardiovascular diseases (CVDs), type 2 diabetes (T2D), hypertension (HTN), and obesity. Materials and Methods: In our study, we have performed a systematic analysis of the functional gene network of four chronic stress-related lifestyle diseases by retrieving genes from published databases. We have tried to systematically construct a functional protein-protein interaction (PPI) network. The goals of establishing this network were the functional enrichment study of interacting partners as well as functional disease ontology annotation (FunDO) of the enriched genes. Results: This study enabled the identification of key genes involved in these stress-related lifestyle diseases by prioritizing candidate genes based on their degree of involvement. In this systematic analysis, we have found key genes for these diseases based on their involvement and association at the gene network level and PPI. Conclusion: We have deciphered a group of genes that in combination play a crucial role and may impact the function of the whole genome in the four lifestyle disorders mentioned.

Nontargeted effects include radiation-induced genomic instability (RIGI) which is observed in the progeny of cells exposed to ionizing radiation and can be manifested in different ways, including chromosomal instability and micronucleus (MN) formation. Since genomic instability is commonly observed in tumors and has a role in tumor progression, RIGI has the potential of being an important mechanism for radiation-induced cancer. The work presented explores the role of dose and dose rate on RIGI, determined using a MN assay, in normal primary human fibroblast (HF19) cells exposed to either 0.1 Gy or 1 Gy of X-rays delivered either as an acute (0.42 Gy/min) or protracted (0.0031 Gy/min) exposure. While the expected increase in MN was observed following the first mitosis of the irradiated cells compared to unirradiated controls, the results also demonstrate a significant increase in MN yields in the progeny of these cells at 10 and 20 population doublings following irradiation. Minimal difference was observed between the two doses used (0.1 and 1 Gy) and the dose rates (acute and protracted). Therefore, these nontargeted effects have the potential to be important for the low-dose and dose-rate exposure. The results also show an enhancement of the cellular levels of reactive oxygen species after 20 population doublings, which suggests that ionising radiation (IR) could potentially perturb the homeostasis of oxidative stress and so modify the background rate of endogenous DNA damage induction. In conclusion, the investigations have demonstrated that normal primary human fibroblast (HF19) cells are susceptible to the induction of early DNA damage and RIGI, not only after a high dose and high dose rate exposure to low linear energy transfer, but also following low dose, low dose rate exposures. The results suggest that the mechanism of radiation induced RIGI in HF19 cells can be correlated with the induction of reactive oxygen species levels following exposure to 0.1 and 1 Gy low-dose rate and high-dose rate x-ray irradiation.