Effect of changing the radiation dose range on the in vitro cytogenetic dose response to gamma-rays.

PURPOSE: To examine the distortion of the linear quadratic (LQ) model of in vitro cytogenetic dose response over an extended range of γ-ray doses by analyzing the available literature data, and to establish the dose ranges, in which the LQ dose response curve (DRC) can be most accurately fitted for biological dosimetry. MATERIALS AND METHODS: Data on yields of dicentrics (Dic) or dicentrics plus centric rings (Dic + CR) induced in vitro in human lymphocytes by acute γ-rays were extracted from 108 open sources. The overall dose response dataset in the dose range up to 50 Gy was fitted to a fractional-rational (FR) model, which included a 'basic' LQ function in the numerator, and a reduction factor dependent on the square of the dose in the denominator. Cytogenetic dose response data obtained at Grigoriev Institute for Medical Radiology, Kharkiv, Ukraine (GIMRO) in the range 0.1 - 20.3 Gy acute γ-rays were fitted to the LQ model with the progressive changing minimum or maximum radiation dose. RESULTS: The overall dose response, as expected, followed the LQ function in the dose range ≤5 Gy, but in the extended dose range appeared to be S-shaped, with intensive saturation and a plateau at doses ≥22 Gy. Coefficients of the 'basic' LQ equation in FR model were very close to many published DRCs; calculated asymptote was 17. Fitting of the GIMRO dataset to the LQ model with the shift of the dose range showed the increase in linear coefficient with the increment of either minimum or maximum radiation dose, while the decline of the quadratic coefficient was regulated mostly by the increase of the highest dose. The best goodness of fit, assessed by lower χ2 values, occurred for dose ranges 0.1 - 1.0 Gy; 0.5 - 5.9 Gy; 1.0 - 7.8 Gy; 2.0 - 9.6 Gy, 3.9 - 16.4 Gy and 5.9 - 20.3 Gy. The 'see-saw' effect in changes of LQ coefficients was confirmed by re-fitting datasets published by other laboratories. CONCLUSIONS: The classical LQ model with fixed coefficients appears to have limited applicability for cytogenetic dosimetry at radiation doses >5 Gy due to the saturation of the dose response. Different response of the LQ coefficients to the changes of the dose range must be considered during the DRC construction. Proper selection of minimum and maximum dose in calibration experiments makes it possible to improve the goodness of fit of the LQ DRC.

Radiation Exposure Biomarkers in the Practice of Medical Radiology: Cooperative Research and the Role of the International Atomic Energy Agency (IAEA) Biodosimetry/Radiobiology Laboratory.

The strategy toward personalized medicine in radiation oncology, nuclear medicine, and diagnostic and interventional radiology demands a specific set of assays for individualized estimation of radiation load for safety concerns and prognosis of normal tissue reactions caused by ionizing radiation. Apparently, it seems reasonable to use validated radiation dosimetric biomarkers for these purposes. However, a number of gaps in knowledge and methodological limitations still have to be resolved until dosimetric biomarkers will start to play a valuable role in clinical practice beyond radiation protection and radiation medicine. An extensive international multicenter research is necessary to improve the methodology of clinical applications of biodosimetry. That became a rationale for launching the IAEA Coordinated Research Project E35010 MEDBIODOSE: "Applications of Biological Dosimetry Methods in Radiation Oncology, Nuclear Medicine, and Diagnostic and Interventional Radiology." At the 2 Coordination Meeting on MEDBIODOSE (18-22 February 2019, Recife, Brazil), participants reported progress in the usage of biological dosimetry for genotoxicity assessment and/or individualization of radiotherapy treatment plans. Another avenue of research was the prognosis of normal tissue toxicity and cancer risk prediction using biomarkers' yield measured in vivo or after ex vivo irradiation of patients' cells. Other important areas are mechanisms of cytogenetic radiation response, validation of new radiation biomarkers, development of innovative techniques, automated and high-throughput assays for biodosimetry, and the overall improvement of biodosimetry service. An important aspect of clinical application of biodosimetry is standardization of techniques and unification of approaches to data interpretation. The new IAEA Biodosimetry/Radiobiology Laboratory, which is being established, will provide support for this activity. The declared lab's mission includes, among other tasks, a harmonization of the biodosimetry applications with relevant international standards, guidelines on good laboratory practice, and the IAEA EPR-Biodosimetry manual.

The assessment of radiation hazardous areas considering the spectral analysis of the neutron component of a tactical neutron bomb detonation.

This study provides an improved background for the operational assessment of the radiation hazardous areas with induced radioactivity, based on the analysis of spectral characteristics of the neutron component of the penetrating radiation. The interrelationships between the nuclear munitions explosion parameters, environmental conditions and dosimetric characteristics of the secondary gamma-ray emission from the activated soil are analyzed. The interrelationships of these parameters are used in the methodology of the assessment of the dosimetric characteristics of the radiation hazardous areas.

Optimizing the Microscopy Time Schedule for Chromosomal Dosimetry of High-dose and Partial-body Irradiations.

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

A new Bayesian model applied to cytogenetic partial body irradiation estimation.

A new zero-inflated Poisson model is introduced for the estimation of partial body irradiation dose and fraction of body irradiated. The Bayes factors are introduced as tools to help determine whether a data set of chromosomal aberrations obtained from a blood sample reflects partial or whole body irradiation. Two examples of simulated cytogenetic radiation exposure data are presented to demonstrate the usefulness of this methodology in cytogenetic biological dosimetry.

Review of Bayesian statistical analysis methods for cytogenetic radiation biodosimetry, with a practical example.

Classical methods of assessing the uncertainty associated with radiation doses estimated using cytogenetic techniques are now extremely well defined. However, several authors have suggested that a Bayesian approach to uncertainty estimation may be more suitable for cytogenetic data, which are inherently stochastic in nature. The Bayesian analysis framework focuses on identification of probability distributions (for yield of aberrations or estimated dose), which also means that uncertainty is an intrinsic part of the analysis, rather than an 'afterthought'. In this paper Bayesian, as well as some more advanced classical, data analysis methods for radiation cytogenetics are reviewed that have been proposed in the literature. A practical overview of Bayesian cytogenetic dose estimation is also presented, with worked examples from the literature.

A comparison of six statistical distributions for analysis of chromosome aberration data for radiation biodosimetry.

The Poisson distribution is the most widely recognised and commonly used distribution for cytogenetic radiation biodosimetry. However, it is recognised that, due to the complexity of radiation exposure cases, other distributions may be more properly applied. Here, the Poisson, gamma, negative binomial, beta, Neyman type-A and Hermite distributions are compared in terms of their applicability to 'real-life' radiation exposure situations. The identification of the most appropriate statistical model in each particular exposure situation more correctly characterises data. The results show that for acute, homogeneous (whole-body) exposures, the Poisson distribution can still give a good fit to the data. For localised partial-body exposures, the Neyman type-A model was found to be the most robust. Overall, no single distribution was found to be universally appropriate. A distribution-specific method of analysis of cytogenetic data is therefore recommended. Such an approach may lead potentially to more accurate biological dose estimates.

Cytogenetic dose-response in vitro for biological dosimetry after exposure to high doses of gamma-rays.

The dose response for dicentrics plus centric rings and total unstable chromosome-type aberrations was studied in the first mitoses of cultured human peripheral blood lymphocytes irradiated in vitro to doses of ∼2, 4, 6, 8, 10, 16 and 20 Gy of acute (60)Со gamma-rays. A dose-dependent increase of aberration yield was accompanied by a tendency to the underdispersion of dicentrics and centric rings among cells distributions compared with Poisson statistics at doses ≥6 Gy. The formal fitting of the data to a linear-quadratic model resulted in an equation with the linear and quadratic coefficients ranged 0.098-0.129×cell(-1)×Gy(-1) and 0.039-0.034×cell(-1)×Gy(-2), respectively, depending on the fitting method. The actual radiation-induced aberration yield was markedly lower than expected from a calibration curve, generated earlier within a lower dose range. Interlaboratory variations in reported dicentric yields induced by medium-to-high radiation doses in vitro are discussed.

Limitations associated with analysis of cytogenetic data for biological dosimetry.

The scientific literature concerning cytogenetic biodosimetry has been reviewed to identify the range of scenarios of radiation exposure where biodosimetry has been carried out. Limitations in the existing standardized statistical methodology have been identified and categorized, and the reasons for these limitations have been explored. Statistical problems generally occur due to either low numbers of aberrations leading to large uncertainties or deviations in aberration-per-cell distributions leading to over- or under-dispersion with respect to the Poisson model. A number of difficulties also stem from limitations of the classical statistical methodology, which requires that chromosome aberration yields be considered as something "fixed" and thus provides a deterministic estimate of radiation dose and associated confidence limits (because an assignment of a probability to an event is based solely on the observed frequency of occurrence of the event). Therefore, it is suggested that solutions to the listed problems should be based in the Bayesian framework. This will allow the investigator to take a probabilistic approach to analysis of cytogenetic data, which can be considered highly appropriate for biological dose estimation.

Delayed chromosomal instability in lymphocytes of cancer patients after radiotherapy.

PURPOSE: To assess possible delayed chromosomal instability (DCI) expressed as elevated chromatid breakage in cells containing previously formed chromosome type aberrations in cultured blood lymphocytes of cancer patients after radiotherapy (RT). MATERIALS AND METHODS: Twenty patients treated for uterine cancer with external Co(60) RT, without chemotherapy, were selected. Blood was taken before, 1-2 days after RT and one year later. Lymphocytes were cultured for 50 and 100 h. Metaphases were stained with fluorescence-plus-Giemsa and analysed for chromosome and chromatid aberrations in 1st (M1) and 3rd plus later (M3+) mitoses. RESULTS: RT caused a significant increase of radiation-specific chromosome aberrations in patients' lymphocytes together with DCI, which was observed as an excessive yield of cells containing both chromosome and chromatid aberrations (defined as C(acs&act)). This DCI passed successfully through mitoses in vitro, and at the end of RT a mean yield of 'extra' C(acs&act) was 3 x 10(-3) x cell(-1) amongst either M1 or M3+ cells. At the end of RT and one year later DCI in M1 lymphocytes appeared at random amongst patients, but some inter-individual variation was found for DCI presence in M3+ cells at both post-irradiation samplings. As time passed, the mean yield of lymphocytes exhibiting DCI decreased in vivo and one year after RT reached the pre-treatment level of 1 x 10(-3) x cell(-1). CONCLUSIONS: DCI was demonstrated in descendants of human lymphocytes after therapeutic irradiation. The effect diminished one year later, suggesting that the progeny of patients' irradiated stem cells did not produce new daughter lymphocytes exhibiting DCI during the studied post-irradiation period.