A follow-up cytogenetic study of workers highly exposed inside the Chernobyl sarcophagus.

Data are presented on some of the engineers and scientists who regularly entered highly radioactive areas of the sarcophagus chamber constructed around the ruins of the Chernobyl reactor. Previous investigations on six of them by unstable chromosomal aberrations, quartz fibre electrometers and, in some cases, tooth electron spin resonance have all indicated high doses accumulated over several years of work inside the sarcophagus. Here, the authors present the data on eleven of the men who agreed to be monitored cytogenetically although two have since died aged 45 and 66 y. The present data were combined with the previous to examine the time-courses of these individuals' changes in their aberration frequencies. As expected, dicentric aberrations showed a clear drop down to 2-3 per 100 cells since the men ceased working inside the sarcophagus. In contrast, the translocation yields remained at a high level showing no tendency to decrease and so proved reliable for retrospective biodosimetry. These data are particularly useful in demonstrating the value of FISH long after high but protracted and fractionated exposure.

The influence of brachytherapy dose heterogeneity on estimates of alpha/beta for prostate cancer.

The sensitivity of estimates of alpha/beta for prostate tumours to dose heterogeneity in 125I brachytherapy implants, as well as to variation in selected radiobiological parameters, is analysed. The tumour control probabilities of brachytherapy and external beam radiotherapy are equated for ranges of alpha, Tpot, RBE and external beam dose. For each combination of parameters, the equality is used to derive the value of alpha/beta. Different clinical (non-uniform) brachytherapy dose distributions, and three uniform brachytherapy dose distributions (120, 144 and 160 Gy) are used. For 'nominal' input parameter values of Tpot = 45 days, alpha = 0.2 Gy(-1), RBE = 1.4, and an external beam dose of 70 Gy, the values obtained for alpha/beta ranged between 2.1 and 12.3 Gy for all of the clinical DVHs, between 2.1 and 3.8 Gy for the better quality clinical implants and between 1.0 and 1.8 Gy for the uniform brachytherapy doses. When only 2% of the volume receiving the lowest dose is omitted from the clinical DVHs, the estimated alpha/beta values ranged between 1.4 and 2.1 Gy. When ranges of input parameters were also considered, the overall range of alpha/beta values for the clinical brachytherapy dose distributions lay between 1.1 and 12.3 Gy for the three best clinical implants, and between 0.7 and 6.3 Gy for uniform doses. We conclude that estimation of alpha/beta without taking into account dose heterogeneity and inter-patient variation may underestimate the actual value alpha/beta.

Monte Carlo simulations of DNA damage from incorporated cold iodine following photoelectrically induced Auger electron cascades.

Radiation-induced damage in nucleosomal DNA from Auger electron cascades due to incorporated cold IUdR has been modelled through Monte Carlo simulations. Probabilities of DNA double strand break (DSB) production following a vacancy in the K, L, M and N shells of iodine are estimated. DSB complexity from the base damage accompanying a break was also estimated. Multiple DSB events were analysed for correlated breaks due to nucleosome periodicity. The probability of an Auger cascade causing at least one DSB strongly depended on the shell in which the initial vacancy was produced. This probability was approximately 0.35 for K and L shells and fell to 0.02 for the N shell. As expected, DSBs were predominantly induced in a nucleosome containing incorporated iodine and were accompanied with extensive base damage. Analysis of multiple DSB events showed that approximately 14% of the DSBs produced following a vacancy in the L1 orbital can be interpreted as correlated with base pair separation attributable to the nucleosome periodicity. The data generated in this work provide a basis for the development of photon-activation therapy using kilovoltage X rays incident upon IUdR sensitised tumours.

Calculation of radiation-induced DNA damage from photons and tritium beta-particles. Part I: Model formulation and basic results.

Radiation-induced damage in nucleosomal DNA was modelled by Monte Carlo means. An atomistic representation of DNA with a first hydration shell was used. DNA single- and double-strand break (SSB and DSB) yields were calculated for 137Cs photons, x-rays and tritium beta-particles. Monte Carlo-generated electron tracks for liquid water were used to model energy deposition. Chemical evolution of a track and interactions between species and DNA following water radiolysis were modelled in an encounter-controlled manner. The effects of varying the scavenging capacity of the environment, the extent of DNA protection by histones and the setting of a threshold for direct energy depositions on DNA break yields were all systematically studied. DSB complexity was assessed in terms of DNA breaks and base damage accompanying a DSB. Model parameters were adjusted to make predictions consistent with experimental data on DSB yields and yield modification by a variety of factors including changing DNA conformation and incorporation of scavengers. An embedded model of nucleosomal DNA by histones was required to explain experimentally observed modification of DSB yield by removal of bound histones. Complex DSB, defined as DSB accompanied by an additional strand breakage, exhibited high association with base damage. It is shown that hydroxyl radical interactions with bases are a major contributor to DSB complexity. On average there were 1.15 and 2.69 OH-base interactions accompanying simple and complex DSB, respectively for 137Cs. Over 80% of complex DSB had at least one OH-base interaction associated with a DNA break.

Calculation of radiation-induced DNA damage from photons and tritium beta-particles. Part II: Tritium RBE and damage complexity.

Yields of DNA single- and double-strand breaks (SSB and DSB) in nucleosomal DNA were calculated for 137Cs, 70 keV photons and tritium beta-particles by Monte Carlo means. Monte Carlo-generated electron tracks for liquid water were used to model energy deposition. Chemical evolution of a track and interactions between species and DNA following water radiolysis were modelled in an encounter-controlled manner. The calculated relative biological effectiveness (RBE) for DSB production for tritium against 137Cs was 1.2 for the total DSB yield. Tritium beta-particles were slightly more efficient compared to 137Cs in producing complex DSB, defined as DSB accompanied by additional strand breaks. The RBE for complex DSB formation was 1.3. Most complex DSB exhibited associated base damage; the extent of the base damage was similar for all the radiation types considered. Correlated DSB conforming to nucleosome periodicity were observed. However, their frequency was low, of the order of 2% of total DSB. For all the DNA damage endpoints considered and their response to variation of the scavenging environment or DNA conformation no difference was observed between 70 keV photons and tritium beta-particles.

Monte Carlo simulations and measurement of DNA damage from x-ray-triggered auger cascades in iododeoxyuridine (IUdR).

We investigated the DNA damage from Auger electrons emitted from incorporated stable iodine (127I), following photoelectric absorption of external x-rays. The effectiveness of the Auger electrons in producing DNA double-strand breaks (DSB) was determined theoretically, using Monte Carlo simulations of the radiation physics and chemistry, and was shown to be in reasonable agreement with DNA damage measured using the comet assay. The DSB yields were measured in CHO cells for 60Co (as a non-Auger-promoting radiation) and for tungsten-filtered 100 kVp x-rays capable of producing Auger electron emission. The theoretical study showed that on average, 2.5 Auger electrons were emitted for N-shell orbital vacancies and up to 10 Auger electrons were emitted from L1-shell vacancies. These Auger bursts produced approximately 0.03 DSB per N-shell vacancy and 0.3 DSB per K-shell or L-shell vacancy. The calculated yield of DSB from Auger cascades per unit dose (1 Gy) in water was approximately 1.7 for tungsten-filtered 100 kVp x-rays, assuming 20% IUdR substitution of thymidine. The comet assay yielded an experimental value of 3.6+/-1.6 per 1 Gy for the same conditions. The Monte Carlo simulations also demonstrated a high complexity of DSB produced by Auger cascades with virtually all DSB from inner shell orbitals (i.e. K, L shells) accompanied by compounded strand breakage and base damage, indicating a difficult lesion to repair. This finding agrees well with comet assay results of DNA repair, where an increase in the DSB yield in IUdR-sensitized cells was shown to persist after a time of 24 h. We conclude that Auger cascades in iodine produce a modest increase in the number of initial strand breaks of the order of 10% but the complex nature of these DSB makes them very difficult to repair or potentially prone to misrepair. The accentuated DNA damage may have major consequences for cell survival and may be exploitable in kilovoltage photon activation therapy (PAT) of tumors sensitized with iodine.

In-field and out-of-field effects in partial volume lung irradiation in rodents: possible correlation between early dna damage and functional endpoints.

PURPOSE: Recent observations have shown that there are regional variations in radiation response in mouse lung as measured by functional assays. Furthermore, there are both in-field and out-of-field effects in radiation-induced lung damage as observed by DNA assay in rats. The purpose of this work is: (a) to examine mice lethality data following partial volume lung irradiation to assess the possibility of directional or regional effects, (b) to evaluate the correlation between mice lethality data and DNA damage assayed by micronuclei production in rat lung, and (c) to re-interpret mice lethality considering the existence of directional effects in lung cellular response to partial volume irradiation. METHODS AND MATERIALS: The lethality data for mice, generated at the M. D. Anderson Cancer Center, Houston, and micronuclei yield data for rats obtained at Princess Margaret Hospital, Toronto, were used. A radiobiological model that allows for out-of-field and in-field effects for lung cell damage and lung response was developed. This model is based on the observation of DNA damage in shielded parts of rat lung that was assumed relevant to cell lethality and consequently overall lung response. RESULTS: While the experimental data indicated directional or regional volume effects, the applicability of dose and volume as sole predictors of lung response to radiation was found to be unreliable for lower lung (base) irradiation in mice. This conforms well to rat lung response where micronuclei were observed in shielded apical parts of lung following base irradiation. The radiobiological model, which was specifically developed to account for the lung response outside of primary irradiated volume, provides a good fit to mice lethality data, using parameters inferred from rat micronuclei data. CONCLUSION: Response to lung irradiation in rodents, in particular, elevated sensitivity to base irradiation, can be interpreted with a hypothesis of in-field and out-of-field effects for cellular response. If the existence of these effects for lung is subsequently proven in humans, it will require the incorporation of geometrical and directional information in normal tissue complication probability calculations for lung. These considerations are ignored in present approaches based only on conventional dose-volume histograms.

The cellular environment in computer simulations of radiation-induced damage to DNA.

Radiation-induced DNA single- and double-strand breaks were modeled for 660 keV photon radiation and scavenger capacity mimicking the cellular environment. Atomistic representation of DNA in B form with a first hydration shell was utilized to model direct and indirect damage. Monte Carlo generated electron tracks were used to model energy deposition in matter and to derive initial spatial distributions of species which appear in the medium following radiolysis. Diffusion of species was followed with time, and their reactions with DNA and each other were modeled in an encounter-controlled manner. Three methods to account for hydroxyl radical diffusion in a cellular environment were tested: assumed exponential survival, time-limited modeling and modeling of reactions between hydroxyl radicals and scavengers in an encounter-controlled manner. Although the method based on modeling scavenging in an encounter-controlled manner is more precise, it requires substantially more computer resources than either the exponential or time-limiting method. Scavenger concentrations of 0.5 and 0.15 M were considered using exponential and encounter-controlled methods with reaction rate set at 3x10(9) dm3 mol(-1) s(-1). Diffusion length and strand break yields, predicted by these two methods for the same scavenger molarity, were different by 20%-30%. The method based on limiting time of chemistry follow-up to 10(-9) s leads to DNA damage and radical diffusion estimates similar to 0.5 M scavenger concentration in the other two methods. The difference observed in predictions made by the methods considered could be tolerated in computer simulations of DNA damage.

Modelling DNA damage induced by different energy photons and tritium beta-particles.

PURPOSE: To model the production of single- and double-strand breaks (ssb and dsb) in DNA by ionizing radiations. To compare the predicted effectiveness of different energy photon radiations and tritium beta-particles. MATERIALS AND METHODS: Modelling is carried out by Monte Carlo and includes consideration of direct energy depositions in DNA molecules, the production of species, their diffusion and interactions with each other and DNA. Computer-generated electron tracks in liquid water are used to model energy deposition and to derive the initial positions of chemical species. Atomistic representation of the DNA in B form with a first hydration shell is used. Photon radiations in the energy range 70keV-1MeV and tritium beta-particles are considered. RESULTS: A tentative increase for dsb yield has been predicted for 70 keV photons and tritium compared with 137Cs. This increase is more pronounced for complex dsb. Double-strand breaks are much more prone compared with ssb to combine with additional strand breaks and base damage, which contributes to break complexity. At least half of DNA breaks are hydroxyl radical mediated. CONCLUSIONS: The developed model makes predictions compatible with features of available experimental data. Break complexity has to be addressed in biophysical modelling when the relative effectiveness of radiations in DNA damage is studied. Obtained data strongly argue against the dominance of direct radiation action in DNA damage in the cellular environment predicted by some theoretical studies.

Energy deposition pattern from tritium and different energy photons--a comparative study.

Energy deposition patterns of beta particles from tritium decay have been analyzed in comparison with photon radiation in the energy range from 12 to 1250 keV. Energy deposition was modeled by Monte Carlo means using electron tracks in water vapor with a complete follow up of delta electrons. Ionizations have been used as representative of track structure. Frequencies of clusters on a nanometer scale and their spatial distribution in spherical targets representing cell nuclei have been derived. Cluster analysis has been carried out by implementing a k-means method. Frequencies of ionization clusters from tritium are similar to 60-100 keV photons. Spatial distribution of clusters from tritium closely match approximately 70 keV photons up to 2 microm separation.

The influence of track structure on the understanding of relative biological effectiveness for induction of chromosomal exchanges in human lymphocytes.

A biophysical model has been applied to describe the production of exchange chromosomal aberrations (dicentrics) in human lymphocytes by radiations of different qualities. The model includes a detailed description of the energy deposition pattern in the form of computer-generated tracks. Energy deposition events are further converted to DNA double-strand breaks (DSBs). Formation of chromosomal exchanges is modeled in competition with repair in a distance-dependent manner with breaks in proximity being most likely to interact. We demonstrate that an assumption of an RBE > 1 for production of DSBs at higher LET leads to a significant increase with LET of both the linear and the quadratic coefficients of the dose response for exchange formation. The latter is not supported experimentally and argues against high RBE values for production of DSBs, at least for those breaks involved in chromosomal exchanges. Assuming that the RBE for production of DSBs is unity, the calculated dose-response curves conformed to experimental data for 60Co gamma rays, 250 kVp X rays and 8.7 MeV protons. The linear coefficient for 23.5 MeV 3He ions is underpredicted. The model predicts that a quadratic term in the dose response for exchange aberrations should be observed at LET values of 20-30 keV/microm. The curvature is not observed experimentally, and the contradiction is discussed.

On the mechanism of the formation of chromosomal aberrations by ionising radiation.

The results of applying a biophysical model to describe the production of chromosomal aberrations in human lymphocytes are presented. The model describes energy deposition in cell nuclei, the conversion to DNA double-strand breaks, and the repair and misrepair of those breaks to form aberrations. The repair and misrepair of double-strand breaks are expressed as a competition process based on the concept that the probability of exchange depends upon the spatial separation of the breaks. Results are restricted to photon irradiations. We show that the model leads to the familiar linear-quadratic equation for the dependence of exchange aberration yield on dose. Exchanges between two DNA breaks along the same track determine the linear term, and exchanges between those in different tracks determine the quadratic term. We demonstrate the importance of electron track structure in the prediction of the linear term and show that the low-dose RBE between x- and gamma-rays depends not only on the physical description of the track but also the biological repair function. For intratrack exchanges, we show that the double-strand breaks are very close, on average about 30 nm apart. For intertrack exchanges, the mean separation of breaks is calculated to be about 2 mu m. There is a clear separation of the two modes of action. In addition, the increased effectiveness of the track ends of electrons is shown.

Modelling the kinetics of chromosome exchange formation in human cells exposed to ionising radiation.

A biophysical model has been applied to study the kinetics of chromosome exchange formation in human cells. Chromosomal exchange induction (for example dicentrics) by ionising radiation was modelled by means of the Monte Carlo technique. This involved energy deposition by electrons, production of chromosomal breaks (assumed to be DNA double-strand breaks) and their repair and exchange. Exchanges were assumed to result from pairwise interaction between two DNA breaks in a distance-dependent manner. The rate at which exchanges are formed was found to depend upon how the exchange to no-exchange probability ratio varied with time. The assumption that this ratio did not alter with time produced a time constant for the formation of exchanges which was exactly half that of the repair time constant. Longer time constants could not be accommodated unless the probability ratio for exchange increases with time. Different time constants for inter- and intratrack exchanges could be achieved on the basis of DNA double-strand break separation.

[The results of the cytogenetic examination of children and adolescents living in radionuclide-contaminated regions of Kaluga Province]. / Rezul'taty tsitogeneticheskogo obsledovaniia detei i podrostkov, prozhivaiushchikh v zagriaznennykh radionuklidami raionakh Kaluzhskoi oblasti.

After the Chernobyl accident in a period from 1989 to 1992 blood samples were collected and analysed for chromosomal aberrations in lymphocytes from 490 children born and living continuously in two districts of Kaluga Province with radioactive contamination. The results of cytogenetic examinations show the frequency of unstable aberrations in children from Zhizdra District to be higher than that in children from Ulyanovo District and in the control group. No correlation was found between the thyroid absorbed doses ranging from 0 to 5 Gy and the aberration frequencies. No differences in chromosomal aberration yields were observed in children born before or after the Chernobyl accident. Repeat examinations of the same children cohorts revealed no significant changes with time in the level of unstable aberrations.

[The results of the cytogenetic examination of the inhabitants of population centers adjacent to the Semipalatinsk nuclear testing grounds (at the period of the work of the interdepartamental commission in 1989)]. / Rezul'taty tsitogeneticheskogo obsledovaniia zhitelei naselennykh punktov, prilegaiushchikh k Semipalatinskomu ispytatel'nomu iadernomu poligonu (v period raboty mezhvedomstvennoi komissii v 1989 g.).

Within the frames of activity of joint commission the cytogenetic studies of 98 persons from different areas of Semipalatinsk Province have been carried out in 1989. The studies revealed the higher level of chromosome aberrations in the settlements adjacent to firing ground. The possible connection of discovered cytogenetic lesions to the influence of radiation aftermath of ground and air nuclear weapon test is discussed.

[The results of the cytogenetic examination of children and adolescents living in radionuclide-contaminated regions of Bryansk Province]. / Rezul'taty tsitogeneticheskogo obsledovaniia detei i podrostkov, prozhivaiushchikh v zagriaznennykh radionuklidami raionakh Brianskoi oblasti.

The paper presents the data on cytogenetic studies of 663 children inhabited radionuclide contaminated territories of Bryansk Province. Statistically significant difference with control was observed only for the territory with the highest radioactive contamination level of 137Cs (1000 kBq/m2). For two other territories with lesser contamination (100 and 700 kBq/m2) the rate of chromosome aberrations is nearly the same and does not differ significantly from control one. No difference in the level of cytogenetic damage was discovered for children affected by radiation pre- or postnatally.

[A comparative study of the frequency of stable and unstable chromosome aberrations in the gamma irradiation of human lymphocytes in vitro]. / Sravnitel'noe issledovanie chastoty stabil'nykh i nestabil'nykh abberatsii khromosom pri gamma-obluchenii limfotsitov krovi cheloveka in vitro.

Comparative studies of the yield of stable and unstable chromosome aberrations have been carried out in human lymphocytes gamma irradiated in vitro within the doze range 0.1--1.0 Gy. It was observed that the yield of unstable chromosome aberrations (dicentrics and centric rings) is satisfactorily described for discovered dose range with linear-quadratic approximation. For stable chromosome aberrations in G-banded chromosome the dose response dependence was less expressed. The studies revealed non-random involvement of different chromosomes in translocations, and the most frequently involved are 1 and 13 chromosomes.

[The possibilities of using biological dosimetry methods for the retrospective assessment of dosages in relation to the sequelae of the accident at the Chernobyl Atomic Electric Power Station. An assessment of the dosages based on an analysis of unstable chromosome aberrations]. / Vozmozhnosti primeneniia metodov biologicheskoi dozimetrii dlia retrospektivnoi otsenki doz v sviazi s posledstviiami avarii na Chernobyl'skoi AES. Otsenka doz na osnove analiza nestabil'nykh khromosomnykh aberratsii.

The problem of retrospective dose assessment in connection with the Chernobyl accident is discussed. Existing methods of dose quantification based on the analysis of unstable aberrations in blood lymphocytes are discussed in details. Starting from the own data it is concluded that unstable aberration technique could not be applied to verify individual doses for ameliorators as well as for inhabitants of contaminated areas.

[The possibilities of using biological dosimetry methods for the retrospective assessment of dosages in relation to the sequelae of the accident at the Chernobyl Atomic Electric Power Station. An assessment of the dosages based on an analysis of stable chromosome aberrations]. / Vozmozhnosti primeneniia metodov biologicheskoi dozimetrii dlia retrospektivnoi otsenki doz v sviazi s posledstviiami avarii na Chernobyl'skoi AES. Otsenka doz na osnove analiza stabil'nykh khromosomnykh aberratsii.

The problem of retrospective dose assessment in connection with the Chernobyl accident is discussed. Reasons justifying the applicability and advantages of technique based on the analysis of stable aberrations in blood lymphocytes are given. Two modern methods for registration aberrations of these registration, namely fluorescence in situ hybridization and full karyotyping of G-banded chromosomes are considered in details.

Modelling of DNA breaks and the formation of chromosome aberrations.

We present results of some calculations that convert double-strand breaks to dicentrics by means of a repair-misrepair model. The method uses Monte Carlo techniques to determine the positions of the double-strand breaks and to determine the dicentric yields for comparison with those obtained experimentally in human lymphocytes following irradiation in G0. The model is able to describe the linear-quadratic curves of dicentric yield following low LET irradiation, but underpredicts by a factor of about 1.5-2.0 the high LET neutron curves. The effects of including corrections for interphase death are demonstrated. The reasons for the discrepancy at high LET are discussed and suggestions for an improvement to the fit are made.