Relative biological effectiveness of 50-MV X rays on jejunal crypt survival in vivo.

Earlier in vitro studies of relative biological effectiveness (RBE) of 50-MV X rays show an RBE of approximately 1.1 compared to 4 MV. No difference in RBE has been found for 20-MV X rays or 50-MeV electrons. The higher RBE for 50 MV can be explained to some extent by the small high linear energy transfer contribution from photonuclear reactions at high X-ray energies. To investigate the validity of the results in vitro, a study of the RBE of 50-MV X rays has been performed in vivo using the jejunal crypt microcolony assay in mice. The reference radiation used in this case was 20-MV X rays. The results confirm the earlier in vitro studies. The RBE for 50-MV X rays was estimated to be 1.06, calculated as the ratio between the slopes of the response curves.     

Renal damage in the mouse: the effect of d(4)-Be neutrons

A further study on the response of the mouse kidney to d(4)-Be neutrons (EN = 2.3 MeV) is described. The results confirm and augment the work published previously by Stewart et al. [Br. J. Radiol. 57, 1009-1021 (1984)]; the present paper includes the data from a "top-up" design of experiment which extends the measurements of neutron RBE (relative to 240 kVp X rays) down to X-ray doses of 0.75 Gy per fraction. The mean RBE for these neutrons increases from 5.8 to 7.3 as X-ray dose per fraction decreases from 3.0 to 1.5 Gy in the kidney. This agrees with the predictions from the linear quadratic (LQ) model, based on the renal response to X-ray doses above 4 Gy per fraction. The mean RBE estimate from a single dose group at 0.75 Gy per fraction of X rays is, however, 3.9. This is below the LQ prediction and may indicate increasing X-ray sensitivity at low doses. Data from this study and from those published previously have been used to determine more accurately the shape of the underlying response to d(4)-Be neutrons; an alpha/beta ratio of 20.5 +/- 3.7 Gy was found. The best value of alpha/beta for X rays determined from these experiments was 3.04 +/- 0.35 Gy, in agreement with previous values.     

Accelerated heavy particles and the lens. VI. RBE studies at low doses

We report on the prevalence, hazard, and relative biological effectiveness (RBE) for various stages of lens opacification in rats induced by very low doses of fast argon ions of LET 88 keV/microns, compared to those for X rays. Doses of argon ions from 0.01 to 0.25 Gy were used and RBEs of these ions relative to X rays estimated using a nonparametric technique. At the end of the follow-up period, which encompasses a significant fraction of the animals' lifetime, 90% confidence intervals for the RBE of the argon ions relative to X rays were 4-8 at 0.25 Gy, 10-40 at 0.05 Gy, and 50-100 at 0.01 Gy. Our results are consistent with the point-estimate neutron RBEs in Japanese A-bomb survivors, though broad confidence bounds are present in the Japanese results. If a reasonable extrapolation to higher doses is used, our results are also consistent with data reported earlier at higher doses for argon-ion cataractogenesis in rats, mice, and rabbits. We conclude from these results that at very low doses the RBE for cataractogenesis from HZE particles in space is considerably more than 20, and use of a quality factor of at least 50 would be prudent.     

Occurrence of mammary tumors in rats after exposure to tritium beta rays and 200-kVp X rays

The RBE for tritium was estimated in reference to 200-kVp X rays, using acceleration of breast tumor appearance in the female Sprague-Dawley rat as the end-point. Chronic X-ray doses of 0.3-2.0 Gy were delivered over 10 days. Intraperitoneal injections of tritiated water ranging in concentrations from 45 to 370 MBq/100 g body wt were administered, followed by four additional injections at 2-day intervals and half of the initial concentrations. Seventy-five percent of the total tritium dose was delivered to the mammary gland within the first 10 days and 95% within the first 20 days after the start of the tritium exposure. RBE estimations were based on various criteria including the tumor incidence per Gy at 450 days postirradiation and the time required to induce tumors in 50% of the animals at risk. The results suggest that tritium beta rays are about 1.1-1.3 times more effective than chronic 200-kVp X rays for acceleration of the appearance of rat mammary tumors. However, the uncertainties involved in these calculations are such that the effects of tritium beta rays could not be reliably distinguished from those of chronic 200-kVp X rays. Measured differences in RBE values were slightly larger for the comparison between acute and chronic X rays than for the comparison between chronic tritium beta rays and chronic X rays.     

RBE of 10 kV X rays determined for the human mammary epithelial cell line MCF-12A

The dependence of relative biological effectiveness (RBE) on photon energy is a topic of extensive discussions. The increasing amount of in vitro data in the low-energy region indicates this to be a complex dependence that is influenced by the end point and cell line studied. In the present investigation, the RBE of 10 kV X rays (W anode) was determined relative to 200 kV X rays (W anode, 0.5 mm copper filter) for cell survival in the dose range 1-10 Gy and for induction of micronuclei in the range 0.5-3.6 Gy for MCF-12A human mammary epithelial cells. The RBE for cell survival was found to increase with decreasing dose, being 1.21+/-0.03 at 10% survival. Considerably higher values were obtained for micronucleus induction, where the RBE(M) obtained from the ratio of the linear coefficients of the dose-effect curves was 2.6+/-0.4 for the fraction of binucleated cells with micronuclei and 4.1+/-1.0 for the number of micronuclei per binucleated cell. These values, together with our previous data, support a monotonic increase in RBE with decreasing photon energy down to the mean energy of 7.3 keV used in the present study.     

Enhanced neoplastic transformation by mammography X rays relative to 200 kVp X rays: indication for a strong dependence on photon energy of the RBE(M) for various end points.

The fundamental assumption implicit in the use of the atomic bomb survivor data to derive risk estimates is that the gamma rays of Hiroshima and Nagasaki are considered to have biological efficiencies equal to those of other low-LET radiations up to 10 keV/microm, including mammography X rays. Microdosimetric and radiobiological data contradict this assumption. It is therefore of scientific and public interest to evaluate the efficiency of mammography X rays (25-30 kVp) to induce cancer. In this study, the efficiency of mammography X rays relative to 200 kVp X rays to induce neoplastic cell transformation was evaluated using cells of a human hybrid cell line (CGL1). For both radiations, a linear-quadratic dose-effect relationship was observed for neoplastic transformation of CGL1 cells; there was a strong linear component for the 29 kVp X rays. The RBE(M) of mammography X rays relative to 200 kVp X rays was determined to be about 4 for doses < or = 0.5 Gy. A comparison of the electron fluences for both X rays provides strong evidence that electrons with energies of < or = 15 keV can induce neoplastic transformation of CGL1 cells. Both the data available in the literature and the results of the present study strongly suggest an increase of RBE(M) for carcinogenesis in animals, neoplastic cell transformation, and clastogenic effects with decreasing photon energy or increasing LET to an RBE(M) approximately 8 for mammography X rays relative to 60Co gamma rays.     

Neutron-energy-dependent oncogenic transformation of C3H 10T1/2 mouse cells

The relative biological effectiveness (RBE) of a range of neutron energies relative to 250-kVp X rays has been determined for oncogenic transformation and cell survival in the mouse C3H 10T 1/2 cell line. Monoenergetic neutrons at 0.23, 0.35, 0.45, 0.70, 0.96, 1.96, 5.90, and 13.7 MeV were generated at the Radiological Research Accelerator Facility of the Radiological Research Laboratories, Columbia University, and were used to irradiate asynchronous cells at low absorbed doses from 0.05 to 1.47 Gy. X irradiations covered the range 0.5 to 8 Gy. Over the more than 2-year period of this study, the 31 experiments provided comprehensive information, indicating minimal variability in control material, assuring the validity of comparisons over time. For both survival and transformation, a curvilinear dose response for X rays was contrasted with linear or nearly linear dose responses for the various neutron energies. RBE increased as dose decreased for both end points. Maximal RBE values for transformation ranged from 13 for cells exposed to 5.9-MeV neutrons to 35 for 0.35-MeV neutrons. This study clearly shows that over the range of neutron energies typically seen by nuclear power plant workers and individuals exposed to the atomic bombs in Japan, a wide range of RBE values needs to be considered when evaluating the neutron component of the effective dose. These results are in concordance with the recent proposals in ICRU 40 both to change upward and to vary the quality factor for neutron irradiations.     

Induction of chromosome aberrations in G0 human lymphocytes by low doses of ionizing radiations of different quality

Human peripheral blood lymphocytes from two donors were exposed to low doses (0.05 to 2.0 Gy) of gamma rays, X rays, or fast neutrons of different energies. Chromosome aberrations were analyzed in metaphase of first-division cells after a culture time of 45-46 hr. At this time, less than 5% of the cells were found in second division. Different dose-response relationships were fitted to the data by using a maximum likelihood method; best fits for radiation-induced dicentric aberrations were obtained with the linear-quadratic law for all radiations. The linear component of this equation predominated, however, for neutrons in the range of doses studied, and the frequency of dicentrics induced by d(16)+Be neutrons up to 1.0 Gy could also be described by a linear relationship. The relative biological efficiency (RBE) of X rays and d(16)+Be, d(33)+Be, and d(50)+Be neutrons compared to 60Co gamma rays in the low dose range was calculated from the dose-effect relationships for the dicentrics produced. The RBE increased with decreasing neutron dose and with decreasing neutron energy from d(50)+Be to d(16)-+Be neutrons. The limiting RBE at low doses (RBEo) was calculated to be about 1.5 for X rays and 14.0, 6.2, and 4.7 for the d(16)+Be, d(33)+Be, and d(50)+Be neutrons, respectively.     

Lung carcinomas in Sprague-Dawley rats after exposure to low doses of radon daughters, fission neutrons, or gamma rays

The effectiveness of radon-daughter inhalation and irradiation with fission neutrons and gamma rays in the induction of lung carcinomas in Sprague-Dawley rats at low doses is compared. Earlier reports which compared radon-daughter inhalations and neutron irradiations over a wider range of doses were based on dosimetry for the radon-daughter inhalations which has recently been found to be faulty. In the present analysis, low-dose experiments were designed to derive revised equivalence ratios between radon-daughter exposures, and fission neutron or gamma irradiations. The equivalence is approximately 15 working level months (WLM) of radon daughters to 10 mGy of neutrons (the earlier value was 30 WLM to 10 mGy). The relative biological effectiveness (RBE) of neutrons is 50 or more at a gamma-ray dose of 1 Gy. In these experiments with low doses and exposures, the lifetime incidences can be estimated from the raw incidences, while the derivation of the time dependence of the prevalence is essential for the estimation of RBE values and equivalence ratios.     

Relative cataractogenic effects of X rays, fission-spectrum neutrons, and 56Fe particles: a comparison with mitotic effects

The eyes of Sprague-Dawley rats were irradiated with doses of 2.5-10 Gy 250-kVp X rays, 1.25-2.25 Gy fission-spectrum neutrons (approximately 0.85 MeV), or 0.1-2.0 Gy 600-MeV/A 56Fe particles. Lens opacifications were evaluated for 51-61 weeks following X and neutron irradiations and for 87 weeks following X and 56Fe-particle irradiations. Average stage of opacification was determined relative to time after irradiation, and the time required for 50% of the irradiated lenses to achieve various stages (T50) was determined as a function of radiation dose. Data from two experiments were combined in dose-effect curves as T50 experimental values taken as percentages of the respective T50 control values (T50-% control). Simple exponential curves best describe dose responsiveness for both high-LET radiations. For X rays, a shallow dose-effect relationship (shoulder) up to 4.5 Gy was followed at higher doses by a steeper exponential dose-effect relationship. As a consequence, RBE values for the high-LET radiations are dose dependent. Dose-effect curves for cataracts were compared to those for mitotic abnormalities observed when quiescent lens epithelial cells were stimulated mechanically to proliferate at various intervals after irradiation. Neutrons were about 1.6-1.8 times more effective than 56Fe particles for inducing both cataracts and mitotic abnormalities. For stage 1 and 2 cataracts, the X-ray Dq was 10-fold greater and the D0 was similar to those for mitotic abnormalities initially expressed after irradiation.     

The effect of 29 kV X rays on the dose response of chromosome aberrations in human lymphocytes

The induction of chromosome aberrations in human lymphocytes irradiated in vitro with X rays generated at a tube voltage of 29 kV was examined to assess the maximum low-dose RBE (RBE(M)) relative to higher-energy X rays or 60Co gamma rays. Since blood was taken from the same male donor whose blood had been used for previous irradiation experiments using widely varying photon energies, the greatest possible accuracy was available for such an estimation of the RBE(M), avoiding the interindividual variations in sensitivity or differences in methodology usually associated with interlaboratory comparisons. The magnitude of the linear coefficient alpha of the linear-quadratic dose-effect relationship obtained for the production of dicentric chromosomes by 29 kV X rays (alpha = 0.0655 +/- 0.0097 Gy(-1)) confirms earlier observations of a strong increase in alpha with decreasing photon energy. Relating this value to previously published values of alpha for the dose-effect curves for dicentrics obtained in our own laboratory, RBE(M) values of 1.6 +/- 0.3 in comparison with weakly filtered 220 kV X rays, 3.0 +/- 0.7 compared to heavily filtered 220 kV X rays, and 6.1 +/- 2.5 compared to 60Co gamma rays have been obtained. These data emphasize that the choice of the reference radiation is of fundamental importance for the RBE(M) obtained. A special survey of the RBE(M) values obtained by different investigators in the narrow quality range from about 30 to 350 kV X rays indicates that the present RBE is in fairly good agreement with previously published findings for the induction of chromosome aberrations or micronuclei in human lymphocytes but differs from recently published findings for neoplastic transformation in a human hybrid cell line.     

The maximum low-dose RBE of 17.4 and 40 keV monochromatic X rays for the induction of dicentric chromosomes in human peripheral lymphocytes

Schmid et al. recently reported on the maximum low-dose RBE for mammography X rays (29 kV) for the induction of dicentrics in human lymphocytes. To obtain additional information on the RBE for this radiation quality, experiments with monochromatized synchrotron radiation were performed. Monochromatic 17.4 keV X rays were chosen for comparison with the diagnostic mammography X-ray spectrum to evaluate the spectral influence, while monochromatic 40 keV X rays represent a higher-energy reference radiation, within the experiment. The induction of dicentric chromosomes in human lymphocytes from one blood donor irradiated in vitro with 17.4 keV and 40 keV monochromatic X rays resulted in alpha coefficients of (3.44 +/- 0.87) x 10(-2) Gy(-1) and (2.37 +/- 0.93) x 10(-2) Gy(-1), respectively. These biological effects are only about half of the alpha coefficients reported earlier for exposure of blood from the same donor with the broad energy spectra of 29 kV X rays (mean energy of 17.4 keV) and 60 kV X rays (mean energy of 48 keV). A similar behavior is evident in terms of RBEM. Relative to weakly filtered 220 kV X rays, the RBEM for 17.4 and 40 keV monochromatic X rays is 0.86 +/- 0.23 and 0.59 +/- 0.24, respectively, which is in contrast to the RBEM of 1.64 +/- 0.27 for 29 kV X rays and 1.10 +/- 0.19 for 60 kV X rays. It is evident that the monochromatic radiations are less effective in inducing dicentric chromosomes than broad-spectrum X rays with the corresponding mean energy value. Therefore, it can be assumed that, for these X-ray qualities with broad energy spectra, a large fraction of the effects should be attributed predominantly to photons with energies well below the mean energy.     

Repopulation kinetics of rat rhabdomyosarcoma tumors following single and fractionated doses of low-LET and high-LET radiation

Measurements were made of clonogenic cell survival in rat rhabdomyosarcoma tumors as a function of time following in situ irradiation with single or fractionated doses of 225-kVp X rays or with 557-MeV/u neon ions in the distal position of a 4-cm extended-peak ionization region. Single doses of 20 Gy of X rays or 7 Gy of peak neon ions reduced the initial surviving fraction to approximately 0.025 for each modality. Daily fractionated doses (four fractions in 3 days) of either peak neon ions (1.75 Gy per fraction) or X rays (6 Gy per fraction) achieved a cell survival of approximately 0.02-0.03 after the fourth dose of radiation. In the single-dose experiments, significant 5- and 10-fold decreases in the fraction of clonogenic cells were observed between the third and fourth days after irradiation with peak neon ions and X rays, respectively. After the sixth day postirradiation, the residual clonogenic cells exhibited a rapid burst of proliferation leading to doubling times for the surviving cell fractions of approximately 1.5 days. Radiation-induced growth delay was consistent with the cellular repopulation dynamics. In the fractionated-dose experiments with both radiation modalities, a large delayed decrease in cell survival was observed at 1-3 days after completion of the fractionated-dose schedule. Cellular repopulation was consistent with postirradiation tumor volume regression and regrowth for both radiation modalities. The extent of decrease in survival following the four-fraction radiation schedule was approximately two times greater in X-irradiated than in neon-ion-irradiated tumors that produced the same survival level immediately after the fourth dose. Mechanisms underlying the marked reduction in cell survival 3-4 days postirradiation are discussed, including the possible role of a toxic host cell response against the irradiated tumor cells.     

Relative biological effectiveness (RBE) of thermal neutron capture therapy of cultured B-16 melanoma cells preincubated with 10B-paraboronophenylalanine

An experimental study of the relative biological effectiveness (RBE) of thermal neutron capture therapy (TNCT) for melanoma cell inactivation using 10B1-paraboronophenylalanine (10B1-BPA) was carried out to demonstrate a high therapeutic effect of TNCT, compared with that of fast neutron. Cells preincubated with or without 10B1-BPA at a concentration of 50 micrograms/ml for 20 h were irradiated with 60Co gamma-ray, fast neutron or thermal neutron. The absorbed dose of the cells from thermal neutron was calculated by Kobayashi's model. The D0 value of fast neutron was 1.07 Gy, and the D0S of thermal neutron radiation with or without preincubation of the cells with 10B1-BPA were 0.46 Gy or 0.67 Gy, respectively. The RBEs of fast neutron, thermal neutron beams, and neutron capture therapy relative to 60Co gamma-ray were calculated as 2.78, 4.18, and 6.15 at 0.1 surviving fraction, respectively. These results indicate radiologically that thermal neutron capture therapy using 10B1-BPA is an excellent radiation therapy for malignant melanoma.     

Measurement of the initial levels of DNA damage in human lymphocytes induced by 29 kV X rays (mammography X rays) relative to 220 kV X rays and gamma rays

Experiments using the alkaline comet assay, which measures all single-strand breaks regardless of their origin, were performed to evaluate the biological effectiveness of photons with different energies in causing these breaks. The aim was to measure human lymphocytes directly for DNA damage and subsequent repair kinetics induced by mammography 29 kV X rays relative to 220 kV X rays, 137Cs gamma rays and 60Co gamma rays. The level of DNA damage, predominantly due to single-strand breaks, was computed as the Olive tail moment or percentage DNA in the tail for different air kerma doses (0.5, 0.75, 1, 1.5, 2 and 3 Gy). Fifty cells were analyzed per slide with a semiautomatic imaging system. Data from five independent experiments were transformed to natural logarithms and fitted using a multiple linear regression analysis. Irradiations with the different photon energies were performed simultaneously for each experiment to minimize interexperimental variation. Blood from only one male and one female was used. The interexperimental variation and the influence of donor gender were negligible. In addition, repair kinetics and residual DNA damage after exposure to a dose of 3 Gy were evaluated in three independent experiments for different repair times (10, 20, 30 and 60 min). Data for the fraction of remaining damage were fitted to the simple function F(d) = A/(t + A), where F(d) is the fraction of remaining damage, t is the time allowed for repair, and A (the only fit parameter) is the repair half-time. It was found that the comet assay data did not indicate any difference in the initial radiation damage produced by 29 kV X rays relative to the reference radiation types, 220 kV X rays and the gamma rays of 137Cs and 60Co, either for the total dose range or in the low-dose range. These results are, with some restrictions, consistent with physical examinations and predictions concerning, for example, the assessment of the possible difference in effectiveness in causing strand breaks between mammography X rays and conventional (150-250 kV) X rays, indicating that differences in biological effects must arise through downstream processing of the damage.     

Radiobiology of ultrasoft X rays. IV. Flat and round-shaped hamster cells (CHO-10B, HS-23)

The results reported earlier in this series indicated that the relative biological effectiveness (RBE) of ultrasoft X rays decreases with decreasing cell thickness, approaching unity for the thinnest cells used, plateau-phase human skin fibroblasts (HSF). The possible dependence of RBE on the configuration of the cell nucleus is investigated further in this paper using two CHO cell lines that attach well and have similar intrinsic radiosensitivities to 60Co gamma rays. One of the lines forms monolayers similar to V79 cells, while the other remains more spherical during growth. We find an increasing RBE with decreasing X-ray energy for both of these cell lines, consistent with our results using V79 cells. Also consistent with our results obtained with 10T1/2 and HSF cells, we find an increasing RBE with increasing cell thickness. The possible dependence of RBE on radiosensitivity and the use of the concept of mean dose for ultrasoft X rays is discussed.     

Lung cancer risk in mice: analysis of fractionation effects and neutron RBE with a biologically motivated model

Data from Argonne National Laboratory on lung cancer in 15,975 mice with acute and fractionated exposures to gamma rays and neutrons are analyzed with a biologically motivated model with two rate-limiting steps and clonal expansion. Fractionation effects and effects of radiation quality can be explained well by the estimated kinetic parameters. Both an initiating and a promoting action of neutrons and gamma rays are suggested. While for gamma rays the initiating event is described well with a linear dose-rate dependence, for neutrons a nonlinear term is needed, with less effectiveness at higher dose rates. For the initiating event, the neutron RBE compared to gamma rays is about 10 when the dose rate during each fraction is low. For higher dose rates this RBE decreases strongly. The estimated lifetime relative risk for radiation-induced lung cancers from 1 Gy of acute gamma-ray exposure at an age of 110 days is 1.27 for male mice and 1.53 for female mice. For doses less than 1 Gy, the effectiveness of fractionated exposure to gamma rays compared to acute exposure is between 0.4 and 0.7 in both sexes. For lifetime relative risk, the RBE from acute neutrons at low doses is estimated at about 10 relative to acute gamma-ray exposure. It decreases strongly with dose. For fractionated neutrons, it is lower, down to about 4 for male mice.     

Neutron RBEs for mouse skin at low doses per fraction

The effect of low doses of 240 kVp X rays or of 3 MeV neutrons has been investigated using skin reactions on mouse feet as the biological system. Eight or nine repeated small doses of radiation were used, followed by graded "top-up" doses to bring the reactions into a detectable range. By comparing dose-response curves, the RBE has been determined for neutron doses per fraction ranging from 0.25-1.0 Gy. The data are consistent with a limiting RBE of between 7 and 10 at very low doses. A review of other published RBE values for low doses per fraction shows a wide range of RBEs . Very few studies show a plateau value for the RBE. These findings are more consistent with dose-response data that fit a linear-quadratic model than with a multitarget single-hit model.     

Radiobiology of ultrasoft X rays. II. Cultured C3H mouse cells (10T1/2)

In the first paper of this series (Radiat. Res. 110, 396-412 (1987], using V79 cells, we reported that the relative biological effectiveness (RBE) of ultrasoft X rays was found to increase with decreasing energy, and the oxygen enhancement ratio (OER) was found to decrease with decreasing energy. In this report, we present RBE and OER results for 10T1/2 cells that are known to grow uniformly flat and are considerably thinner than V79 cells. Thus the variation in dose across the cell nucleus is considerably reduced. The OER results agree well with our earlier V79 results. However, the RBE values for 10T1/2 cells compared to V79 cells are systematically less for all soft X rays and especially for 0.28 keV carbon-K (1.3 compared to 3.4 for V79 cells). Some plausible explanations are presented to reconcile the apparent discrepancy between V79 and 10T1/2 results.     

RBE of X rays of different energies: a cytogenetic evaluation by FISH

Mammography using 26-30 kVp X rays is routinely used in breast cancer screening. Discussion about the radiation-related risk associated with this methodology is ongoing. For radioprotection purposes, a quality factor of 1 has been assigned for all photon energies. However, the relative biological effectiveness (RBE) could increase as the photon energy decreases. Analyzing different biological parameters, for 30 kVp X rays, RBE values from 1 to 8 have been estimated. In the present study, a cytogenetic FISH evaluation of the RBE of 30, 80 and 120 kVp X rays has been done. Blood samples were irradiated with 10 doses from 0.05 to 3 Gy for each energy studied. The yields of translocations and dicentrics were determined by fluorescence in situ hybridization (FISH) using whole chromosome probes for chromosomes 1, 4 and 11 together with a pancentromeric probe. The alpha coefficients of the dose-effect curves for dicentrics, minimum number of breaks needed to produce exchange-type aberrations, and apparently simple translocations were used to estimate the RBE. Using the curves obtained for 120 kVp as a reference, the RBE values for dicentrics were 1.08+/-0.43 and 1.73+/-0.59 for 80 and 30 kVp X rays, respectively; for minimum number of breaks these values were 1.38+/-0.39 and 1.42+/-0.41, and for apparently simple translocations they were 1.26+/-0.40 and 1.51+/-0.47, respectively. Moreover, the induction of complex aberrations by these energies was compared. The percentage of complex aberrations relative to total aberrations showed a significant tendency to increase as X-ray energy decreased: 7.8+/-1.19, 9.8+/-1.6 and 14.1+/-1.9 for 120, 80 and 30 kVp, respectively (P<0.02).