Development of a simulation method for dynamics of electrons ejected from DNA molecules irradiated with X-rays

Abstract

Purpose: To develop a method for simulating the dynamics of the photoelectrons and Auger electrons ejected from DNA molecules irradiated with pulsed monochromatic X-rays.

Materials and methods: A 30-base-pair (bp) DNA molecule was used as the target model, and the X-rays were assumed to have a Gaussian-shaped time distribution. Photoionization and Auger decay were considered as the atomic processes. The atoms from which the photoelectrons or Auger electrons were emitted were specified in the DNA molecule (or DNA ion) using the Monte Carlo method, and the trajectory of each electron in the electric field formed around the positively charged DNA molecule was calculated with a Newtonian equation. The kinetics of the electrons produced by irradiation with X-rays at an intensity ranging from 1 × 10¹² to 1 × 10¹⁶ photons/mm² and energies of 380 eV (below the carbon K-edge), 435 eV (above the nitrogen K-edge), and 560 eV (above the oxygen K-edge) were evaluated.

Results: It was found that at an X-ray intensity of 1 × 10¹⁴ photons/mm² or less, all the produced electrons escaped from the target. However, above an X-ray intensity of 1 × 10¹⁵ photons/mm² and an energy of 560 eV, some photoelectrons that were ejected from the oxygen atoms were trapped near the target DNA.

Conclusions: A simulation method for studying the trajectories of electrons ejected from a 30-bp DNA molecule irradiated with pulsed monochromatic X-rays has been developed. The present results show that electron dynamics are strongly dependent on the charged density induced in DNA by pulsed X-ray irradiation.     

Characterization of lesions induced in linear-formed plasmid DNA by valence ionization and Auger decay at carbon,nitrogen and oxygen

Purpose: To study the DNA lesions induced by the Auger decay of carbon, nitrogen, and oxygen using ultrasoft X-rays (USX) that are expected to be important with the DNA repair system of living cells.

Materials and methods: pUC19 plasmid DNA dry samples were irradiated with USX photons at 270 and 560 eV and ⁶⁰Co γ-rays in vacuum at room temperature. The amounts of unaltered base release by the direct radiation effects were quantified using high-performance liquid chromatography. To quantify and characterize the strand break termini the rate at which snake venom phosphodiesterase (SVPD) digested irradiated DNA pretreated with and without calf intestine alkaline phosphatase was measured. Moreover, the piperidine-labile base lesions and abasic sites of the irradiated DNA were estimated using the SVPD method.

Results: The yields of unaltered base release for 270, 560 eV photons and ⁶⁰Co γ-rays were 0.016, 0.014, and 0.018 μmol/J, respectively. The total 3′ termini for the three kinds of photons were around 0.1 μmol/J. The production of 3′ termini with phosphate was found to be predominant with respect to that of 3′OH termini for the three kinds of radiation. The yield of piperidine-labile sites for 270 eV (～0.1 μmol/J) was slightly larger than that for 560 eV (～0.07 μmol/J) and also for γ-rays (～0.082 μmol/J).

Conclusion: Although the Auger process in DNA-constituent atoms was expected to induce Auger-specific lesions in the molecule the chemical endpoints would have been covered with a large number of lesions produced from secondary electrons in the surrounding bulk DNA molecules. The present results, however, suggest that a low-energy electron field produced by the USX photons in the bulk DNA is basically not at all specific to DNA damage being produced when compared with the high-energy electron field produced by ⁶⁰Co γ-rays.     

DNA double-strand break signalling: X-ray energy dependence of residual co-localised foci of γ-H2AX and 53BP1

Purpose:?The application of ionising radiation for medical purposes requires the investigation of induced and persistent DNA damages, especially for soft X-rays that are assumed to be more effective than higher energy photons. Therefore, we examined the energy dependent time and dose response of residual DNA damage foci for soft X-rays in comparison to 200 kV photons.

Materials and methods:?DNA damage present in cell line 184A1 within 48?h after irradiations with 10 kV, 25 kV and 200 kV photons was analysed by immunochemical detection of co-localised γ-H2AX (phosphorylated histone H2AX) and 53BP1 (tumour protein 53 binding protein) foci.

Results:?The dose dependencies of the colocated foci revealed significant energy dependent differences with increasing amounts of residual foci at decreasing X-ray energy independent on postirradiation time. Dose-dependent RBE (relative biological effectiveness) values ranging from 4 to 7 were determined for 10 kV relative to 200 kV X-rays based on the 24 hour dose responses. For 25 kV photons, ratios considerably higher than one were obtained only for doses above 2 Gy.

Conclusions:?The expected energy dependence with increasing DNA damage at decreasing photon energy was confirmed for the residual co-localised foci measured at different time points after irradiation.     

Repair of the double-strand breaks induced by low energy electrons: A modelling approach

Abstract

Purpose: We propose a biochemical mathematical model for the repair of double-strand breaks (DSB) induced by low energy electron tracks, and determine the repair time for simple and complex DSB.

Materials and methods: The track structure code KURBUC_liq was used to simulate electron tracks in liquid water. All possible sites of energy depositions and reactions of water radicals in the nucleobases of an atomistic model of DNA were located, and the types of damage were determined. The initial induced DSB were subjected to a mechanistic model of Nonhomologous end-joining (NHEJ) repair.

Results: Data are presented for the initial and residual yield of DSB induced by low energy electrons. The model of repair was verified by comparing the kinetics of the unrejoined DSB with the experimental data for the V79?-?4 hamster cells irradiated with 15 Gy of Carbon-K (C_K) 278 eV ultrasoft X-rays. The residual unrepaired DSB in the duplex DNA is presented in the time interval up to 3 hours. The calculated repair time for the simple and complex DSB are presented.

Conclusion: With the hypothesis that complex DSB take longer time to repair than the simple type DSB, the model provides an estimate of DSB repair kinetics of experimental data.     

Biological effects and inter-individual variability in peripheral blood lymphocytes of healthy donors exposed to 60 MeV proton radiotherapeutic beam

Purpose: The aim of our study was to investigate the amount of initial DNA damage and cellular repair capacity of human peripheral blood lymphocytes exposed to the therapeutic proton beam and compare it to X-rays.

Materials and methods: Lymphocytes from 10 healthy donors were irradiated in the Spread Out Bragg Peak of the 60?MeV proton beam or, as a reference, exposed to 250?kV X-rays. DNA damage level was assessed using the alkaline version of the comet assay method. For both sources of radiation, dose–DNA damage response (0–4?Gy) and DNA repair kinetics (0–120?min) were estimated. The observed DNA damage was then used to calculate the relative biological effectiveness (RBE) of the proton beam in comparison to that of X-rays.

Results: Dose–response relationships for the DNA damage level showed linear dependence for both proton beam and X-rays (R²?=?0.995 for protons and R²?=?0.993 for X-rays). Within the dose range of 1–4?Gy, protons were significantly more effective in inducing DNA damage than were X-rays (p?<?.05). The average RBE, calculated from the proton and X-ray doses required for the iso-effective, internally standardized tail DNA parameter (sT-DNA) was 1.28?±?0.57. Similar half-life time of residual damage and repair efficiency of induced DNA damage for both radiation types were observed. In the X-irradiated group, significant inter-individual differences were observed.

Conclusions: Proton therapy was more effective at high radiation doses. However, DNA damage repair mechanism after proton irradiation seems to differ from that following X-rays.     

Low‐energy Auger‐ and Photo‐electron Effects on the Degradation of Thymine by Ultrasoft X‐irradiation

Purpose: To investigate quantitatively and qualitatively the production of thymine radicals produced by monochromatic ultrasoft X (USX) ‐ or ⁶⁰Co γ‐rays using electron paramagnetic resonance (EPR) spectroscopy.

Materials and Methods: Thymine was chosen as the DNA component for the irradiation. The EPR experiments of irradiated thymine were performed using an X‐band EPR device installed in a soft X‐ray beamline (BL23SU) in SPring‐8. Sample pellets were irradiated with USX photons in a microwave cavity in a vacuum chamber. EPR measurements of thymine powder pellets irradiated with USX photons at energies of 407 and 538?eV were performed at 77?K or room temperature. For reference, ⁶⁰Co γ‐irradiation to a pellet was also performed at room temperature.

Results: The following three features were found: 1) comparison between the two energies shows that the EPR dose‐response curves are clearly distinguishable from each other: the curve for 407?eV saturated at a lower dose and spin number than that for 538?eV. 2) no evident qualitative difference between the radical species produced at the two energies was observed. 3) the EPR signal of the 538?eV USX‐irradiated sample measured after annealing for 12 days is similar to that obtained with ⁶⁰Co γ‐irradiation.

Conclusions: The difference observed in the EPR dose‐response relationship reflects the difference in the K‐absorption cross‐sections of carbon, nitrogen and oxygen in the thymine molecule which govern the photo‐/Auger electron energy spectrum.     

Chromosome aberrations and cell inactivation induced in mammalian cells by ultrasoft X‐rays: correlation with the core ionizations in DNA

Purpose: To study the frequency of chromosome aberrations induced by soft X‐rays. To see if the core ionization of DNA atoms is involved in this end‐point as much as it appears to be in cell killing.

Materials and methods: V79 hamster cells were irradiated by synchrotron radiation photons iso‐attenuated in the cell (250, 350, 810?eV). The morphological chromosome aberrations detected in the first post‐irradiation cell division (dicentrics and centric rings) were studied by Giemsa staining.

Results: The chromosome aberrations at 350?eV were, respectively, 2.6±0.8 and 2.1±0.8 times more numerous than at 250 and 810?eV for the same average dose absorbed by the nucleus. These relative effectivenesses are comparable with the ones already measured for cell killing. Moreover, they roughly vary such as the relative numbers of core ionizations (including in the phosphorus L‐shell) produced in DNA and its bound water (water being involved only at 810?eV through the oxygen atoms). In particular, they reproduce the characteristic twofold enhancement at 350?eV, above the carbon K threshold.

Conclusions: Correlations suggest that the core ionization process is likely a common and essential mechanism initiating both chromosome aberration and cell killing end‐points at these photon energies.     

On the biological efficiency of I-123 and I-125 decay on the molecular level

Purpose: To evaluate DNA damage of Auger emitters by numerical modelling at the molecular level.

Material and methods: Energy emission spectra of I-123 and I-125 were used as input data for a computer code that simulates the complete transport of electrons and photons from the physical stage up to the primary chemical stage at 10^?7 s. The simulation was performed in a complex environment of liquid water, DNA structures and scavengers. Electron and photon interactions with the DNA molecules were carefully managed. Simulations were carried out with both I-123 and I-125 bound to a pBR322 plasmid or free in its vicinity.

Results: The distributions of direct and indirect single strand breaks (SSB) and double strand breaks (DSB) as a function of the kinetic energy of the emitted Auger electrons show that damage is caused primarily by electrons with energies lower than 800 eV, while higher energy electrons are mainly involved in indirect effects. The yields per unit energy emitted strengthen this fact. When compared to experimental values, the calculated yields of linearization (LE) and relaxation (RE) events show good agreement as well as does the ratio LE/RE for each radionuclide and the ratio I-125/I-123 in the case of LE.     

A comparison of X-ray and proton beam low energy secondary electron track structures using the low energy models of Geant4

Abstract

Purpose: Lethal cell damage by ionising radiation is generally initiated by the formation of complex strand breaks, resulting from ionisation clusters in the DNA molecule. A better understanding of the effect of the distribution of ionisation clusters within the cell and particularly in regard to DNA segments could be beneficial to radiation therapy treatment planning. Low energy X-rays generate an abundance of low energy electrons similar to that associated with MeV protons. The study and comparison of the track structure of photon and proton beams could permit the substitution of photon microbeams for single cell ion irradiations at proton facilities used to predict the relative biological effectiveness (RBE) of charged particle fields. Materials and methods: The track structure of X-ray photons is compared with proton pencil beams in voxels of approximate DNA strand size (2 × 2 × 5 nm). The Very Low Energy extension models of the Monte Carlo simulation toolkit GEometry ANd Tracking 4 (Geant4) is used. Simulations were performed in a water phantom for an X-ray and proton beam of energies 100 keV and 20 MeV, respectively. Results: The track structure of the photon and proton beams are evaluated using the ionisation cluster size distribution as well as the radial dose deposition of the beam. Conclusions: A comparative analysis of the ionisation cluster distribution and radial dose deposition obtained is presented, which suggest that low energy X-rays could produce similar ionisation cluster distributions to MeV protons on the DNA scale of size at depths greater than ～10 μm and at distances greater than ～1 μm from the beam centre. Here the ionisation cluster size for each beam is less than ～100. The radial dose deposition is also approximately equal at large depths and at distances greater than 10 μm from the beam centre.     

Observation of cleavage in DNA and nucleotides following oxygen K-shell ionization by measuring X-ray absorption near edge structure

Abstract

Purpose: To investigate which type of bond is more likely to be cleaved in functional groups in DNA including the nucleobases by the K-shell ionization of oxygen of DNA, and to determine whether the production of propenal is specific to the oxygen resonant excitation. To investigate the degradation pattern which depends on the type of nucleobase in the DNA monomer.

Materials and methods: Calf thymus DNA film and four nucleotides (dAMP, TMP, dGMP, and dCMP) films were used as samples. Soft X-rays with energy of 560 eV were used to irradiate the samples, and the changes in the X-ray absorption near edge structure (XANES) spectra during the irradiation were measured. The XANES measurements were performed by using a 0.02 eV scanning photon energy step.

Results: The difference spectra for DNA and nucleotides were similar to those for pyridine deprotonation. The oxygen K-edge regions in the difference spectra were all similar apart from the spectrum obtained at the resonant excitation energy of oxygen in DNA. The spectral change did not depend on the type of nucleotide.

Conclusion: (1) Deprotonation of the nucleobase -NH is usually induced by core ionization of carbon, nitrogen, and oxygen; (2) propenal production is specific to the oxygen K-shell resonant excitation; and (3) the pattern of XANES spectral changes does not significantly depend on the type of nucleobase.     

DNA damage intensity in fibroblasts in a 3-dimensional collagen matrix correlates with the Bragg curve energy distribution of a high LET particle

Purpose:?The DNA double-strand break (DSB) damage response induced by high energy charged particles on lung fibroblast cells embedded in a 3-dimensional (3-D) collagen tissue equivalents was investigated using antibodies to the DNA damage response proteins gamma-histone 2AX (γ-H2AX) and phosphorylated DNA-PKcs (p-DNA-PKcs).

Materials and methods:?3-D tissue equivalents were irradiated in positions across the linear distribution of the Bragg curve profiles of 307.7 MeV/nucleon, 556.9 MeV/nucleon, or 967.0 MeV/nucleon ⁵⁶Fe ions at a dose of 0.30 Gy.

Results:?Patterns of discrete DNA damage streaks across nuclei or saturated nuclear damage were observed, with saturated nuclear damage being more predominant as samples were positioned closer to the physical Bragg peak. Quantification of the DNA damage signal intensities at each distance for each of the examined energies revealed a biological Bragg curve profile with a pattern of DNA damage intensity similar to the physical Bragg curve for the particular energy. Deconvolution microscopy of nuclei with streaked or saturated nuclear damage pattern revealed more details of the damage, with evidence of double-strand breaks radially distributed from the main particle track as well as multiple discrete tracks within saturated damage nuclei.

Conclusions:?These 3-D culture systems can be used as a biological substrate to better understand the interaction of heavy charged particles of different energies with tissue and could serve as a basis to model space-radiation-induced cancer initiation and progression.     

Chromosome aberrations and cell inactivation induced in mammalian cells by ultrasoft X-rays: correlation with the core ionizations in DNA

PURPOSE: To study the frequency of chromosome aberrations induced by soft X-rays. To see if the core ionization of DNA atoms is involved in this end-point as much as it appears to be in cell killing. MATERIALS AND METHODS: V79 hamster cells were irradiated by synchrotron radiation photons iso-attenuated in the cell (250, 350, 810eV). The morphological chromosome aberrations detected in the first post-irradiation cell division (dicentrics and centric rings) were studied by Giemsa staining. RESULTS: The chromosome aberrations at 350eV were, respectively, 2.6 +/- 0.8 and 2.1 +/- 0.8 times more numerous than at 250 and 810eV for the same average dose absorbed by the nucleus. These relative effectivenesses are comparable with the ones already measured for cell killing. Moreover, they roughly vary such as the relative numbers of core ionizations (including in the phosphorus L-shell) produced in DNA and its bound water (water being involved only at 810eV through the oxygen atoms). In particular, they reproduce the characteristic twofold enhancement at 350eV, above the carbon K threshold. CONCLUSIONS: Correlations suggest that the core ionization process is likely a common and essential mechanism initiating both chromosome aberration and cell killing end-points at these photon energies.     

Synchrotron-produced Ultrasoft X-rays: A Tool for Testing Biophysical Models of Radiation Action

Summary

Ultrasoft X-rays are useful for mechanistic studies of ionizing radiation damage in living cells due to the localized nature of their energy depositions. To date radiobiology experiments in this energy region have relied on characteristic X-rays (mainly Al_k and C_k) from X-ray tubes. However, limitations in the photon intensity and the available energies from X-ray tube sources prevent a definitive characterization of the relationship between photon energy and biological damage. Synchrotron radiation has the potential to avoid these limitations, since it produces X-rays with high intensity over a continuous spectrum. We have established a synchrotron-based system for radiation biology studies using the ES-0 exposure station of the Center for X-ray Lithography at the University of Wisconsin Synchrotron Radiation Center storage ring, Aladdin. A characterization of the system including spectral and intensity properties of the photon beam is presented. The first mammalian cell survival curve for synchrotron-produced ultrasoft X-rays was generated and is presented. Cell survival curves of C3H/10T1/2 cells using synchrotron radiation of 1·48 keV agree with previous data using Al_k X-rays (1·49 keV). An RBE of 1·47 ± 0·30 at the 10% survival level was measured with reference to 250 kVp X-rays.     

“In situ” observation of guanine radicals induced by ultrasoft X‐ray irradiation around the K‐edge regions of nitrogen and oxygen

Purpose: In order to understand the molecular mechanism of nucleobase damage caused by ultrasoft X‐ray irradiation, guanine radicals have been studied using an X‐band EPR (electron paramagnetic resonance) spectrometer installed in a synchrotron soft X‐ray beamline.

Materials and Methods: Guanine pellets were irradiated under vacuum with ultrasoft X‐rays obtained from a soft X‐ray beamline (BL23SU) in SPring‐8. The energy regions around the nitrogen (0.4?keV) and oxygen (0.5?keV) K‐edges were chosen for the irradiation. The ultrasoft X‐ray irradiation and EPR measurements were carried out simultaneously at low temperature, 20?K and 77?K.

Results: The EPR spectrum observed during irradiation was clearly distinguishable from that of the stable radical, which still exists after exposure to ultrasoft X‐rays at 77?K. The spectrum of the short‐lived radicals consisted of two components, which exhibited different EPR microwave power saturation. The EPR signal intensities increased linearly with increasing dose rate (photon flux density). These signals immediately disappeared when the beam was turned off, even when irradiated at lower temperature (20?K). At the energy of the oxygen K‐resonance excitation (539?eV) the signal intensity was clearly increased to more than five times that obtained on the lower energy side (526?eV). On the other hand, the enhancement was insignificant above and below the nitrogen K‐edge (401?eV). The singlet EPR signal of the stable radical was similar to that reported previously in the literature for γ‐irradiated guanine.

Conclusions: The short‐lived radical species observed were mainly induced as a result of the final state of the resonant Auger process on oxygen atoms existing solely in the carbonyl group in guanine. Auger events at the other atoms in guanine (namely, carbon and nitrogen) do not induce this radical process to any great extent, even though the abundance of these atoms (i.e. the sum of their photoabsorption cross sections) is dominant in the guanine molecule.     

DNA strand breaks by direct energy deposition by Auger and photo‐electrons ejected from DNA constituent atoms following K‐shell photoabsorption

Purpose: To study DNA strand breaks induced by direct energy deposition by photo‐ and Auger electrons ejected following K‐shell photoabsorption of DNA constituent atoms (carbon, nitrogen, oxygen and phosphorus).

Method: Using a Monte Carlo code which has been developed to simulate the photoelectric effect on plasmid DNA pBR322, the energy deposition pattern of secondary electrons ejected after photoabsorption in DNA constituent atoms (not including the hydration shell) was calculated. Experimentally obtained X‐ray absorption near edge structures were considered of the cross‐sections at the K‐shell resonant absorption (1s→σ*) of carbon, nitrogen and oxygen, and the K‐shell resonant absorption (1s→t2*) of phosphorus. Direct energy deposition by secondary electrons was scored using two different DNA models with and without the hydrated shell. The yields of SSB, DSB per photoabsorption events as well as break complexity were estimated for monochromatic X‐rays around the K‐edges of DNA constituent atoms (200–3000?eV).

Results: Higher SSB and DSB yields were obtained below the carbon, nitrogen and oxygen K‐edges compared to at or above the resonance, and at the K‐shell resonant absorption of phosphorus compared to below the resonance. The number of electrons with sufficient energy to induce strand breaks was found to change depending on the photon energy. Electrons with 120 and 250?eV are shown to be rather more effective in SSB and DSB induction than electrons with higher energy. Inclusion of hydrated water in the DNA volume did not affect the photon energy dependence of the strand break yields.

Conclusion: The small difference of photon energies around K‐absorption edges of the carbon, nitrogen, oxygen and phosphorus is indicated to induce variation in strand break yields by direct effect. Higher SSB and DSB induction efficiencies could be due to a higher yield of more than two electrons with around 120?eV to 250?eV per photoabsorption event.     

Differential response to acute low dose radiation in primary and immortalized endothelial cells

Abstract

Purpose: The low dose radiation response of primary human umbilical vein endothelial cells (HUVEC) and its immortalized derivative, the EA.hy926 cell line, was evaluated and compared.

Material and methods: DNA damage and repair, cell cycle progression, apoptosis and cellular morphology in HUVEC and EA.hy926 were evaluated after exposure to low (0.05–0.5 Gy) and high doses (2 and 5 Gy) of acute X-rays.

Results: Subtle, but significant increases in DNA double-strand breaks (DSB) were observed in HUVEC and EA.hy926 30 min after low dose irradiation (0.05 Gy). Compared to high dose irradiation (2 Gy), relatively more DSB/Gy were formed after low dose irradiation. Also, we observed a dose-dependent increase in apoptotic cells, down to 0.5 Gy in HUVEC and 0.1 Gy in EA.hy926 cells. Furthermore, radiation induced significantly more apoptosis in EA.hy926 compared to HUVEC.

Conclusions: We demonstrated for the first time that acute low doses of X-rays induce DNA damage and apoptosis in endothelial cells. Our results point to a non-linear dose-response relationship for DSB formation in endothelial cells. Furthermore, the observed difference in radiation-induced apoptosis points to a higher radiosensitivity of EA.hy926 compared to HUVEC, which should be taken into account when using these cells as models for studying the endothelium radiation response.     

Measurement of inelastic cross sections for low-energy electron scattering from DNA bases

Abstract

Purpose: To determine experimentally the absolute cross sections (CS) to deposit various amount of energies into DNA bases by low-energy electron (LEE) impact.

Materials and methods: Electron energy loss (EEL) spectra of DNA bases were recorded for different LEE impact energies on the molecules deposited at very low coverage on an inert argon (Ar) substrate. Following their normalisation to the effective incident electron current and molecular surface number density, the EEL spectra were then fitted with multiple Gaussian functions in order to delimit the various excitation energy regions. The CS to excite a molecule into its various excitation modes were finally obtained from computing the area under the corresponding Gaussians.

Results: The EEL spectra and absolute CS for the electronic excitations of pyrimidine and the DNA bases thymine, adenine, and cytosine by electron impacts below 18 eV were reported for the molecules deposited at about monolayer coverage on a solid Ar substrate.

Conclusions: The CS for electronic excitations of DNA bases by LEE impact were found to lie within the 10²¹⁶ to 10²¹⁸ cm² range. The large value of the total ionisation CS indicated that ionisation of DNA bases by LEE is an important dissipative process via which ionising radiation degrades and is absorbed in DNA.     

Effects of K-shell X-ray Absorption of Intracellular Phosphorus on Yeast Cells

Summary

The effects of K-shell absorption of phosphorus atoms on yeast cells were investigated using synchroton X-rays that were tuned to the resonance absorption peak (2153 eV). Three types of cellular effect (cell inactivation, induction of gene conversion at the trp-5 locus, and cell membrane impairment (changes in the permeability)) were measured. It was demonstrated that the enhancement factor was 1·4 at the resonance peak regarding both lethality and the induction of gene conversion in reference of off-peak irradiation (2146 and 2160 eV). No difference was found between the two off-peak irradiation energies. No cell membrane impairment was detected, irrespective of the X-ray photon energies employed within the fluence range tested. These results strongly suggest that K-shell X-ray absorption in the resonance mode by cellular phosphorus atoms causes significantly more cellular effects than the off-resonance mode of absorption, probably via some specific changes induced in the phosphates of the DNA strand. Calculations using the number of phosphorus atoms in a defined size of the trp locus (2127 base pairs) on the DNA and the absorption cross-section of the resonance mode of phosphorus showed that gene conversion is inducible at a rate of 0·13 per X-ray photon absorption per locus. These results are discussed regarding the modes of K-shell photoabsorption.     

Relation between DNA double-strand breaks and energy spectra of secondary electrons produced by different X-ray energies

Purpose: In a radiological examination, low-energy X-radiation is used (<100?keV). For other radiological procedures, the energy used is several MeV. ICRP in publication 103 has currently considered that photons irrespective of their energy have the same radiation weighting factor. Nevertheless, there are topological differences at the nanoscale of X-ray energy deposition as a function of its energy spectrum, meaning that the different interactions with living matter could vary in biological efficacy.

Materials and methods: To study these differences, we characterized our irradiation conditions in terms of initial photon energies, but especially in terms of energy spectra of secondary electrons at the cell nucleus level, using Monte Carlo simulations. We evaluated signaling of DNA damage by monitoring a large number of γH2A.X foci after exposure of G0/G1-phase synchronized human primary endothelial cells from 0.25 to 5?Gy at 40?kV, 220?kV and 4?MV X-rays. Number and spatial distribution of γH2A.X foci were explored. In parallel, we investigated cell behavior through cell death and ability of a mother cell to produce two daughter cells. We also studied the missegregation rate after cell division.

Results: We report a higher number of DNA double-strand breaks signaled by γH2A.X for 40?kVp and/or 220?kVp compared to 4?MVp for the highest tested doses of 2 and 5?Gy. We observed no difference between the biological endpoint studies with 40?kVp and 220?kVp X-ray spectra. This lack of difference could be explained by the relative similarity of the calculated energy spectra of secondary electrons at the cell monolayer.

Conclusion: The energy spectrum of secondary electrons seems to be more closely related to the level of DNA damage measured by γH2A.X than the initial spectrum of photon energy or voltage settings. Our results indicate that as the energy spectrum of secondary electrons increases, the DNA damage signaled by γH2A.X decreases and this effect is observable beyond 220?kVp.