Justification of dose limits-historical development, ethical basis, and the way forward.

The dose limits recommended by the International Commission on Radiological Protection have undergone considerable changes in the course of the past 90 years, and different arguments have been put forward for their justification. This has been largely due to new insights of radiation research, but changing perceptions of the related ethical values have also played a role. The current paper reviews important stages in this development and attempts to derive some implications for the Commission's next general recommendations. Above all, it suggests that it is essential to present clear and consistent justification strategies for dose limits (and related values), compatible with the core values of the system of radiological protection, especially prudence and justice.

The role of empathy in ethics of radiological protection.

Radiological protection is often considered a matter of scientific and technological facts only, not of value judgements. This perception is now gradually changing, especially with ICRP Publication 138, which addressed the ethical foundation of the system of radiological protection. It identified values which have guided the Commission's recommendations over the decades, but have not always been made explicit. Four core values are discussed (beneficence/non-maleficence, prudence, justice, dignity) as well as three procedural values (accountability, transparency, inclusivity). The latter are considered critical to the practical implementation of the system of radiological protection. Here we are exploring empathy as a procedural values complementing the three identified in ICRP Publication 138. Empathy can be defined as the 'capability (or disposition) to immerse oneself in and to reflect upon the experiences, perspectives and contexts of others'. It is often understood as a skill that one either has or has not, but research has shown it can be taught and therefore can be required as an attitude of those working in health care, education, design, and technology. We suggest it is an essential prerequisite to the assessment and management of any radiological situation and the health problems accruing from it. The concerns of people affected, their needs and wishes need to be taken seriously from the very beginning of any decision-making process. Even if they are considered unfounded and exaggerated, the insights they provide will be valuable for the understanding of the overall situation. Without empathy, our practice of beneficence and non-maleficence as well as solidarity would be oddly limited.

Incidence of spontaneous abortions and congenital anomalies in the vicinity of a uranium processing plant.

OBJECTIVES: The exposure of embryos or foetuses to ionising radiation can cause serious detriments to health. Thus, an enhanced incidence of spontaneous abortions and congenital anomalies might be expected in the vicinity of a uranium processing plant. We analysed the situation in the vicinity of MAPE Mydlovary, a facility about 20 km from Ceské Budejovice, South Bohemia, Czech Republic, which was in operation from 1963 to 1992. METHODS: No relevant data are available for the period of operation of the uranium processing plant. Statistical data have only been collected since 1994. As sanitation work in the area was initiated at around that time and has yet to be completed, we considered a study of possible prenatal effects in the vicinity of MAPE Mydlovary to still be of interest. Data were provided by the Institute of Health Information and Statistics of the Czech Republic for the years 1994-2013. RESULTS: We tested whether there are demonstrable, statistically significant differences between the microregions of the four closest villages (Mydlovary, Olesník, Zahájí, and Zliv), the District of Ceské Budejovice, the South-Bohemian Region, and the Czech Republic. CONCLUSIONS: No increase was found in the incidence of spontaneous abortions and congenital anomalies in the vicinity of this former uranium processing plant compared to the surrounding District of Ceské Budejovice, the South Bohemian Region, or the Czech Republic as a whole.

Towards a strategic research agenda for social sciences and humanities in radiological protection.

Reflecting a change in funding strategies for European research projects, and a commitment to the idea of responsible research and innovation in radiological protection (RP), a collective of research institutes and universities have developed a prospective Strategic Research Agenda (SRA) for Social Sciences and Humanities (SSH) in radiological protection. This is the first time such a research agenda has been proposed. This paper identifies six research lines of interest and concern: (1) Effects of social, psychological and economic aspects on RP behaviour; (2) Holistic approaches to the governance of radiological risks; (3) Responsible research and innovation in RP; (4) Stakeholder engagement and participatory processes in RP research, development, policy and practice; (5) Risk communication; and (6) RP cultures. These topics were developed through broad stakeholder consultation, in conjunction with activities carried out in the framework of various projects and initiatives (EU H2020 CONCERT programme, the EU FP7 projects OPERRA, PREPARE and EAGLE, the 2015-2018 RICOMET series of conferences, and the 2014 and 2016 International Symposia on Ethics of Environmental Health); as well as through dialogues with members of the European radiation protection research communities. The six research lines open opportunities to integrate a range of key social and ethical considerations into RP, thereby expanding research opportunities and programmes and fostering collaborative approaches to research and innovation.

Micronucleus frequency and content in healthy relatives of cancer patients.

OBJECTIVE: To compare chromosomal damage in lymphocytes of individuals who did or did not report first-degree relatives with cancer. MATERIALS AND METHODS: Cases and controls (68 each) were matched for sex, age and radon exposure. Chromosomal damage was quantified as frequency of micronucleus-containing cells and proportion of centromere-free micronuclei. RESULTS: Individuals not reporting cancer in their families showed lower values of both the frequency of micronucleus-containing cells (n.s.) and the proportion of centromere-free micronuclei (p < 0.05) in some subgroups, but not in all. CONCLUSION: The chromosomal damage observed may be due to inheritable genomic instability, but environmental influences cannot be excluded.

Radioactivity in mushrooms from selected locations in the Bohemian Forest, Czech Republic.

137Cs is one of the most important radionuclides released in the course of atmospheric nuclear weapon tests and during accidents in nuclear power plants such as that in Chernobyl, Ukraine, or Fukushima, Japan. The aim of this study was to compare 137Cs and 40K concentrations in particular species of mushrooms from selected locations in the Bohemian Forest (Czech: Sumava), Czech Republic, where a considerable contamination from the Chernobyl accident had been measured in 1986. Samples were collected between June and October 2014. Activities of 137Cs and 40K per dry mass were measured by means of a semiconductor gamma spectrometer. The 137Cs values measured range from below detection limit to 4300 ± 20 Bq kg-1, in the case of 40K from 910 ± 80 to 4300 ± 230 Bq kg-1. Differences were found between individual locations, due to uneven precipitation in the course of the movement of the radioactive cloud after the Chernobyl accident. There are, however, also differences between individual species of mushrooms from identical locations, which inter alia result from different characteristics of the soil and depths of mycelia. The values measured are compared with established limits and exposures from other radiation sources present in the environment. In general, it can be stated that the values measured are relatively low and the effects on the health of the population are negligible compared to other sources of ionizing radiation.

Analysis of Genetic Damage in Lymphocytes of Former Uranium Processing Workers.

The frequency of cells containing micronuclei (MN) and the presence of centromeres in these MN were analyzed in lymphocytes of 98 men from Southern Bohemia. Forty-six of them had worked at the uranium processing plant 'MAPE Mydlovary' which was closed in 1991, and 52 men were controls from the same area. FISH using human pan-centromeric chromosome paint was employed to detect centromere-positive (CEN+) and -negative (CEN-) MN. A total of 1,000 binucleated cells (BNC) per participant were analyzed after cytochalasin B treatment. All BNC with MN (CEN+ or CEN-) were recorded. No differences were found between formerly exposed workers and the control group, neither in the total frequency of cells with MN per 1,000 BNC (mean levels ± SD, 9.1 ± 3.1 and 9.8 ± 2.5, respectively) nor in the percentage of CEN- MN, which were equal (50 ± 18 and 49 ± 17, respectively). Also, there was no difference between individuals living in the 3 villages closest to the uranium processing plant and those living further away. Considering the fact that effective doses of the workers at MAPE Mydlovary were overall similar to those of former uranium miners in whom higher frequencies of CEN- MN have been found more than 10 years after they had finished working underground, these results are somewhat surprising. A more detailed analysis of the exposures indicates that uranium miners received a greater percentage of their effective dose from the inhalation of radon and its daughters, whereas uranium processing workers received it from the incorporation of long-lived radioactive nuclides such as uranium. If, as has been suggested before, the higher level of DNA damage in miners is due to induced genomic instability, then this phenomenon may be related to radon exposure rather than exposure to uranium.

G2-block after irradiation of cells with different p53 status.

BACKGROUND: Although it is clear that functional p53 is not required for radiation-induced G2 block, certain experimental findings suggest a role for p53 in this context. For instance, as we also confirm here, the maximum accumulation in the G2 compartment after X-ray exposure occurs much later in p53 mutants than in wild types. It remains to be seen, however, whether this difference is due to a longer block in the G2 phase itself. MATERIAL AND METHODS: We observed the movement of BrdU-labeled cells through G2 and M into G1. From an analysis of the fraction of labeled cells that entered the second posttreatment cell cycle, we were able to determine the absolute duration of the G2 and M phases in unirradiated and irradiated cells. RESULTS: Our experiments with four cell lines, two melanomas and two squamous carcinomas, showed that the radiation-induced delay of transition through the G2 and M phases did not correlate with p53 status. CONCLUSION: We conclude that looking at the accumulation of cells in the G2 compartment alone is misleading when differences in the G2 block are investigated and that the G2 block itself is indeed independent of functional p53.

Differential S-phase progression after irradiation of p53 functional versus non-functional tumour cells.

BACKGROUND: Many pathways seem to be involved in the regulation of the intra-S-phase checkpoint after exposure to ionizing radiation, but the role of p53 has proven to be rather elusive. Here we have a closer look at the progression of irradiated cells through S-phase in dependence of their p53 status. MATERIALS AND METHODS: Three pairs of tumour cell lines were used, each consisting of one p53 functional and one p53 non-functional line. Cells were labelled with bromodeoxyuridine(BrdU) immediately after irradiation, they were then incubated in label-free medium, and at different times afterwards their position within the S-phase was determined by means of flow cytometry. RESULTS: While in the p53 deficient cells progression through S-phase was slowed significantly over at least a few hours, it was halted for just about an hour in the p53 proficient cells and then proceeded without further delay or even at a slightly accelerated pace. CONCLUSIONS: It is clear from the experiments presented here that p53 does play a role for the progress of cells through the S-phase after X-ray exposure, but the exact mechanisms by which replicon initiation and elongation is controlled in irradiated cells remain to be elucidated.

Micronuclei in lymphocytes from radon spa personnel in the Czech Republic.

PURPOSE: To assess the radiation exposure of radon spa personnel in Jáchymov, Czech Republic. METHODS: The frequency of micronucleus-containing cells and the percentage of centromere-free micronuclei (micronuclei containing only acentric fragments) was determined in peripheral blood lymphocytes of 42 individuals working at the Jáchymov spa and 42 control individuals. RESULTS: There was a highly significant increase in the frequency of micronucleus-containing cells as well as the percentage of centromere-free micronuclei in the lymphocytes of spa personnel versus controls. No individual dosimetry data were available. A comparison with results from currently active uranium miners suggests that the individuals examined at the Jáchymov spa had accumulated effective doses in the order of several tens of mSv, very similar to those of the miners. CONCLUSION: The spa personnel in Jáchymov needs to be monitored on an individual level.

Micronuclei in lymphocytes from currently active uranium miners.

Micronuclei can be used as markers of past radiation exposure, but only few studies have dealt with uranium miners. In this paper, we report on micronuclei in lymphocytes from individuals currently working at Rozná, Czech Republic, the last functioning uranium mine in the European Union. A modified micronucleus-centromere test was applied to assess the occurrence of micronuclei in stimulated lymphocytes, as well as their content in terms of whole chromosomes or fragments. Compared with unexposed individuals, the miners had higher frequencies of micronucleus-containing lymphocytes and higher percentages of micronuclei without centromeres, and the differences were significant for both parameters (0.74 ± 0.60 vs. 0.50 ± 0.42, p = 0.017 and 49 ± 44 vs. 12 ± 21, p = 0.0002; means ± standard deviations). There were also significant correlations between one or other of these parameters on the one hand and various dose values on the other, in particular with a 'retrievable' dose, that is, a dose whose effect should still be recognisable in lymphocytes assuming a half-life of 3 years. The 'retrievable' dose at which a doubling of the micronucleus frequency was observed was around 35 mSv, corresponding to a total dose of 90 mSv received while working in the mines. Altogether, our data show that the micronucleus-centromere test is a valuable tool for the assessment of past radiation exposure in uranium miners. The scatter in the data is of course far too great to allow individual dosimetry, but for groups of a few dozen exposed individuals, the method can be used to monitor doses clearly below 100 mSv.

CZECH RADIOGRAPHERS' PERCEPTION OF ETHICAL ASPECTS OF RADIOLOGICAL PROTECTION.

This study assessed the perception of ethics by 40 radiographers and 30 radiography students in the Czech Republic. Most respondents confirmed that they considered the topic to be important for their profession. The majority of radiographers, but only few students, had encountered some ethical dilemma during their work. Almost all were aware of the existence of a Code of Ethics for radiographers, and the majority had at least some ethics education during their professional training. Our findings differ from those of similar surveys in the USA published in 2013 and 2015. In these, two-thirds of the respondents stated that they had not received any instruction in ethics during their training, and one-third indicated that they had witnessed unethical behaviour in their profession. We conclude that the ethics education of Czech radiographers makes them more aware of ethical problems when they encounter them.

Solar Ultraviolet Radiation Risk Estimates-A Comparison of Different Action Spectra and Detector Responsivities.

Studies assessing the dose-response relationship for human skin cancer induction by solar ultraviolet radiation (UVR) apply a range of methods to quantify relevant UVR doses, but information about the comparability of these datasets is scarce. We compared biologically weighted effectivities applying the most relevant UVR action spectra in order to test the ability of certain UVR detectors to mimic these biological effects at different times during the day and year. Our calculations were based on solar spectra measured at Dortmund, Germany (51.5° N) and at Townsville, Australia (19.3° S), or computed for latitudes 20° S and 50° N. Convolutions with the CIE action spectra for erythema and non-melanoma skin cancer (NMSC) and with ICNIRP's weighting function showed comparable solar zenith angle (SZA) dependences with little influence of season or latitude. A different SZA dependence was found with Setlow's action spectrum for melanoma induction. Calculations for a number of UVR detector responsivities gave widely discrepant absolute irradiances and doses, which were nevertheless related to those calculated with both CIE spectra by correction factors largely independent of the SZA. Commonly used detectors can thus provide quite accurate estimates of NMSC induction by solar UVR, whereas they may be inadequate to mimic melanoma induction.

Empathy as an ethical principle for environmental health.

PURPOSE: Environmental health ethics is a relatively young field of study, drawing on experience from medical ethics, public health ethics, and the ethics of radiological protection. Fundamental to all of these in one way or another are the four "principles of biomedical ethics", originally proposed by Beauchamp and Childress (1979) as a guide for decision making in clinical practice. Suggestions have been made of various other principles which should be added to address the specifics of the individual disciplines under consideration. Here we are exploring empathy as a principle complementing those hitherto applied in environmental health practice. RESULTS AND CONCLUSIONS: Empathy can be defined as the "capability (or disposition) to immerse oneself in and to reflect upon the experiences, perspectives and contexts of others". It is often understood as a skill that one either has or has not, but research has shown it can be taught and therefore can be required as an attitude of those working in health care, education, design, and even politics. We suggest to consider it a procedural principle on a par with inclusiveness, accountability, and transparency. It should drive the assessment of any environmental situation and the health problems accruing from it.

Effective Doses of Employees at the Former Uranium Processing Plant MAPE Mydlovary, Czechoslovakia.

Results are presented of a survey of almost 1000 dosimetric records of employees at the former uranium processing plant MAPE Mydlovary. Located ~20 km to the north-west of Ceské Budejovice in South Bohemia, it was the place where most of the uranium ore mined in Czechoslovakia in the years from 1962 to 1991 was processed. The records refer to incorporation of short-lived radon progeny and long-lived radionuclides as well as external gamma exposure. The average annual doses calculated from the recorded data were 2.7 ± 1.4, 5.0 ± 3.1 and 1.7 ± 0.9 mSv from these three sources, respectively. Thus, the relative contributions of these components to the effective dose were 29 ± 6, 53 ± 14 and 18 ± 13% , respectively. This is different from the findings in an earlier study for the exposure of uranium miners, where the overall doses were similar, but over 50% was contributed by short-lived radon progeny. No legal limits applicable at the time were exceeded, as the inclusion of long-lived radionuclides in the dose calculations was not yet obligatory.

Adaptation of the human population to the environment: Current knowledge, clues from Czech cytogenetic and "omics" biomonitoring studies and possible mechanisms.

The human population is continually exposed to numerous harmful environmental stressors, causing negative health effects and/or deregulation of biomarker levels. However, studies reporting no or even positive impacts of some stressors on humans are also sometimes published. The main aim of this review is to provide a comprehensive overview of the last decade of Czech biomonitoring research, concerning the effect of various levels of air pollution (benzo[a]pyrene) and radiation (uranium, X-ray examination and natural radon background), on the differently exposed population groups. Because some results obtained from cytogenetic studies were opposite than hypothesized, we have searched for a meaningful interpretation in genomic/epigenetic studies. A detailed analysis of our data supported by the studies of others and current epigenetic knowledge, leads to a hypothesis of the versatile mechanism of adaptation to environmental stressors via DNA methylation settings which may even originate in prenatal development, and help to reduce the resulting DNA damage levels. This hypothesis is fully in agreement with unexpected data from our studies (e.g. lower levels of DNA damage in subjects from highly polluted regions than in controls or in subjects exposed repeatedly to a pollutant than in those without previous exposure), and is also supported by differences in DNA methylation patterns in groups from regions with various levels of pollution. In light of the adaptation hypothesis, the following points may be suggested for future research: (i) the chronic and acute exposure of study subjects should be distinguished; (ii) the exposure history should be mapped including place of residence during the life and prenatal development; (iii) changes of epigenetic markers should be monitored over time. In summary, investigation of human adaptation to the environment, one of the most important processes of survival, is a new challenge for future research in the field of human biomonitoring that may change our view on the results of biomarker analyses and potential negative health impacts of the environment.

Pragmatic ethical basis for radiation protection in diagnostic radiology.

OBJECTIVE: Medical ethics has a tried and tested literature and a global active research community. Even among health professionals, literate and fluent in medical ethics, there is low recognition of radiation protection principles such as justification and optimization. On the other hand, many in healthcare environments misunderstand dose limitation obligations and incorrectly believe patients are protected by norms including a dose limit. Implementation problems for radiation protection in medicine possibly flow from apparent inadequacies of the International Commission on Radiological Protection (ICRP) principles taken on their own, coupled with their failure to transfer successfully to the medical world. Medical ethics, on the other hand, is essentially global, is acceptable in most cultures, is intuitively understood in hospitals, and its expectations are monitored, even by managements. This article presents an approach to ethics in diagnostic imaging rooted in the medical tradition, and alert to contemporary social expectations. ICRP and the International Radiation Protection Association (IRPA), both alert to growing ethical concerns, organized a series of consultations on ethics for general radiation protection in the last few years. METHODS: The literature on medical ethics and implicit ICRP ethical values were reviewed qualitatively, with a view to identifying a system that will help guide contemporary behaviour in radiation protection of patients. Application of the system is illustrated in six clinical scenarios. The proposed system is designed, as far as is possible, so as not to be in conflict with the conclusions emerging from the ICRP/IRPA consultations. RESULTS AND CONCLUSION: A widely recognized and well-respected system of medical ethics was identified that has global reach and claims acceptance in all cultures. Three values based on this system are grouped with two additional values to provide an ethical framework for application in diagnostic imaging. This system has the potential to be robust and to reach conclusions that are in accord with contemporary medical, social and ethical thinking. The system is not intended to replace the ICRP principles. Rather, it is intended as a well-informed interim approach that will help judge and analyse situations that arouse ethical concerns in radiology. Six scenarios illustrate the practicality of the value system in alerting one to possible deficits in practice. ADVANCES IN KNOWLEDGE: Five widely recognized values and the basis for them are identified to support the contemporary practice of diagnostic radiology. These are essential to complement the widely used ICRP principles pending further development in the area.

Mapping the factors affecting the frequency and types of micronuclei in an elderly population from Southern Bohemia.

The micronucleus assay is one of the most common methods used to assess chromosomal damage (losses or breaks) in human peripheral blood lymphocytes (PBL) in genetic toxicology. Most studies have focused on analyzing total micronuclei (MN), but identifying the content of MN can provide more detailed information. The main aim of this study was to map the factors affecting the frequency and types of micronuclei in binucleated cells (BNC) in elderly population. Fluorescence in situ hybridization using Human Pan Centromeric Chromosome Paint was used to identify centromere positive (CEN+) or centromere negative (CEN-) MN. A group of 95 men from Southern Bohemia, Czech Republic (average age 68.0±6.8 years) was followed repeatedly, in spring and fall 2014. The study participants were former workers of the uranium plant "MAPE Mydlovary" (processing uranium ore from 1962 to 1991), and controls. The general profile of individual types of MN, and the effect of the season, former uranium exposure, age, smoking status, weight, and X-ray examination on the level and type of MN were analyzed. The results of this study showed: (i) a stable profile of BNC with MN based on the number of MN during two seasons; (ii) an increase of the number of CEN+ MN from spring to fall; (iii) a lower frequency of the total MN in the exposed group than in controls with a significant difference in the percentage of aberrant cells (%AB.C.) in the fall; (iv) no clear effect of age, smoking and BMI on DNA damage in this group; (v) lower DNA damage levels in former uranium workers who received X-ray examination later in life. In summary, the results indicate a trend of seasonal changes of individual types of MN and suggest that former exposure can have a protective effect on the level of DNA damage in case of future exposure.

Increased radiosensitivity with chronic hypoxia in four human tumor cell lines.

PURPOSE: It is well known that the radiosensitivity of tumor cells can be significantly reduced under hypoxic conditions. However, most of the reports in the literature refer to an experimental setup in which the supply of oxygen is kept low for a short period of time only. In tumors, chronic hypoxia would seem to be the more typical situation, because of an insufficient vascularization and the limited diffusion of oxygen into the tissue. Under such conditions, certain changes in the proliferation patterns of tumor cells, in which the cell cycle checkpoint protein p53 seems to play a role, have been shown to occur. We therefore decided to study radiosensitivity and cell cycle progression under conditions of chronic hypoxia in several human tumor cell lines differing in their p53 status. METHODS AND MATERIALS: Four human tumor cell lines (melanomas Be11 and MeWo and squamous carcinomas 4197 and 4451) were incubated for 3 h, 24 h, and 72 h under either oxic or hypoxic conditions and subsequently exposed to graded doses of X-rays. In some cases, cells were kept under hypoxia for the same periods of time, but then reoxygenated immediately before irradiation. Cell survival was assessed with the usual colony formation assay, and cell cycle distributions were determined by two-parameter flow cytometry after labeling with bromodeoxyuridine (BrdU). RESULTS: As expected, the oxygen enhancement ratio at 3 h was 2.0 or more in all cases. Differences, however, became evident with longer incubation times. At 24 h, the sensitivity of cells kept under hypoxic conditions both before and during irradiation was practically unchanged with cell lines Be11, 4197, and 4451, but clearly increased with MeWo. This resulted in an oxygen enhancement ratio of only 1.1 for the latter cell line when the sensitivity of aerated cells was used as reference. Cells kept under hypoxia for 24 h and reoxygenated shortly before irradiation, however, also showed an increase in sensitivity, so that the oxygen enhancement ratio based on differences in irradiation atmosphere alone was still around 2.0. At 72 h, the two p53 wild-type cell lines were not available for experiments, because they quickly degenerated under hypoxic conditions. Both mutant cell lines now showed similar results, the sensitivity being increased with irradiation under continued hypoxia as well as after reoxygenation. The oxygen enhancement ratios with reference to aerated cells were 1.3 and 1.5 for MeWo and 4451, respectively. Flow cytometric measurements after labeling with BrdU revealed that in all cell lines, the fraction of active S-phase cells during incubation tended to decrease under hypoxic conditions. Only in the p53 mutant cell lines, however, was this accompanied by an increase of the percentage of S-phase cells that were not actively incorporating BrdU. CONCLUSIONS: It is suggested that these quiescent cells in the S-phase compartment develop because of a general breakdown of cellular energy metabolism. In the p53 mutant cells, this may lead to a cessation of cell cycle progression in all phases alike, because checkpoint control has been lost; p53 wild-type cells, on the other hand, settle down preferentially in G(1) under the same conditions. Independently of the p53 status, however, energy depletion may be the cause of a decreased ability to cope with radiation damage and thus the cause of the observed increase in radiosensitivity. This would become more easily apparent in the p53 mutant cell lines, because they are less sensitive than the p53 wild types to hypoxia as such.

Effect of hypothermia on radiation-induced micronuclei and delay of cell cycle progression in TK6 cells.

PURPOSE: 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. MATERIALS AND METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.