Modelling chromosomal aberration induction by ionising radiation: the influence of interphase chromosome architecture

Several advances have been achieved in the knowledge of nuclear architecture and functions during the last decade, thus allowing the identification of interphase chromosome territories and sub-chromosomal domains (e.g. arm and band domains). This is an important step in the study of radiation-induced chromosome aberrations; indeed, the coupling between track-structure simulations and reliable descriptions of the geometrical properties of the target is one of the main tasks in modelling aberration induction by radiation, since it allows one to clarify the role of the initial positioning of two DNA lesions in determining their interaction probability. In the present paper, the main recent findings on nuclear and chromosomal architecture are summarised. A few examples of models based on different descriptions of interphase chromosome organisation (random-walk models, domain models and static models) are presented, focussing on how the approach adopted in modelling the target nuclei and chromosomes can influence the simulation of chromosomal aberration yields. Each model is discussed by taking into account available experimental data on chromosome aberration induction and/or interphase chromatin organisation. Preliminary results from a mechanistic model based on a coupling between radiation track-structure features and explicitly-modelled, non-overlapping chromosome territories are presented.

Surgical treatment of chordee without hypospadias

OBJECTIVE

The aim of this study was to retrospectively assess the efficacy of the surgical techniques commonly used in three types of chordee without hypospadias (Devine and Horton classification).

METHODS

Twenty-six patients, ranging in age from 3 to 14 years, had chordee without hypospadias and underwent Nesbit dorsal plication (9 cases out of 12, type III), associated extensive mobilization of the urethra (10 cases out of 10, type II), and vascularized neourethra (3 cases out of 4, type I).

RESULTS

Eleven subjects undergoing follow-up investigation for the milder forms of chordee were satisfied with the outcome achieved; there was no impediment of any kind in the sex lives of 5 adult subjects. Of the 11 patients undergoing follow-up after more complex surgery for Devine and Horton type I-II chordee without hypospadias, a residual abnormal curvature was present in 4 subjects. These patients were submitted to one or more reoperations with outcomes defined as satisfactory.

CONCLUSION

To avoid the risks of persistent chordee, the authors suggest more radical and accurate operations for the treatment of type I and II pseudohypospadias.

Stochastic aspects and uncertainties in the prechemical and chemical stages of electron tracks in liquid water: a quantitative analysis based on Monte Carlo simulations

A new physical module for the biophysical simulation code PARTRAC has recently been developed, based on newly derived electron inelastic-scattering cross-sections in liquid water. In the present work, two modules of PARTRAC describing the production, diffusion and interaction of chemical species were developed with the specific purpose of quantifying the role of the uncertainties in the parameters controlling the early stages of liquid water radiolysis. A set of values for such parameters was identified, and time-dependent yields and frequency distributions of chemical species produced by electrons of different energies were calculated. The calculated yields were in good agreement with available data and simulations, thus confirming the reliability of the code. As the primary-electron energy decreases down to 1 keV, the *OH decay kinetics were found to get faster, reflecting variations in the spatial distribution of the initial energy depositions. In agreement with analogous works, an opposite trend was found for energies of a few hundred eV, due to the very small number of species involved. The spreading effects shown at long times by *OH frequency distributions following 1 keV irradiation were found to be essentially due to stochastic aspects of the chemical stage, whereas for 1 MeV tracks the physical and pre-chemical stages also were found to play a significant role. Relevant differences in the calculated e(aq) -yields were found by coupling the physics of PARTRAC with descriptions of the pre-chemical and chemical stages adopted in different models. This indicates a strict interrelation of the various stages, and thus a strong dependence of the parameter values on the assumptions made for the preceding and subsequent stages of the process. Although equally acceptable results can be obtained starting from different assumptions, it is necessary to keep control of such uncertainties, since they can significantly influence the modeling of radical attack on DNA and, more generally, radiobiological damage estimation. This study confirms the need for new, independently derived data on specific steps of water radiolysis, to be included in comprehensive biophysical simulation codes.

Radiation pneumonitis after breast cancer irradiation: analysis of the complication probability using the relative seriality model

BACKGROUND

Toxicity of the respiratory system is quite common after radiotherapy of thoracic tumors; breast cancer patients represent one of the groups for which there is also a long expected survival. The quantification of lung tissue response to irradiation is important in designing treatments associated with a minimum of complications and maximum tumor control.

METHODS

The study population consisted of 68 patients who received irradiation for breast cancer at Stage II. Radiation pneumonitis was retrospectively assessed on the basis of clinical symptoms and radiological findings. For each patient, a measure of the exposure (i.e., the lung dose-volume histogram [DVH]) and a measure of the outcome was available. Based on these data, a maximum likelihood fitting to the relative seriality model was performed. The uncertainties of the model parameters were calculated and their impact on the dose-response curve was studied. The optimum parameter set was then applied to 5 other patient groups treated for breast cancer, and the normal tissue complication probability (NTCP) was calculated. Each group was individuated by the radiotherapy treatment technique used; the dose distribution in the lung was described by a mean DVH and the incidence of radiation pneumonitis in each group was known. Lung radiosensitivity was assumed to be homogeneous through all of the calculations.

RESULTS

The relative seriality model could describe the dataset. The volume effect was found to be relevant in the description of radiation pneumonitis. Age was found to be associated with increased risk of radiation pneumonitis. Two distinct dose-response curves were obtained by splitting the group according to age. The impact of the parameter uncertainties on the dose-response curve was quite large. The parameter set determined could be used predictively on 3 of the 5 patient groups.

CONCLUSION

The complication data could be modeled with the relative seriality model. However, further independent datasets, classified according to the same endpoint, must be analyzed before introducing NTCP modeling in clinical practice.

Simulation of DNA fragment distributions after irradiation with photons

The Monte Carlo track structure code PARTRAC has been further improved by implementing electron scattering cross-sections for liquid water and by explicitly modelling the interaction of water radicals with DNA. The model of the genome inside a human cell nucleus in its interphase is based on the atomic coordinates of the DNA double helix with an additional volume for the water shell. The DNA helix is wound around histone complexes, and these nucleosomes are folded into chromatin fibres and further to fibre loops, which are interconnected to build chromosomes with a territorial organisation. Simulations have been performed for the irradiation of human fibroblast cells with carbon K and aluminium K ultrasoft x-rays, 220 kVp x-rays and 60Co gamma-rays. The ratio single-strand breaks to double-strand breaks (ssb/dsb) for both types of ultrasoft x-rays is lower than for gamma-rays by a factor of 2. The contributions of direct and indirect effects to strand break induction are almost independent of photon energy. Strand break patterns from indirect effects reflect differences in the susceptibility of the DNA helix to OH* attack inside the chromatin fibre. Distributions of small DNA fragments (<3 kbp) are determined by the chromatin fibre structure irrespective of whether direct or indirect effects are causing the breaks. In the calculated fragment size distributions for larger DNA fragments (>30 kbp), a substantial deviation from random breakage is found only for carbon K irradiation, and is attributed to its inhomogeneous dose distribution inside the cell nucleus. For the other radiation qualities, the results for larger fragments can be approximated by random breakage distributions calculated for a yield of dsb which is about 10% lower than the average for the whole genome. The excess of DNA fragments detected experimentally in the 8-300 kbp region after x-ray irradiation is not seen in our simulation results.

Modelling radiation-induced biological lesions: from initial energy depositions to chromosome aberrations

The development of biophysical models of chromosome aberration induction has undergone considerable improvements in the past few years. This is mainly due to the development of new experimental techniques, such as fluorescence in situ hybridization (FISH) and premature chromosome condensation (PCC), and to a better knowledge of track structure characteristics (both in the physical and chemical stages). In particular, track structure simulations, providing a detailed description of the spatial distribution of energy depositions and relevant DNA lesions, represent a useful starting point for the development of 'ab initio' models. Various aspects of the processes determining the induction and the formation kinetics of chromosome aberrations are still under debate, concerning in particular the target description (interphase chromosome organization), the characterization of relevant DNA lesions, the possibility of inducing exchanges starting from single radiation-induced lesions, the rejoining mechanisms (proximity effects and possible induction of incomplete exchanges, i.e. one-way exchanges) and the influence of specific scoring criteria adopted both in experiments and models. Starting from Lea's breakage-and-reunion theory and Revell's exchange theory, an overview is given of various models recently reported in the literature. The assumptions adopted by the authors concerning the various processes involved in aberration formation are analysed in detail, in order to clarify the different approaches adopted in treating the open questions outlined above.

Chromosome aberrations induced by light ions: Monte Carlo simulations based on a mechanistic model

PURPOSE

To investigate the mechanisms underlying the induction of chromosome aberrations by ionizing radiation, focusing attention on DNA damage severity, interphase chromosome geometry and the distribution of DNA strand breaks.

METHODS

An ab initio biophysical model of aberration induction in human lymphocytes specific for light ions was developed, based on the assumption that 'complex lesions' (clustered DNA breaks) produce aberrations, whereas less severe breaks are repaired. It was assumed that interphase chromosomes are spatially localized and that chromosome break free-ends rejoin pairwise randomly; the unrejoining of a certain fraction of free-ends was assumed to be possible, and small fragments were neglected in order to reproduce experimental conditions. The yield of different aberrations was calculated and compared with some data obtained using Giemsa or FISH techniques.

RESULTS

Dose-response curves for dicentrics and centric rings (Giemsa) and for reciprocal, complex and incomplete exchanges (FISH) were simulated; the ratio between complex and reciprocal exchanges was also calculated as a function of particle type and LET. The results showed agreement with data from lymphocyte irradiation with light ions.

CONCLUSIONS

The results suggest that clustered DNA breaks are a critical damage type for aberration induction and that interphase chromosome localization plays an important role. Moreover, the effect of a given particle type is related both to the number of induced complex lesions and to their spatial distribution.

[Rare anorectal malformations. Intermediate-type anal agenesis with a rectocutaneous fistula]

A case of anal agenesia with recto-cutaneous fistula is presented. The recto-cutaneous fistula in an intermediate or high anomaly seldom noticed so that it is not included in the common classifications. The surgical approach performed by us was that described by Mollard. The anterior perineal pull-through, under proximal protective enterostomy, allow to reduce the rate of postoperative complications and sequences and obtain--as in our patient--a normal continence and sphincteric function with an excellent esthetic outcome.

[Unusual anorectal anomalies. A clinical contribution]

Anorectal malformations (ARM) include a spectrum of anomalies which have been subdivided as "high", "intermediate" and "low"; a fourth group, defined as "miscellaneous", collects the forms which are most rarely observed. It is important, for a specialist, to know the last ones under the diagnostic and/or therapeutic profile. In this paper, the various problems observed in five cases of ARM considered particularly rare are analysed: a case of anorectal agenesia with recto-cloacal fistula and short uro-genital sinus; a rectal atresia with normal anal canal; a case of anorectal stenosis; an incomplete anal membrane; an anal agenesia with scrotal fistula.

A Monte Carlo calculation of cell inactivation by light ions

This study simulates the exposure of V79 Chinese hamster fibroblasts to low-energy protons, deuterons and alpha-particles in the LET range 10-200 keV/microm. The starting assumption is that the induction of clustered lesions in DNA is a fundamental step for cell inactivation. A non-homogeneous cell population was simulated by a computer program, using as input measured morphological parameters reported in the literature. Variations in the number of traversals through each cell of the population and in the length of the traversal, depending on actual nuclear thickness and position of the traversal, the energy spread of the incident beam, and the change of LET along the tracks were included in the simulation. Microdosimetric spectra were computed and compared with spectra obtained neglecting particle slowing-down and stochastic aspects of cell morphology. Simulated cell survival was estimated under the assumption that surviving cells are those with no clustered DNA lesions or no passages. The main features of experimental RBE versus LET and particle type were reproduced by the simulations. The influence of stochastic aspects of target-cell morphology and of the energy of the incident particles on survival were investigated under different assumptions about the correlation between morphological parameters. Results support the hypothesis of a relevant role of clustered DNA damage in cell killing and point out the importance of target-cell morphology and its variability in beam dosimetry and computer simulations of low-energy particle radiation effects.

Inactivation of individual mammalian cells by single alpha-particles

PURPOSE

To measure clonogenic death of Chinese hamster V79 cells following exposure to a defined number of 4.3 MeV alpha-particles (track-averaged LET = 105 keV/micron).

MATERIALS AND METHODS

Cells were irradiated at the radiobiological facility installed at the TTT-3 Tandem accelerator in Naples by using a 'Biostack' approach, which allows the positions of incident tracks relative to cells to be carefully determined. Subcellular structure was identified by fluorescence microscopy, while tracks were visualized by LR-115 solid state nuclear track detectors.

RESULTS

Particle hits in the cytoplasm did not significantly affect cell survival, yet survival probability decreased exponentially as a function of the number of nuclear traversals. Measured probability of surviving to exactly one 4.3 MeV alpha-particle traversal in the cell nucleus was 0.67 +/- 0.10. Inactivation cross-section was substantially higher than expected from conventional survival curves. However, folding of the data with Poisson statistics showed that survival level expected if a mean of one alpha-particle goes through a nucleus is higher than the measured value after exactly one particle traversal.

CONCLUSIONS

V79 cells have about 67% probability to survive a single alpha-particle traversal in the cell nucleus. Single-particle survival curves are consistent with conventional dose-survival relationships, once Poisson distribution of traversals is taken into account.

DNA complex lesions induced by protons and alpha-particles: track structure characteristics determining linear energy transfer and particle type dependence

The yield of DNA double-strand breaks (dsb) and DNA complex lesions induced by protons and alpha-particles of various energies was simulated using a Monte Carlo track structure code (MOCA15) and a simple model of the DNA molecule. DNA breaks of different complexity were analysed. The linear energy transfer (LET) and particle-type dependence of lesions of higher complexity seems to confirm the importance of clustered damage in DNA as a relevant step leading to biological endpoints such as cell inactivation. The detailed structure of proton and alpha-particle tracks was analysed to identify the main characteristics possibly responsible for such a dependence. The role of the primary ion and of its secondary electrons in inducing dsb and complex lesions is described, showing that the relative contribution of secondary electron tracks alone in inducing clustered lesions is almost negligible at high LET, but tends to dominate below = 10 keV/micron. This is consistent with the observed similar effectiveness of low-LET fast particle radiation and sparsely ionizing radiation such as x-rays. The dependence on LET and particle type is mainly due to energy deposition events of the primary ion together with short range electrons surrounding the ion track; the yield of complex lesions due to secondary electron tracks alone is substantially LET independent. The radial distributions of the energy contributing to the induction of complex lesions were analyzed and compared with the radial distributions of energy deposition of the full tracks. The results suggest that the stochastic behaviour (i.e. cluster properties) of the energy deposition pattern within a radius of a few nanometers around the ion track plays a relevant role in determining the biological radiation effectiveness.

[Morgagni's hernia in infancy]

Morgagni's hernia is an uncommon type of diaphragmatic hernia fairly exceptional in pediatric age. A case of Morgagni's hernia, observed by chance in a 15-month-old infant, is presented. The authors present the fundamental embryogenic stages of the diaphragm and stress the clinical manifestations and diagnostic procedures to reach the diagnosis, as well as the advantages of early treatment in order to prevent possible complications.

A methodology for biokinetic studies using stable isotopes: results of repeated molybdenum investigations on a healthy volunteer

A method for biokinetic studies in humans using stable isotopes is presented. The technique is based on double tracer administration and on proton activation as the analytical method. As an application, the results of investigations on molybdenum metabolism in humans are reported. The contents of 95Mo and 96Mo in biological samples were determined by inducing (p,n) reactions and by analysing the gamma-rays emitted by the radioactive products. The minimum detectable quantity was 2 ng/mL plasma for both Mo isotopes. Four investigations on molybdenum metabolism were performed on a healthy volunteer subject in the course of 3 yr. Two absorption studies with different amounts of tracers in aqueous solution were performed by giving 96Mo orally and 95Mo intravenously. Two investigations were performed with single oral administration of 96Mo in aqueous solution and of a 96Mo solution mixed with an infant formula respectively. The stability with time of the biokinetic parameters was tested. The fractional absorption values measured in this volunteer were 0.84, 0.98 and 0.95 for three studies with Mo in HCl and 0.51 for a single study with Mo administered in an infant formula, these data are discussed.

Long-term cardiac mortality after radiotherapy of breast cancer--application of the relative seriality model

Effects on the heart constitute a potentially significant and serious clinical problem in primary radiation therapy of early breast cancer. Increased cardiac mortality among irradiated patients may offset the potential benefit in terms of a reduced risk of recurrence or of death from breast cancer. Clinical data on long-term cardiac mortality among breast cancer patients included in two randomized trials (the Stockholm and Oslo studies) of radiation therapy as an adjunct to primary surgery were analysed using the relative seriality model of radiation response. Five different radiation therapy techniques were used in the trials. The original treatment plans were recalculated on a group of model patients using a three-dimensional treatment planning system. A mean dose-volume histogram (DVH) was calculated for each treatment technique. Both heart and myocardium, i.e. excluding circulating blood within the heart, were separately investigated as risk organs. Model parameters, (D50, i.e. the dose giving 50% complication probability; gamma, i.e. the maximum relative slope of the dose-response curve; s, describing the organ relative seriality) were determined by a chi 2 fitting of the calculated probability of excess cardiac mortality, based on the DVHs, to the incidence data. Computed complication probabilities for each treatment technique were modelled within the 95% confidence interval (CI) of the clinical incidence data. It was shown that the relative seriality model, assuming a homogeneous radiation sensitivity within the volume of the heart/myocardium can be used to describe the incidence data. A small dependence on the volume was found. The results do not, however, exclude the possibility that more sensitive structures within the myocardium are the main target for radiation.

[Diagnosis of food allergy caused by fruit and vegetables in children with atopic dermatitis]

Atopic dermatitis (A.D.) is a frequent, complex and multifactorial disease: Food Allergy (F.A.), probably underestimated, especially for fruits and vegetables, seems to play an important pathogenetic role in children. The purpose of this study is to estimate, on a sample of children with A.D., the prevalence of F.A. (for fruits and vegetables), and the reliability of diagnosis of Prick+Prick test compared with the usual Prick test, RAST and challenge. Twentysix patients (17 M and 9 F), ranging in age from 5 months to 8 years, were enrolled in the study. All fulfilled the criteria of Hanifin and Rajka for the diagnosis of A.D. Food RAST, prick tests with inhalant and food extracts and Prick+Prick tests with fresh fruits and vegetables were carried out. In the case of positive result to fruits and vegetables with skin tests and/or RAST, open challenge for every type of food considered responsible was carried out, after healing or improvement of dermatitis. Three children (11.53%) suffered from F.A. for fruits and vegetables: allergy to celery of one patient was discovered only by usual Prick test; allergy to tomato and kiwi in another patient was spotted by Prick+Prick only; while in another case by both tests. In this last patient Prick+Prick test revealed a real allergy for 5 aliments (carrot, tomato, celery, cucumber, fennel) of which only 2 (carrot and celery) also caused a reaction with the Prick test. The combined use of both tests made it possible to increase the diagnosis of F.A. both for the number of patients and for a complete identification of implicated foods.

The quality of DNA double-strand breaks: a Monte Carlo simulation of the end-structure of strand breaks produced by protons and alpha particles

The quality of DNA damage induced by protons and alpha-particles of various linear energy transfer (LET) was studied. The aim was to single out specific lesions in the DNA molecule that might lead to biological endpoints such as inactivation. A DNA model coupled with a track structure code (MOCA-15) were used to simulate the lesions induced on the two helixes. Four categories of DNA breaks were considered: single-strand breaks (ssb), blunt-ended double-strand breaks (dsb, with no or few overlapping bases), sticky-ended double-strand breaks (with cohesive free ends of many bases), and deletions (complex lesions which involve at least two dsb within a small number of base pairs). Calculations were carried out assuming various sets of parameters characterizing the production of these different DNA breaks. No large variations in the yields of ssb and blunt- or sticky-ended dsb were found in the LET range between 10 and 200 keV/mu m. On the other hand, the yield of deletions increases up to about 100 keV/mu m and seems to reach a plateau at higher LET values. In the LET interval from 30 to 60 keV/mu m, protons proved to be more efficient than alpha-particles in inducing deletions. The induction of these complex lesions is thus dependent not simply on LET but also on the characteristics of the track structure. Comparison with RBE values for cell killing shows that this special class of dsb might play an important role in radiation-induced cell inactivation.

Influence of the administered mass of tellurium on plasma clearance in rabbits

The combination of analytical techniques such as PNA and SIMS with a compartmental approach enables the study of the metabolism and biokinetics in humans of several elements by using stable isotopes as tracers. The techniques developed for Te require the administration of greater masses than those used for similar studies performed with radioactive tracers, therefore a test was carried out in rabbits in order to assess the possible influence of the administered amounts on the determination of the biokinetic parameters. The behaviour of the tracers was found to be similar for Te administration of up to 70 micrograms/kg of body weight. An inter-individual variability in the size of the transfer compartment was observed and has to be taken into account.

Proton activation analysis of stable isotopes for a molybdenum biokinetics study in humans

Molybdenum is a trace element essential to life. Nevertheless, little information is available on its metabolism in humans. A methodology based on stable isotope administration that combines compartmental analysis, simultaneous use of two tracers, and proton nuclear activation (PNA) is presented. A four-compartment metabolic model was adopted. The compartments are stomach, small intestine, transfer compartment, and unquantified tissue pool. The employment of two different stable isotopes of the element under investigation as tracers was made possible by PNA. Optimization of the technique for molybdenum determination in plasma led to the choice of 95Mo and 96Mo as tracers. Their concentrations in plasma can be determined measuring the disintegration gamma lines of the corresponding technetium radioisotopes produced via (p,n) reaction. In the adopted experimental conditions, a minimum detectable concentration of 2 ng isotope/ml plasma was attained. A kinetics study was performed on two healthy volunteers. To both subjects one tracer was orally administered, and the other intravenously injected. Venous blood samples were withdrawn at different postinjection times and the concentrations for both isotopes determined. The model parameters describing molybdenum kinetics were obtained for the two individuals. Total absorbed fraction was found to be 0.84 +/- 0.03 and 0.86 +/- 0.07, respectively.

Immunoactivation by pidotimod in children with recurrent respiratory infections

The therapeutic activity and safety of pidotimod ((R)-3-[(S)-(5-oxo-2-pyrrolidinyl) carbonyl]-thiazolidine-4-carboxylic acid, PGT/1A, CAS 121808-62-6), a new synthetic "biological response modifier", were examined in a double-blind, placebo-controlled, multicentre trial in 101 children, including 53 boys and 48 girls aged 2-13 years (mean +/- SD: 4.7 +/- 2.1 years) with a history of recurrent respiratory infections (RRI). Pidotimod (400 mg/day) or placebo were administered orally for 60 consecutive days, followed by a further 60-day follow-up period. The trial was completed by 89.1% of patients. The results indicate that pidotimod has a beneficial effect in children with recurrent respiratory infections: the percentage of patients presenting symptoms affecting the upper and lower airways was significantly lower after treatment with the active drug than after treatment with placebo. Relevant side effects were not reported during the trial. An evaluation of the expression of CD25 (after in vitro stimulation of circulating mononuclear cells with PHA) before and after treatment with the two products revealed a significant increase in CD25+ cells in the group treated with pidotimod but not in the group treated with placebo.

Adolescence and paediatrics in Europe

Adolescence is an important period of growth and development. It is often claimed that adolescent care and assistance should be assigned to paediatricians who have followed children during their growth, are prepared to deal with growth and puberty problems, and are used to discussing with parents problems related to school, sports and habits. There is serious doubt as to the extent to which this actually occurs. An inquiry in various European countries demonstrates that there are wide differences in adolescent care and guidelines. Useful viewpoints for planning have emerged from the inquiry.

Ectopic anus in childhood

The treatment of ten cases of ectopic anus is presented in this paper. The association of constipation with perineal anus due to alignment disturbance of the external sphincter triple loop system during evacuation has been described by Hendren (1978), Leape and Ramenofsky (1978), Upadhyaya (1984) et al. Hendren, Leape and Ramenofsky described two valid surgical techniques for this association. In this paper we describe the effect of non-invasive treatment in patients affected by symptomatic "primary" perineal anus. We also describe the effect of posterior anal transplant via the perineum in six infants and children with vulvar anus. This approach gave good results within three months without having to perform more complex operative techniques (Peña for example). Posterior anal transplant leads not only to an esthetic improvement but also to an improvement in fecal continence, which is sometimes deficient due to reduced anorectal angulation. It appears to us that an adequate perineal spur plays an important complementary role in the complex mechanism which controls fecal continence.

Radiation-induced chromosomal aberrations in mouse 10T1/2 cells: dependence on the cell-cycle stage at the time of irradiation

Cell-cycle stage radiosensitivity for the induction of chromosome aberrations has been investigated in C3H 10T1/2 cells. Exponentially growing cells were irradiated with 3 Gy X-rays (80 kVp) or 0.6 Gy alpha-particles (LET = 101 keV/micron). The two doses produce the same survival level (37%) in the asynchronous population. Cells were harvested at four different times following irradiation and cell-cycle phase at the time of irradiation was assessed by using the differential replication staining technique. The frequency of chromosome aberrations produced in a given stage of the cell cycle was not constant as a function of the sampling time, but this could not be simply related to the existence of subphases exhibiting different radiosensitivity, because of cell-cycle perturbation introduced by radiation. X-radiation induced more exchanges than deletions, whereas a predominance of isochromatid deletions was observed after alpha-irradiation. This can be interpreted on the basis of the different patterns of energy deposition of densely- and sparsely-ionizing radiation. Both X- and alpha-rays produced a significant increase in the frequency of Robertsonian translocations when cells were exposed in G1 or S phase, but not in G2 phase.