Impact of the measurement conditions and compression paddle on mammography dosimeter response.

The effect of mammography measurement conditions was investigated to evaluate their impact on measurement uncertainties in clinical practice. The most prominent physical X-ray beam quantities i.e., - air kerma, half-value layer, and X-ray tube voltage - were examined by measuring the response of two ionization chambers and six X-ray multimeters (XMMs) of different models. Measurements were performed using several anode/filter combinations and both with and without the compression paddle in the X-ray beam. Maximum differences of higher than 6 % were found for all quantities when the dosimeter displayed value was compared with the reference value or the variation within the clinical anode/filter combinations Mo/Mo and Mo/Rh were considered. The study showed that the calibration procedure with the W/Al anode/filter combination was reliable only for ionization chambers, and the response of XMMs varies in such a way that the calibration coefficient cannot be predicted between various measurement conditions used in calibration and clinical practices. XMM calibrations are typically performed without a compression paddle in the beam, and the response of the XMM changes when radiation quality is slightly altered. If XMM specific data is not available, based on this study, an additional uncertainty of 2 % (k = 1) could be used as a typical estimate, at least for air kerma measurements. XMMs should be used for clinical measurements in mammography only with correct settings. If the correct settings are not available, the XMMs should not be used or used only with extreme caution.

Manual versus automated γ-H2AX foci analysis across five European laboratories: can this assay be used for rapid biodosimetry in a large scale radiation accident?

The identification of severely exposed individuals and reassurance of the 'worried well' are of prime importance for initial triage following a large scale radiation accident. We aim to develop the γ-H2AX foci assay into a rapid biomarker tool for use in accidents. Here, five laboratories established a standard operating procedure and analysed 100 ex vivo γ-irradiated, 4 or 24h incubated and overnight-shipped lymphocyte samples from four donors to generate γ-H2AX reference data, using manual and/or automated foci scoring strategies. In addition to acute, homogeneous exposures to 0, 1, 2 and 4Gy, acute simulated partial body (4Gy to 50% of cells) and protracted exposures (4Gy over 24h) were analysed. Data from all laboratories could be satisfactorily fitted with linear dose response functions. Average yields observed at 4h post exposure were 2-4 times higher than at 24h and varied considerably between laboratories. Automated scoring caused larger uncertainties than manual scoring and was unable to identify partial exposures, which were detectable in manually scored samples due to their overdispersed foci distributions. Protracted exposures were detectable but doses could not be accurately estimated with the γ-H2AX assay. We conclude that the γ-H2AX assay may be useful for rapid triage following a recent acute radiation exposure. The potentially higher speed and convenience of automated relative to manual foci scoring needs to be balanced against its compromised accuracy and inability to detect partial body exposures. Regular re-calibration or inclusion of reference samples may be necessary to ensure consistent results between laboratories or over long time periods.

Ionizing radiation biomarkers for potential use in epidemiological studies.

Ionizing radiation is a known human carcinogen that can induce a variety of biological effects depending on the physical nature, duration, doses and dose-rates of exposure. However, the magnitude of health risks at low doses and dose-rates (below 100mSv and/or 0.1mSvmin(-1)) remains controversial due to a lack of direct human evidence. It is anticipated that significant insights will emerge from the integration of epidemiological and biological research, made possible by molecular epidemiology studies incorporating biomarkers and bioassays. A number of these have been used to investigate exposure, effects and susceptibility to ionizing radiation, albeit often at higher doses and dose rates, with each reflecting time-limited cellular or physiological alterations. This review summarises the multidisciplinary work undertaken in the framework of the European project DoReMi (Low Dose Research towards Multidisciplinary Integration) to identify the most appropriate biomarkers for use in population studies. In addition to logistical and ethical considerations for conducting large-scale epidemiological studies, we discuss the relevance of their use for assessing the effects of low dose ionizing radiation exposure at the cellular and physiological level. We also propose a temporal classification of biomarkers that may be relevant for molecular epidemiology studies which need to take into account the time elapsed since exposure. Finally, the integration of biology with epidemiology requires careful planning and enhanced discussions between the epidemiology, biology and dosimetry communities in order to determine the most important questions to be addressed in light of pragmatic considerations including the appropriate population to be investigated (occupationally, environmentally or medically exposed), and study design. The consideration of the logistics of biological sample collection, processing and storing and the choice of biomarker or bioassay, as well as awareness of potential confounding factors, are also essential.

Staff eye lens dose in interventional radiology and cardiology in Finland.

PURPOSE: The aim of this study was to investigate the eye lens and whole-body radiation doses to interventional radiology and cardiology staff in two Finnish hospitals. METHODS: Simultaneous measurements of personal dose equivalent quantities Hp(3) and Hp(10) were conducted in clinical conditions during different radiological and cardiological interventional procedures. In order to study the feasibility to estimate eye lens dose with Hp(10) measured over the protective apron or thyroid shield, the ratio between measured Hp(3) and Hp(10) was investigated. RESULTS AND CONCLUSIONS: Applying the obtained ratio on Hp(10) records from national dose register showed that only a small number of interventional radiologists and cardiologists in Finland may exceed eye lens equivalent dose levels of 20 mSv per year or 100 mSv in five consecutive years, but likely do not exceed 50 mSv in a single year. For the most Finnish interventionalists, the eye lens dose is well below 10 mSv per year. Nurses and radiographers assisting in interventions are, on average, less exposed than interventionalists, and will not exceed 20 mSv per year. Based on our results, Hp(10) measured over the protective apron or thyroid shield provides a conservative estimate of the eye lens dose for interventional radiologists and cardiologists, provided that appropriate protective glasses are used.

Clastogenic plasma factors: a short overview.

A large number of studies have revealed that irradiated subjects produce soluble factors found in their blood plasma which, when transferred into cell cultures from non-irradiated individuals, show clastogenic (chromosome breaking) activity. Increased yields of chromatid-type aberrations have been characteristic in most of these studies. Exposed cohorts of various origins have revealed to possess this feature: from A-bomb survivors to patients treated with radiotherapy. It is apparent that the plasma factors are sustainable for long time periods. On the other hand, they seem to be produced very fast after exposure. Considerable variation in the effect has been found between individuals with similar radiation exposure. Further, the phenomenon is not restricted to irradiated populations. Clastogenic plasma has also been observed in patients with inflammatory diseases or congenital chromosome breakage syndromes as well in subjects exposed to other agents than ionizing radiation. Chromosomal aberration inducing substances have been detected not only in vivo, but also in vitro. A common feature to all the conditions is that they are associated with oxidative stress. Studies on the biochemical nature of the clastogenic factor(s) have been conducted, and tumor necrosis factor alpha and lipid peroxidation products, among others, have been suggested as good candidates. The relevance of the plasma factors to health effects remains open. The aim of the paper is to give a short overview on the phenomenon of clastogenic factors--their occurrence and formation as well as possible effectors.

CONTEMPORARY RADIATION DOSES IN INTERVENTIONAL CARDIOLOGY: A NATIONWIDE STUDY OF PATIENT SKIN DOSES IN FINLAND.

In contemporary interventional cardiology, for typical elderly patients, the most severe radiation-related harm to patients can be considered to come from skin exposures. In this paper, maximum local skin doses in cardiological procedures are explored with Gafchromic film dosimetry. Film and reader calibrations and reading were performed at the Secondary Standards Dosimetry Laboratory of the Radiation and Nuclear Safety Authority (STUK), and data were gathered from seven hospitals in Finland. As alert levels for early transient erythema, 200 Gycm2 kerma area product (KAP) and 2000 mGy air kerma levels for transcatheter aortic valve implantations (TAVI) procedures are proposed. The largest doses were measured in TAVI (4158.8 mGy) and percutaneous coronary interventions (PCI) (941.68 mGy). Accuracies of the GE DoseWatch and Siemens CareMonitor skin dose estimates were reasonable, but more results are needed to reliably assess and validate the tools' capabilities and reliabilities. Uncertainty of the Gafchromic dosimetry was estimated as 9.1% for a calibration with seven data points and 19.3% for a calibration with five data points.

International study of factors affecting human chromosome translocations.

Chromosome translocations in peripheral blood lymphocytes of normal, healthy humans increase with age, but the effects of gender, race, and cigarette smoking on background translocation yields have not been examined systematically. Further, the shape of the relationship between age and translocation frequency (TF) has not been definitively determined. We collected existing data from 16 laboratories in North America, Europe, and Asia on TFs measured in peripheral blood lymphocytes by fluorescence in situ hybridization whole chromosome painting among 1933 individuals. In Poisson regression models, age, ranging from newborns (cord blood) to 85 years, was strongly associated with TF and this relationship showed significant upward curvature at older ages versus a linear relationship (p<0.001). Ever smokers had significantly higher TFs than non-smokers (rate ratio (RR)=1.19, 95% confidence interval (CI), 1.09-1.30) and smoking modified the effect of age on TFs with a steeper age-related increase among ever smokers compared to non-smokers (p<0.001). TFs did not differ by gender. Interpreting an independent effect of race was difficult owing to laboratory variation. Our study is three times larger than any pooled effort to date, confirming a suspected curvilinear relationship of TF with age. The significant effect of cigarette smoking has not been observed with previous pooled studies of TF in humans. Our data provide stable estimates of background TF by age, gender, race, and smoking status and suggest an acceleration of chromosome damage above age 60 and among those with a history of smoking cigarettes.

Automated scoring of dicentric chromosomes differentiates increased radiation sensitivity of young children after low dose CT exposure in vitro.

PURPOSE: Automated detection of dicentric chromosomes from a large number of cells was applied to study age-dependent radiosensitivity after in vitro CT exposure of blood from healthy donors. MATERIALS AND METHODS: Blood samples from newborns, children (2-5 years) and adults (20-50 years) were exposed in vitro to 0 mGy, 41 mGy and 978 mGy using a CT equipment. In this study, automated scoring based on 13,000-31,000 cells/dose point/age group was performed. Results for control and low dose points were validated by manually counting about 26,000 cells/dose point/age group. RESULTS: For all age groups, the high number of analyzed cells enabled the detection of a significant increase in the frequency of radiation induced dicentric chromosomes in cells exposed to 41 mGy as compared to control cells. Moreover, differences between the age groups could be resolved for the low dose: young donors showed significantly increased risk for induced dicentrics at 41 mGy compared to adults. CONCLUSIONS: The results very clearly demonstrate that the automated dicentric scoring method is capable of discerning radiation induced biomarkers in the low dose range (<100 mGy) and thus may open possibilities for large-scale molecular epidemiology studies in radiation protection.

Age-dependent differences in DNA damage after in vitro CT exposure.

PURPOSE: Age dependent radiation sensitivity for DNA damage after in vitro blood exposure by computer tomography (CT) was investigated. MATERIALS AND METHODS: Radiation biomarkers (dicentrics and gammaH2AX) in blood samples of newborns, children under five years and adults after sham exposure (0 mGy), low-dose (41 mGy) and high-dose (978 mGy) in vitro CT exposure were analyzed. RESULTS: Significantly higher levels of dicentric induction were found for the single and combined newborns/children group compared to adults, by a factor of 1.48 (95% CI 1.30-1.68), after exposure to 978 mGy. Although a significant dose response for damage induction and dose-dependent repair was found, the gammaH2AX assay did not show an age-dependent increase in DNA damage in newborns/children compared to adults. This was the case for the gammaH2AX levels after repair time intervals of 30 minutes and 24 hours, after correcting for the underlying background damage. For the low dose of 41 mGy, the power of the dicentric assay was also not sufficient to detect an age-dependent effect in the sample size investigated. CONCLUSION: A 1.5-fold increased level of dicentric aberrations is detected in newborns and children under five years after 1 Gy radiation exposure.

An international case-control study of glutathione transferase and functionally related polymorphisms and risk of primary adult brain tumors.

BACKGROUND: Glutathione transferases (GST) detoxify environmental and endogenous compounds and levels of two polymorphic GST proteins, GSTM3 and GSTP1, are high in the brain. Previous studies of GSTM3 and GSTP1 polymorphisms and adult brain tumor risk have produced inconsistent results, whereas the GSTM3 -63 variant is newly identified and, therefore, has not yet been studied in this context. We therefore examined associations between GSTM3 -63, GSTM3 *A/*B, GSTP1 105, and GSTP1 114 variants and adult brain tumor risk and the interaction of the effects of these same polymorphisms with cigarette smoking. In addition, the enzymes NQO1 and CYP1A1 alter susceptibility to oxidative brain damage. Because there is less previous evidence for a role of NQO1, CYP1A1, GSTM1, and GSTT1 variants, we restricted analysis of these variants to a small preliminary study. METHODS: We genotyped DNA collected for an international population-based case-control study of 725 glioma cases, 329 of which were glioblastoma cases, 546 meningioma cases and 1,612 controls. Study participants were residents of Sweden, southeast England, Denmark, and Finland. RESULTS: We found no associations between the GSTM3, GSTP1, NQO1, CYP1A1, GSTM1, or GSTT1 polymorphisms and adult brain tumor risk with the possible exception of a weak association between the G-C (Val-Ala) GSTP1 105/114 haplotype and glioma [odds ratio (OR), 0.73; 95% confidence interval (95% CI), 0.54, 0.99], nor was there an interaction between the effects of the GSTM3 or GSTP1 polymorphisms and cigarette smoking. CONCLUSIONS: Overall, we observed no strong evidence for an association between GST or related enzyme polymorphisms and adult brain tumor risk.

CABAS: a freely available PC program for fitting calibration curves in chromosome aberration dosimetry.

The aim of biological dosimetry is to estimate the dose and the associated uncertainty to which an accident victim was exposed. This process requires the use of the maximum-likelihood method for fitting a calibration curve, a procedure that is not implemented in most statistical computer programs. Several laboratories have produced their own programs, but these are frequently not user-friendly and not available to outside users. We developed a software for fitting a linear-quadratic dose-response relationship by the method of maximum-likelihood and for estimating a dose from the number of aberrations observed. The program called as CABAS consists of the main curve-fitting and dose estimating module and modules for calculating the dose in cases of partial body exposure, for estimating the minimum number of cells necessary to detect a given dose of radiation and for calculating the dose in the case of a protracted exposure. The program is freely available at http://www.pu.kielce.pl/ibiol/cabas.

The second gamma-H2AX assay inter-comparison exercise carried out in the framework of the European biodosimetry network (RENEB).

PURPOSE: Within the EU RENEB project, seven laboratories have taken part in training and harmonisation activities to strengthen triage gamma-H2AX-based radiation exposure assessment. This has culminated in a second triage biodosimetry exercise. MATERIALS AND METHODS: Whole blood and separated lymphocyte samples were homogenously irradiated with 60Co gamma rays at 0.5, 2.5 (blind samples), 0 and 2 Gy (reference samples). Following post-exposure incubations of 4 and 24 h, 16 samples were shipped on ice packs to each partner. The samples were stained and scored for gamma-H2AX foci, using manual and/or automated fluorescence microscope scoring strategies. Dose estimates were obtained and used to assign triage categories to the samples. RESULTS: Average dose estimates across all the laboratories correlated well with true doses. The most accurate assignment of triage category was achieved by manual scoring of the 4-h blood and lymphocyte samples. Only three samples out of a total of 46 were miscategorized in a way that could have adversely effected the clinical management of a radiation casualty. CONCLUSIONS: This inter-comparison exercise has demonstrated that following a recent acute radiation exposure, the gamma-H2AX assay could be a useful triage tool that can be successfully applied across a network of laboratories.

RENEB biodosimetry intercomparison analyzing translocations by FISH.

PURPOSE: In the framework of RENEB, several biodosimetry exercises were conducted analyzing different endpoints. Among them, the analysis of translocations is considered the most useful method for retrospective biodosimetry due to the relative stability of their frequency with post irradiation time. The aim of this study was to harmonize the accuracy of translocation-based biodosimetry within the RENEB consortium. MATERIALS AND METHODS: An initial telescoring exercise analyzing FISH metaphase images was done to harmonize chromosome aberration descriptions. Then two blind intercomparison exercises (IE) were performed, by sending irradiated blood samples to each partner. Samples were cultured and stained by each partner using their standard protocol and translocation frequency was used to produce dose estimates. RESULTS: The coefficient of variation in the 1st IE (CV = 0.34) was higher than in the 2nd IE (CV = 0.16 and 0.23 in the two samples analyzed), for the genomic frequency of total translocations. Z-score analysis revealed that eight out of 10 and 17 out of 20 dose estimates were satisfactory in the 1st and 2nd IE, respectively. CONCLUSIONS: The results obtained indicate that, despite the problems identified in few partners, which can be corrected, the RENEB consortium is able to carry out retrospective biodosimetry analyzing the frequency of translocations by FISH.

Web based scoring is useful for validation and harmonisation of scoring criteria within RENEB.

PURPOSE: To establish a training data set of digital images and to investigate the scoring criteria and dose assessment of the dicentric assay within the European network of biodosimetry (RENEB), a web based scoring inter-comparison was undertaken by 17 RENEB partners. MATERIALS AND METHODS: Two sets of 50 high resolution images were uploaded onto the RENEB website. One set included metaphases after a moderate exposure (1.3 Gy) and the other set consisted of metaphases after a high dose exposure (3.5 Gy). The laboratories used their own calibration curves for estimating doses based on observed aberration frequencies. RESULTS: The dose estimations and 95% confidence limits were compared to the actual doses and the corresponding z-values were satisfactory for the majority; only the dose estimations from two laboratories were too low or too high. The coefficients of variation were 17.6% for the moderate and 11.2% for the high dose. Metaphases with controversial results could be identified for training purposes. CONCLUSIONS: Overall, the web based scoring of the two galleries by the 17 laboratories produced very good results. Application of web based scoring for the dicentric assay may therefore be a relevant strategy for an operational biodosimetry assistance network.

Capabilities of the RENEB network for research and large scale radiological and nuclear emergency situations.

PURPOSE: To identify and assess, among the participants in the RENEB (Realizing the European Network of Biodosimetry) project, the emergency preparedness, response capabilities and resources that can be deployed in the event of a radiological or nuclear accident/incident affecting a large number of individuals. These capabilities include available biodosimetry techniques, infrastructure, human resources (existing trained staff), financial and organizational resources (including the role of national contact points and their articulation with other stakeholders in emergency response) as well as robust quality control/assurance systems. MATERIALS AND METHODS: A survey was prepared and sent to the RENEB partners in order to acquire information about the existing, operational techniques and infrastructure in the laboratories of the different RENEB countries and to assess the capacity of response in the event of radiological or nuclear accident involving mass casualties. The survey focused on several main areas: laboratory's general information, country and staff involved in biological and physical dosimetry; retrospective assays used, the number of assays available per laboratory and other information related to biodosimetry and emergency preparedness. Following technical intercomparisons amongst RENEB members, an update of the survey was performed one year later concerning the staff and the available assays. CONCLUSIONS: The analysis of RENEB questionnaires allowed a detailed assessment of existing capacity of the RENEB network to respond to nuclear and radiological emergencies. This highlighted the key importance of international cooperation in order to guarantee an effective and timely response in the event of radiological or nuclear accidents involving a considerable number of casualties. The deployment of the scientific and technical capabilities existing within the RENEB network members seems mandatory, to help other countries with less or no capacity for biological or physical dosimetry, or countries overwhelmed in case of a radiological or nuclear accident involving a large number of individuals.

RENEB - Running the European Network of biological dosimetry and physical retrospective dosimetry.

PURPOSE: A European network was initiated in 2012 by 23 partners from 16 European countries with the aim to significantly increase individualized dose reconstruction in case of large-scale radiological emergency scenarios. RESULTS: The network was built on three complementary pillars: (1) an operational basis with seven biological and physical dosimetric assays in ready-to-use mode, (2) a basis for education, training and quality assurance, and (3) a basis for further network development regarding new techniques and members. Techniques for individual dose estimation based on biological samples and/or inert personalized devices as mobile phones or smart phones were optimized to support rapid categorization of many potential victims according to the received dose to the blood or personal devices. Communication and cross-border collaboration were also standardized. To assure long-term sustainability of the network, cooperation with national and international emergency preparedness organizations was initiated and links to radiation protection and research platforms have been developed. A legal framework, based on a Memorandum of Understanding, was established and signed by 27 organizations by the end of 2015. CONCLUSIONS: RENEB is a European Network of biological and physical-retrospective dosimetry, with the capacity and capability to perform large-scale rapid individualized dose estimation. Specialized to handle large numbers of samples, RENEB is able to contribute to radiological emergency preparedness and wider large-scale research projects.

RENEB intercomparisons applying the conventional Dicentric Chromosome Assay (DCA).

PURPOSE: Two quality controlled inter-laboratory exercises were organized within the EU project 'Realizing the European Network of Biodosimetry (RENEB)' to further optimize the dicentric chromosome assay (DCA) and to identify needs for training and harmonization activities within the RENEB network. MATERIALS AND METHODS: The general study design included blood shipment, sample processing, analysis of chromosome aberrations and radiation dose assessment. After manual scoring of dicentric chromosomes in different cell numbers dose estimations and corresponding 95% confidence intervals were submitted by the participants. RESULTS: The shipment of blood samples to the partners in the European Community (EU) were performed successfully. Outside the EU unacceptable delays occurred. The results of the dose estimation demonstrate a very successful classification of the blood samples in medically relevant groups. In comparison to the 1st exercise the 2nd intercomparison showed an improvement in the accuracy of dose estimations especially for the high dose point. CONCLUSIONS: In case of a large-scale radiological incident, the pooling of ressources by networks can enhance the rapid classification of individuals in medically relevant treatment groups based on the DCA. The performance of the RENEB network as a whole has clearly benefited from harmonization processes and specific training activities for the network partners.

Impact of types of lymphocyte chromosomal aberrations on human cancer risk: results from Nordic and Italian cohorts.

The frequency of cells with structural chromosomal aberrations (CAs) in peripheral blood lymphocytes is the first genotoxicity biomarker that has shown an association with cancer risk. CAs are usually divided into chromosome-type (CSAs) and chromatid-type aberrations (CTAs), with different mechanisms of formation. From a mechanistic point of view, it is of interest to clarify whether the cancer predictivity of CAs is different with respect to CSAs and CTAs. We report here cancer risk for cytogenetically tested, healthy subjects with respect to frequency of CAs, CSAs, and CTAs in peripheral blood lymphocytes, using Nordic (1981 subjects with CA data, 1871 subjects with CSA/CTA data) and Italian (1573 subjects with CA data, 877 subjects with CTA/CSA data) cohorts, with a median follow-up of 17 years. High levels of CAs at test were clearly associated with increased total cancer incidence in the Nordic cohorts and increased total cancer mortality in the Italian cohort. In the Nordic cohorts, significantly elevated cancer risks were observed for subjects with both high CSAs and high CTAs at test, and these variables showed equally strong cancer predictivity. The results of the Italian cohort did not indicate any clear-cut difference in cancer predictivity between the CSA and CTA biomarkers. There was no significant effect modification by age at test, gender, country, or time since test. The results suggest that both DNA double-strand breaks and other initial DNA lesions responsible for CSAs and CTAs are associated with cancer risk.

Influence of DNA repair gene polymorphisms on the yield of chromosomal aberrations.

We evaluated the influence of several DNA repair gene polymorphisms on the frequency of chromosomal aberrations (CAs) analyzed in peripheral lymphocytes, using the fluorescence in situ hybridization technique. The CA data were obtained from an earlier study of 84 healthy nonsmokers (48 women and 36 men) carefully characterized for indoor radon exposure. The frequency of translocations showed a wide interindividual variability, which was only partly explained by age. To investigate the potential role of DNA repair polymorphisms in this variation, genotypes of DNA repair genes OGG1 (codon 326), XPD (codon 751), XRCC1 (X-ray repair cross-complementing group 1) (codons 194, 280, and 399), and XRCC3 (X-ray repair cross-complementing group 3) (codon 241) were determined from leukocyte DNA using polymerase chain reaction-based methods. Negative binomial regression models were applied to evaluate the effect of the polymorphisms and other factors (age, gender, radon exposure, and medical exposure) on the frequency of CAs. No interactions between genotypes and radon, medical exposure, or gender were found. Carriers of the XRCC1 codon 280His variant allele had a two-fold increase (frequency ratio [FR] = 2.01, 95% confidence interval [CI] = 1.01-3.98; P = 0.046) in unstable exchanges (dicentrics and ring chromosomes). In addition, the XRCC3 codon 241 homozygous variant genotype (Met/Met) was associated with an increase (FR = 1.70, 95% CI = 1.06-2.74; P = 0.028) in two-way translocations when age was taken into account in the analysis. Our data suggest that the XRCC1 280His and XRCC3 241Met alleles affect individual CA levels, most probably via influencing the DNA repair phenotype.

EPI-CT: in vitro assessment of the applicability of the γ-H2AX-foci assay as cellular biomarker for exposure in a multicentre study of children in diagnostic radiology.

PURPOSE: To conduct a feasibility study on the application of the γ-H2AX foci assay as an exposure biomarker in a prospective multicentre paediatric radiology setting. MATERIALS AND METHODS: A set of in vitro experiments was performed to evaluate technical hurdles related to biological sample collection in a paediatric radiology setting (small blood sample volume), processing and storing of blood samples (effect of storing blood at 4°C), the reliability of foci scoring for low-doses (merge γ-H2AX/53BP1 scoring), as well as the impact of contrast agent administration as potential confounding factor. Given the exploratory nature of this study and the ethical constraints related to paediatric blood sampling, blood samples from adult volunteers were used for these experiments. In order to test the feasibility of pooling the γ-H2AX data when different centres are involved in an international multicentre study, two intercomparison studies in the low-dose range (10-500 mGy) were performed. RESULTS: Determination of the number of X-ray induced γ-H2AX foci is feasible with one 2 ml blood sample pre- and post-computed tomography (CT) scan. Lymphocyte isolation and fixation on slides is necessary within 5 h of blood sampling to guarantee reliable results. The possible enhancement effect of contrast medium on the induction of DNA DSB in a patient study can be ruled out if radiation doses and the contrast agent concentration are within diagnostic ranges. The intercomparison studies using in vitro irradiated blood samples showed that the participating laboratories, executing successfully the γ-H2AX foci assay in lymphocytes, were able to rank blind samples in order of lowest to highest radiation dose based on mean foci/cell counts. The dose response of all intercomparison data shows that a dose point of 10 mGy could be distinguished from the sham-irradiated control (p = 0.006). CONCLUSIONS: The results demonstrate that it is feasible to apply the γ-H2AX foci assay as a cellular biomarker of exposure in a multicentre prospective study in paediatric CT imaging after validating it in an in vivo international pilot study on paediatric patients.