<p>In the event of a radiological or nuclear accident, or when physical dosimetry is not available, the scoring of radiation-induced chromosomal aberrations in lymphocytes constitutes an essential tool for the estimation of the absorbed dose of the exposed individual and for effective triage. Cytogenetic biodosimetry employs different cytogenetic assays including the scoring of dicentrics, micronuclei, and translocations as well as analyses of induced premature chromosome condensation to define the frequency of chromosome aberrations. However, inherent challenges using these techniques include the considerable time span from sampling to result, the sensitivity and specificity of the various techniques, and the requirement of highly skilled personnel. Thus, techniques that obviate these challenges are needed. The introduction of telomere and centromere (TC) staining have successfully met these challenges and, in addition, greatly improved the efficiency of cytogenetic biodosimetry through the development of automated approaches, thus reducing the need for specialized personnel. Here, we review the role of the various cytogenetic dosimeters and their recent improvements in the management of populations exposed to genotoxic agents such as ionizing radiation. Finally, we discuss the emerging potentials to exploit these techniques in a wider spectrum of medical and biological applications, e.g., in cancer biology to identify prognostic biomarkers for the optimal triage and treatment of patients.</p>

Digital object identifier (DOI): 10.3390/ijms24065699

<p>In a large-scale catastrophe, such as a nuclear detonation in a major city, it will be crucial to accurately diagnose large numbers of people to direct scarce medical resources to those in greatest need. Currently no FDA-cleared tests are available to diagnose radiation exposures, which can lead to complex, life-threatening injuries. To address this gap, we have achieved substantial advancements in radiation biodosimetry through refinement and adaptation of the cytokinesis-block micronucleus (CBMN) assay as a high throughput, quantitative diagnostic test. The classical CBMN approach, which quantifies micronuclei (MN) resulting from DNA damage, suffers from considerable time and expert labor requirements, in addition to a lack of universal methodology across laboratories. We have developed the CytoRADx™ System to address these drawbacks by implementing a standardized reagent kit, optimized assay protocol, fully automated microscopy and image analysis, and integrated dose prediction. These enhancements allow the CytoRADx System to obtain high-throughput, standardized results without specialized labor or laboratory-specific calibration curves. The CytoRADx System has been optimized for use with both humans and non-human primates (NHP) to quantify radiation dose-dependent formation of micronuclei in lymphocytes, observed using whole blood samples. Cell nuclei and resulting MN are fluorescently stained and preserved on durable microscope slides using materials provided in the kit. Up to 1,000 slides per day are subsequently scanned using the commercially based RADxScan™ Imager with customized software, which automatically quantifies the cellular features and calculates the radiation dose. Using less than 1 mL of blood, irradiated ex vivo, our system has demonstrated accurate and precise measurement of exposures from 0 to 8 Gy (90% of results within 1 Gy of delivered dose). These results were obtained from 636 human samples (24 distinct donors) and 445 NHP samples (30 distinct subjects). The system demonstrated comparable results during in vivo studies, including an investigation of 43 NHPs receiving single-dose total-body irradiation. System performance is repeatable across laboratories, operators, and instruments. Results are also statistically similar across diverse populations, considering various demographics, common medications, medical conditions, and acute injuries associated with radiological disasters. Dose calculations are stable over time as well, providing reproducible results for at least 28 days postirradiation, and for blood specimens collected and stored at room temperature for at least 72 h. The CytoRADx System provides significant advancements in the field of biodosimetry that will enable accurate diagnoses across diverse populations in large-scale emergency scenarios. In addition, our technological enhancements to the well-established CBMN assay provide a pathway for future diagnostic applications, such as toxicology and oncology.</p>

Digital object identifier (DOI): 10.1667/RADE-20-00030.1

Biodosimetry-based individualized reconstruction of complex irradiation scenarios (partial-body shielding and/or neutron + photon mixtures) can improve treatment decisions after mass-casualty radiation-related incidents. We used a high-throughput micronucleus assay with automated scanning and imaging software on ex-vivo irradiated human lymphocytes to: a) reconstruct partial-body and/or neutron exposure, and b) estimate separately the photon and neutron doses in a mixed exposure. The mechanistic background is that, compared with total-body photon irradiations, neutrons produce more heavily-damaged lymphocytes with multiple micronuclei/binucleated cell, whereas partial-body exposures produce fewer such lymphocytes. To utilize these differences for biodosimetry, we developed metrics that describe micronuclei distributions in binucleated cells and serve as predictors in machine learning or parametric analyses of the following scenarios: (A) Homogeneous gamma-irradiation, mimicking total-body exposures, vs. mixtures of irradiated blood with unirradiated blood, mimicking partial-body exposures. (B) X rays vs. various neutron + photon mixtures. The results showed high accuracies of scenario and dose reconstructions. Specifically, receiver operating characteristic curve areas (AUC) for sample classification by exposure type reached 0.931 and 0.916 in scenarios A and B, respectively. R for actual vs. reconstructed doses in these scenarios reached 0.87 and 0.77, respectively. These encouraging findings demonstrate a proof-of-principle for the proposed approach of high-throughput reconstruction of clinically-relevant complex radiation exposure scenarios.

Digital object identifier (DOI): 10.1038/s41598-020-59695-9

During almost 40 years of use, the micronucleus assay (MN) has become one of the most popular methods to assess genotoxicity of different chemical and physical factors, including ionizing radiation-induced DNA damage. In this minireview, we focus on the position of MN among the other genotoxicity tests, its usefulness in different applications and visibility by international organizations, such as International Atomic Energy Agency, Organization for Economic Co-operation and Development and International Organization for Standardization. In addition, the mechanism of micronuclei formation is discussed. Finally, foreseen directions of the MN development are pointed, such as automation, buccal cells MN and chromothripsis phenomenon.

Digital object identifier (DOI): 10.3390/ijms21041534

Analysis of historical negative control group data from the rat in vivo micronucleus assay.

A database of micronuclei counts for historical negative control data from rat in vivo micronuclei tests performed in 10 different laboratories was established. Data were available from over 4000 negative control rats from 10 laboratories. The mean frequency of micronucleated cells (MN)/1000 cells ranged from 0.44 to 2.22, a 5-fold range. Overall there were no major sex or strain differences in frequency, although there were some small but statistically significant differences within laboratories. There was appreciable variability between experiments compared with variability within experiments in some laboratories. No specific factor was identified which could explain this variability although it was noted that many different vehicles were used in the experiments. It is hoped that these data will help laboratories beginning studies with the rat micronucleus assay and those involved in the assessment of micronucleus assay results.

Digital object identifier (DOI): 10.1016/j.mrgentox.2019.503086

In the framework of the 'Realizing the European Network of Biodosimetry' (RENEB) project, two intercomparison exercises were conducted to assess the suitability of an optimized version of the cytokinesis-block micronucleus assay, and to evaluate the capacity of a large laboratory network performing biodosimetry for radiation emergency triages. Twelve European institutions participated in the first exercise, and four non-RENEB labs were added in the second one. Irradiated blood samples were shipped to participating labs, whose task was to culture these samples and provide a blind dose estimate. Micronucleus analysis was performed by automated, semi-automated and manual procedures. The dose estimates provided by network laboratories were in good agreement with true administered doses. The most accurate estimates were reported for low dose points (≤ 0.94 Gy). For higher dose points (≥ 2.7 Gy) a larger variation in estimates was observed, though in the second exercise the number of acceptable estimates increased satisfactorily. Higher accuracy was achieved with the semi-automated method. The results of the two exercises performed by our network demonstrate that the micronucleus assay is a useful tool for large-scale radiation emergencies, and can be successfully implemented within a large network of laboratories.

Digital object identifier (DOI): 10.1080/09553002.2016.1206231

<p>Ionising radiation causes free radical-mediated damage in cellular DNA. This damage is manifested as chromosomal aberrations and micronuclei (MN) in proliferating cells. Sesamol, present in sesame seeds, has the potential to scavenge free radicals; therefore, it can reduce radiation-induced cytogenetic damage in cells. The aim of this study was to investigate the radioprotective potential of sesamol in bone marrow cells of mice and related haematopoietic system against radiation-induced genotoxicity. A comparative study with melatonin was designed for assessing the radioprotective potential of sesamol. C57BL/6 mice were administered intraperitoneally with either sesamol or melatonin (10 and 20mg/kg body weight) 30min prior to 2-Gy whole-body irradiation (WBI) and sacrificed after 24h. Total chromosomal aberrations (TCA), MN and cell cycle analyses were performed using bone marrow cells. The comet assay was performed on bone marrow cells, splenocytes and lymphocytes. Blood was drawn to study haematological parameters. Prophylactic doses of sesamol (10 and 20mg/kg) in irradiated mice reduced TCA and micronucleated polychromatic erythrocyte frequency in bone marrow cells by 57 % and 50 %, respectively, in comparison with radiation-only groups. Sesamol-reduced radiation-induced apoptosis and facilitated cell proliferation. In the comet assay, sesamol (20mg/kg) treatment reduced radiation-induced comets (% DNA in tail) compared with radiation only (P &lt; 0.05). Sesamol also increased granulocyte populations in peripheral blood similar to melatonin. Overall, the radioprotective efficacy of sesamol was found to be similar to that of melatonin. Sesamol treatment also showed recovery of relative spleen weight at 24h of WBI. The results strongly suggest the radioprotective efficacy of sesamol in the haematopoietic system of mice.</p>

<p>Low temperature (hypothermia) during irradiation leads to a reduced frequency of micronuclei in TK6 cells and it has been suggested that perturbation of cell cycle progression is responsible for this effect. The aim of the study was to test this hypothesis.Human lymphoblastoid TK6 cells were treated by a combination of hypothermia (0.8°C) and ionizing radiation in varying order (hypothermia before, during or after irradiation) and micronuclei were scored. Growth assay and two-dimensional flow cytometry was used to analyze cell cycle kinetics following irradiated of cells at 0.8°C or 37.0°C.The temperature effect was observed at the level of micronuclei regardless of whether cells were cooled during or immediately before or after the radiation exposure. No indication of cell cycle perturbation by combined exposure to hypothermia and radiation could be detected.The protective effect of hypothermia observed at the level of cytogenetic damage was not due to a modulation of cell cycle progression. A possible alternative mechanism and experiments to test it are discussed.</p>

<p>To investigate both the formation of micronuclei (MN) and the induction and subsequent loss of phosphorylated histone H2AX foci (gammaH2AX foci) after in vitro exposure of human lymphocytes to either (60)Co gamma-rays or p(66)+ Be(40) neutrons.MN dose response (DR) curves were obtained by exposing isolated lymphocytes of 10 different donors to doses ranging from 0-4 Gy gamma-rays or 0-2 Gy neutrons. Also, gammaH2AX foci DR curves were obtained following exposure to doses ranging from 0-0.5 Gy of either gamma-rays or neutrons. Foci kinetics for lymphocytes for a single donor exposed to 0.5 Gy gamma-rays or neutrons were studied up to 24 hours post-irradiation.Micronuclei yields following neutron exposure were consistently higher compared to that from (60)Co gamma-rays. All MN yields were over-dispersed compared to a Poisson distribution. Over-dispersion was higher after neutron irradiation for all doses &gt; 0.1 Gy. Up to 4 hours post-irradiation lower yields of neutron-induced gammaH2AX foci were observed. Between 4 and 24 hours the numbers of foci from neutrons were consistently higher than that from gamma-rays. The half-live of foci disappearance is only marginally longer for neutrons compared to that from gamma-rays. Foci formations were more likely to be over-dispersed for neutron irradiations.Although neutrons are more effective to induce MN, the absolute number of induced gammaH2AX foci are less at first compared to gamma-rays. With time neutron-induced foci are more persistent. These findings are helpful for using gammaH2AX foci in biodosimetry and to understand the repair of neutron-induced cellular damage.</p>

<p>The European Union's Seventh Framework Programme-funded project 'Multi-disciplinary biodosimetric tools to manage high scale radiological casualties' (MULTIBIODOSE) has developed a multiparametric approach to radiation biodosimetry, with a particular emphasis on triage of large numbers of potentially exposed individuals following accidental exposures. In November 2012, an emergency exercise took place which tested the capabilities of the MULTIBIODOSE project partners. The exercise described here had a dual purpose: Intercomparison of (i) three biodosimetric assays, and (ii) the capabilities of the seven laboratories, with regards to provision of triage status for suspected radiation exposed individuals.Three biological dosimetry tools - the dicentric, micronucleus and gamma-H2AX (the phosphorylated form of member X of histone H2A, in response to DNA double-strand breaks) foci assays - were tested, in addition to provision of the triage status results (low exposure: 2 Gy) by the MULTIBIODOSE software. The exercise was run in two modes: An initial triage categorisation of samples (based on the first dose estimates for each assay received from each laboratory) followed by collation of the full set of estimated doses (all the results from all modes of each assay carried out by the participating laboratories) calculated using as many modes of operation as possible of the different assays developed during the project. Simulated acute whole body and partial body exposures were included.The results of the initial triage categorisation and the full comparison of assays and methods within and between laboratories are presented here.The data demonstrate that the MULTIBIODOSE approach of applying multiparametric tools to radiation emergencies is valid and effective.</p>

Azidothymidine (Zidovudine, AZT) is part of the standard care of treatment for acquired immunodeficiency syndrome since many years. A great number of studies on the genotoxic potential of AZT have been published, but no comprehensive hypothesis yet explains all observations. We investigated a multitude of genotoxic endpoints, both in vitro and in vivo, with the goal to complete the picture. The mutagenic potential of AZT in bacteria was found to be restricted to strains with an #ochre# target sequence and could be abrogated both by thymidine supplementation and rat liver S9 mix. Single-strand breaks in mammalian cells were detected in the comet assay after short-term treatment (3h) with AZT, which did not induce micronuclei. The latter were mainly seen after prolonged exposure (24 and 48h) and are probably not directly related to AZT incorporation into DNA. Our data demonstrate that short-term exposure to low AZT concentrations does not induce biologically relevant micronucleation. Only treatment with high concentrations of AZT for prolonged time periods manifests in substantial micronucleus induction. Furthermore, we found that high concentrations of thymidine have no effect in the comet assay but increase micronucleus frequency in a manner very similar to AZT. These results lead us to the following hypothesis: AZT is triphosphorylated and then incorporated into DNA strands, leading to mutations and cytotoxicity. Cellular attempts to repair these DNA lesions as well as stalled replication forks due to chain termination are detectable with the comet assay. Increased micronucleus frequency is likely related to nucleotide pool imbalance.

<p>The genotoxic potential of azidothymidine (Zidovudine, AZT), chosen as a model compound for nucleotide analogs, was comprehensively assessed in vivo for gene mutation, clastogenicity, and DNA breakage endpoints. Male Wistar rats were treated by oral gavage over 7 days with AZT at dose levels of 2×0 (control), 2×250, 2×500, and 2×1000mg/kg/day with a final single dose given on day 8. DNA damage was then evaluated with the comet assay in liver, stomach, and peripheral blood and with the micronucleus test in bone marrow and peripheral blood (by flow cytometry) in the same animals. After a treatment-free period of upto 42 days, the Pig-a gene mutation assay was performed in peripheral blood of the high-dose animals. In the comet assay as well as the micronucleus test, AZT caused a considerable dose-dependent increase in DNA damage in all tissues evaluated and was highly cytotoxic to bone marrow and peripheral blood cells. These data are well in line with published results. Surprisingly, AZT did not significantly increase the number of Pig-a mutant cells. We speculate that two factors likely contributed to this negative result: a predominance of large deletions caused by AZT, and the relatively low statistical power of the first-generation scoring method used for this study.</p>

A semi-automated scoring system has been developed to provide rapid, accurate assessment of micronuclei in preparations of mononuclear mouse lymphoma L5178Y cells. Following exposure to a range of test agents, flat, single-cell preparations were produced from exponentially growing cultures by cytocentrifugation. Following staining with 4'-6-diamidino-2-phenylindole (DAPI), cells were scanned by use of the MicroNuc module of Metafer 4 v 3.4.102, after modifying the classifier developed for selecting micronuclei in binucleate cells to increase its sensitivity. The image gallery of all cells was then sorted to bring aberrant cells to the top of the gallery to assess visually the numbers of cells with micronuclei, as distinct from other debris. Slide quality was shown to be paramount in obtaining accurate results from an automated scan and the data obtained compared very well with the incidence of micronuclei scored conventionally by microscopy. Compared with manual scoring the time saving is considerable, as more than 2000 images are captured in approximately 2min, with subsequent visual assessment of aberrant cells in the image gallery taking about 1-2min/slide. By scanning all aberrant cells, the system also captures additional information on necrotic, apoptotic and fragmented cells. Although optimised for mouse lymphoma cells, it should be simple to adapt the method for any cell type growing in suspension.

Automated image analysis scoring of micronuclei (MN) in cells can facilitate the objective and rapid measurement of genetic damage in mammalian and human cells. This approach was repeatedly developed and tested over the past two decades but none of the systems were sufficiently robust for routine analysis of MN until recently. New methodological, hardware and software developments have now allowed more advanced systems to become available. This mini-review presents the current stage of development and validation of the Metasystems Metafer MNScore system for automated image analysis scoring of MN in cytokinesis-blocked binucleated lymphocytes, which is the best-established method for studying MN formation in humans. The results and experience of users of this system from 2004 until today are reviewed in this paper. Significant achievements in the application of this method in research related to mutagen sensitivity phenotype in cancer risk, radiation biodosimetry and biomonitoring studies of air pollution (enriched by new data) are described. Advantages as well as limitations of automated image analysis in comparison with traditional visual analysis are discussed. The current increased use of the Metasystems Metafer MNScore system in various studies and the growing number of publications based on automated image analysis scoring of MN is promising for the ongoing and future application of this approach.

The scoring of micronuclei (MN) is widely used in biomonitoring and mutagenicity testing as a surrogate marker of chromosomal damage inflicted by clastogenic agents or by aneugens. Individual differences in the response to a mutagenic challenge are known from studies on cancer patients and carriers of mutations in DNA repair genes. However, it has not been studied to which extent genetic factors contribute to the observed variability of individual MN frequencies. Our aim was to quantify this heritable genetic component of both baseline and radiation-induced MN frequencies. We performed a twin study comprising 39 monozygotic (MZ) and 10 dizygotic (DZ) twin pairs. Due to the small number of DZ pairs, we had to recruit controls from which 38 age- and gender-matched random control pairs (CPs) were generated. For heritability estimates, we used biometrical modelling of additive genetic, common environmental, and unique environmental components (ACE model) of variance and direct comparison of variance between the sample groups. While heritability estimates from MZ to DZ comparisons produced inconclusive results, both estimation methods revealed a high degree of heritability (h(2)) for baseline MN frequency (h(2) = 0.68 and h(2) = 0.72) as well as for the induced frequency (h(2) = 0.68 and h(2) = 0.57) when MZ were compared to CP. The result was supported by the different intraclass correlation coefficients of MZ, DZ and CP for baseline (r = 0.63, r = 0.31 and r = 0.0, respectively) as well as for induced MN frequencies (r = 0.79, r = 0.74 and r = 0.0, respectively). This study clearly demonstrates that MN frequencies are determined by genetic factors to a major part. The strong reflection of the genetic background supports the idea that MN frequencies represent an intermediate phenotype between molecular DNA repair mechanisms and the cancer phenotype and affirms the approaches that are made to utilise them as predictors of, for example, cancer risk.

Biological dosimetry, based on the analysis of micronuclei (MN) in the cytokinesis-block micronucleus (CBMN) assay can be used as an alternative method for scoring dicentric chromosomes in the field of radiation protection. Biological dosimetry or Biodosimetry, is mainly performed, in addition to physical dosimetry, with the aim of individual dose assessment. Many studies have shown that the number of radiation-induced MN is strongly correlated with dose and quality of radiation. The CBMN assay has become, in the last years, a thoroughly validated and standardised technique to evaluate in vivo radiation exposure of occupational, medical and accidentally exposed individuals. Compared to the gold standard, the dicentric assay, the CBMN assay has the important advantage of allowing economical, easy and quick analysis. The main disadvantage of the CBMN assay is related to the variable micronucleus (MN) background frequency, by which only in vivo exposures in excess of 0.2-0.3 Gy X-rays can be detected. In the last years, several improvements have been achieved, with the ultimate goals (i) of further increasing the sensitivity of the CBMN assay for low-dose detection by combining the assay with a fluorescence in situ hybridisation centromere staining technique, (ii) of increasing the specificity of the test for radiation by scoring nucleoplasmic bridges in binucleated cells and (iii) of making the assay optimally suitable for rapid automated analysis of a large number of samples, viz. in case of a large-scale radiation accident. The development of a combined automated MN-centromere scoring procedure remains a challenge for the future, as it will allow systematic biomonitoring of radiation workers exposed to low-dose radiation.

<p>In the case of a large-scale nuclear or radiological incidents a reliable estimate of dose is an essential tool for providing timely assessment of radiation exposure and for making life-saving medical decisions. Cytogenetics is considered as the #gold##standard# for biodosimetry. The dicentric analysis (DA) represents the most specific cytogenetic bioassay. The micronucleus test (MN) applied in interphase in peripheral lymphocytes is an alternative and simpler approach. A dose-effect calibration curve for the MN frequency in peripheral lymphocytes from 27 adult donors was established after in vitro irradiation at a dose range 0.15-8 Gy of 137Cs gamma rays (dose rate 6 Gy min-1). Dose prediction by visual scoring in a dose-blinded study (0.15-4.0 Gy) revealed a high level of accuracy (R = 0.89). The scoring of MN is time consuming and requires adequate skills and expertise. Automated image analysis is a feasible approach allowing to reduce the time and to increase the accuracy of the dose estimation decreasing the variability due to subjective evaluation. A good correlation (R = 0.705) between visual and automated scoring with visual correction was observed over the dose range 0-2 Gy. Almost perfect discrimination power for exposure to 1-2 Gy, and a satisfactory power for 0.6 Gy were detected. This threshold level can be considered sufficient for identification of sub lethally exposed individuals by automated CBMN assay.</p>

Cytosine arabinoside (a nucleoside analogue that inhibits the gap-filling step of excision repair), vinblastine (an aneugen that inhibits tubulin polymerisation), 5-fluorouracil (a nucleoside analogue with a steep response profile), and 2-aminoanthracene (a metabolism-dependent reference genotoxin) were tested in the in vitro micronucleus assay with L5178Y mouse lymphoma cells, without cytokinesis block. The four chemicals were independently evaluated in two Sanofi Aventis laboratories, one of which used an image analyser to score micronuclei, while the other scored micronucleated cells manually. Very similar results were obtained in the two laboratories, highlighting the robustness of the assay. The four test chemicals induced significant increases in the incidence of micronucleated cells at concentrations that produced no more than a 55+/-5\% reduction in survival growth, as measured with the three parameters recommended in the draft OECD Test Guideline on In Vitro Mammalian Cell Micronucleus Test (MNvit) for chemical testing, namely the relative increase in cell counts, relative population doubling, and the relative cell count. These results support the premise that the relative increase in cell counts and relative population doubling, that take into account both cell death and cytostasis, are appropriate measures of survival growth reduction in the in vitro micronucleus test conducted in the absence of cytokinesis block, as recommended in MNvit.

The measurement of micronuclei (MN) in human peripheral blood lymphocytes is frequently used in molecular epidemiology as one of the preferred methods for assessing chromosomal damage resulting from environmental mutagen exposure. In the present study, we evaluated the effect of exposure to carcinogenic polycyclic aromatic hydrocarbons (c-PAHs), volatile organic compounds (VOC) and smoking on the frequency of MN in a group of 56 city policemen living and working in Prague. The average age of the participants was 34+/-6 years. The study was conducted on the same subjects in February and May 2007. The concentrations of air pollutants were obtained from personal and stationary monitoring. A statistically significant decrease in the levels of pollutants was observed in May when compared with February, with the exception of toluene levels measured by stationary monitoring. The frequency of MN was determined by the automatic image scoring (MetaSystems Metafer 4, version 3.2.1) of DAPI-stained slides. The results of the image analysis indicated a significant difference in the frequency of MN (mean levels 7.32+/-3.42 and 4.67+/-2.92, for February and May, respectively). Our study suggests that automatic image analysis of MN is a highly sensitive method for evaluating the effect of c-PAHs and confirms that there are no differences between smokers and nonsmokers. These results demonstrate the ability of c-PAHs to increase MN frequency, even if the exposure to c-PAHs occurred up to 60 days before the collection of biological material. Our work is the first human biomonitoring study focused on the measurement of MN by automated image analysis for assessing chromosomal damage as a result of environmental mutagen exposure.

The cytokinesis-block micronucleus assay in peripheral blood lymphocytes is a standardised and validated technique for biodosimetry. Automated scoring of micronuclei allows large scale applications as in population triage in case of radiation accidents or malevolent use of radioactive sources. The dose detection limit (95% confidence) of the micronucleus assay for individual dose assessment is restricted to 0.2 Gy but can be decreased to 0.1 Gy by scoring centromeres in micronuclei using fluorescence in situ hybridization (FISH). In the past the micronucleus assay was applied for a number of large scale biomonitoring studies of nuclear power plant workers and hospital workers. Baseline micronucleus frequencies depend strongly on age and gender. The assay was also already used for biodosimetry of radiation accidents. In a multiple endpoint biodosimetry study for dose assessment of a worker exposed accidentally in 2003 to X-rays, a good agreement was obtained between dose estimates resulting from the micronucleus assay, the scoring of dicentrics and translocations. Automated scoring of micronuclei in combination with centromere signals, allowing systematic biodosimetry of exposed populations, remains a challenge for the future.