Electron spectra and the RBE of X rays

For an assessment of the possible difference in effectiveness between mammography X rays and conventional X rays, the energy and LET spectra of the released electrons are examined. At photon energies below 20 keV and above 100 keV, the energy of the electrons increases with increasing photon energy, which implies that higher-energy photons produce less densely ionizing radiation and are therefore somewhat less effective per unit dose. However, in the intermediate energy range from 20 keV to 100 keV-the range that is relevant to medical diagnostics-the change from the photoelectric effect to the Compton effect causes a transient decrease of electron energies. The ionization density is therefore similar for 200 kVp X rays and 30 kVp mammography X rays, and the distributions of dose in LET suggest an RBE of 30 kVp mammography X rays compared to 200 kVp X rays of up to 1.3. This is in line with an earlier assessment by Brenner and Amols in terms of microdosimetric data, but it is strongly at variance with a recent claim that X rays for mammography are about four times more effective at small doses than conventional X rays and that they cause a correspondingly greater risk for breast cancer. Since LET need not be the only relevant factor, general response functions are examined here that specify-at low dose-the effect per electron of initial energy E and account, for example, for a particular role of the electron range. It is shown that, with any response per electron track that is a nondecreasing function of its starting energy, the low-dose RBE of the mammography X rays relative to the 200 kVp X rays must be substantially less than 2. The Auger electron that accompanies most photoelectrons, but only a minority of the Compton electrons, may increase the effectiveness of the mammography X rays somewhat, but it cannot explain the reported high values of the RBE.     

Enhancement of X-ray-induced breaks in DNA bound to molecules containing platinum: a possible application to hadrontherapy.

Complexes made of DNA and chloroterpyridine platinum (PtTC) bound to plasmid DNA were placed in aqueous solution and irradiated with monochromatic X rays tuned to the resonant photoabsorption energy of the L(III) shell of the platinum atom. The number of single- and double-strand breaks (SSBs and DSBs) induced by irradiation on a supercoiled DNA plasmid was measured by the production of the circular-nicked and linear forms. To distinguish the contribution of the direct effects of ionization from the indirect effects due to a free radical attack, experiments were also performed in the presence of a hydroxyl free radical scavenger, dimethyl sulfoxide (DMSO). An enhancement of the number of SSBs and DSBs was observed when the plasmids contained the platinum intercalating molecules. A quantitative analysis was made to evaluate the respective contributions of the direct effects (Auger effect) and the indirect effects (free radical attack) to the number of DNA strand breaks. Even when off-resonant X rays were used, the strand break efficiency remained higher than expected based upon the absorption cross section, suggesting that the platinum bound to DNA might be increasing the yield of strand breaks. A mechanism is suggested that involves photoelectrons generated from the ionization of water which efficiently ionize platinum atoms. If this mechanism is correct, then heavy atoms, with a large cross section for ionization by electrons that are bound to the DNA, should behave as a radiosensitizer. This observation may provide insight into understanding the effects of new radiotherapy protocols, related chemotherapeutic agents such as cisplatin, and conventional radiotherapy for the treatment of tumors. A possible way to deliver the dose selectively in a well-defined volume, which uses the properties of the linear energy transfer of atomic ions interacting with matter, is suggested.     

Dependence of the yield of DNA double-strand breaks in Chinese hamster V79-4 cells on the photon energy of ultrasoft X rays

Induction of DNA DSBs by low-LET radiations reflects clustered damage produced predominantly by low-energy, secondary electron "track ends". Cell inactivation and induction of DSBs and their rejoining, assayed using pulsed-field gel electrophoresis, were determined in Chinese hamster V79-4 cells irradiated as a monolayer with characteristic carbon K-shell (CK) (0.28 keV), aluminum K-shell (AlK) (1.49 keV), and titanium K-shell (TiK) (4.55 keV) ultrasoft X rays under aerobic and anaerobic conditions. Relative to (60)Co gamma rays, the relative biological effectiveness (RBE) for cell inactivation at 10% survival and for induction of DSBs increases as the photon energy of the ultrasoft X rays decreases. The RBE values for cell inactivation and for induction of DSBs by CK ultrasoft X rays are 2.8 +/- 0.3 and 2.7 +/- 0.3, respectively, and by TiK ultrasoft X rays are 1.5 +/- 0.1 and 1.4 +/- 0.1, respectively. Oxygen enhancement ratios (OERs) of approximately 2 for cell inactivation and induction of DSBs by ultrasoft X rays are independent of the photon energy. The time scale for rejoining of DNA DSBs is similar for both ultrasoft X rays and 60Co gamma rays. From the size distribution of small DNA fragments down to 0.48 kbp, we concluded that DSBs are induced randomly by CK and AlK ultrasoft X rays. Therefore, ultrasoft X rays are more efficient per unit dose than gamma radiation at inducing DNA DSBs, the yield of which increases with decreasing photon energy.     

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.     

Strand break induction by photoabsorption in DNA-bound molecules

Dried samples of a DNA-chloroterpyridine platinum complex were irradiated with monochromatic X rays tuned to the photoabsorption resonance of the L(III) inner shell of the platinum atom. The number of single- and double-strand breaks (SSBs and DSBs) triggered by the Auger effect in supercoiled DNA plasmids was measured by the production of circular nicked and linear forms. To probe the specific contribution of the L(III) inner-shell excitation of the platinum atom, photon wavelengths were tuned on the resonance energy (on peak) and below (off peak). The quantum yields of the resonance radiation were typically found to be 11 for the SSBs and 1 for the DSBs. The DSB-to-SSB ratio increased by 20% when switching from off-resonance to on-resonance 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.     

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).     

Microdosimetric analysis of various mammography spectra: lineal energy distributions and ionization cluster analysis

In view of recent recommendations on the frequency and the starting age of mammography screening in healthy women, it is desirable to quantify the enhanced relative biological effectiveness (RBE) of mammography X rays compared to hard X rays. While there is little doubt that the former are more potent in inducing biological damage than the latter, the magnitude of the effect is still hotly debated in the literature. We used Monte Carlo simulations and track structure analysis in micrometer and nanometer volumes to investigate differences in distributions of lineal energy and ionization clusters for a range of mammography X-ray qualities. Dose-averaged lineal energies, (yD), in breast tissue for various mammography qualities were found to result in quality factors about 40% higher than unity. Among the various mammography qualities studied, the popular molybdenum/molybdenum target/filter combination was found to have the highest (yD) in 1-microm spheres (about 5.0 keV/microm near the entrance surface of breast tissue). In 10-nm radius spheres, the mean ionization cluster order was found to be about 35% higher in mammography X rays compared to 300 keV electrons (roughly representing 60Co or 192Ir photon radiation). In even smaller spheres (2 nm radius), no significant differences were observed for the mean ionization cluster order between mammography X rays and 300 keV electrons. We conclude that the potential of mammography X rays to induce biological damage is probably not much higher than a factor of two compared to hard X rays.     

DNA double-strand break misrejoining after exposure of primary human fibroblasts to CK characteristic X rays, 29 kVp X rays and 60Co gamma rays

The efficiency of ionizing photon radiation for inducing mutations, chromosome aberrations, neoplastic cell transformation, and cell killing depends on the photon energy. We investigated the induction and rejoining of DNA double-strand breaks (DSBs) as possible contributors for the varying efficiencies of different photon energies. A specialized pulsed-field gel electrophoresis assay based on Southern hybridization of single Mbp genomic restriction fragments was employed to assess DSB induction and rejoining by quantifying the restriction fragment band. Unrejoined and misrejoined DSBs were determined in dose fractionation protocols using doses per fraction of 2.2 and 4.4 Gy for CK characteristic X rays, 4 and 8 Gy for 29 kVp X rays, and 5, 10 and 20 Gy for 60Co gamma rays. DSB induction by CK characteristic X rays was about twofold higher than for 60Co gamma rays, whereas 29 kVp X rays showed only marginally elevated levels of induced DSBs compared with 60Co gamma rays (a factor of 1.15). Compared with these modest variations in DSB induction, the variations in the levels of unrejoined and misrejoined DSBs were more significant. Our results suggest that differences in the fidelity of DSB rejoining together with the different efficiencies for induction of DSBs can explain the varying biological effectiveness of different photon energies.     

Cross sections for low-energy (10-50 eV) electron damage to DNA

We report direct measurements of the formation of single-, double- and multiple strand breaks in pure plasmid DNA as a function of exposure to 10-50 eV electrons. The effective cross sections to produce these different types of DNA strand breaks were determined and were found to range from approximately 10(-17) to 3 x 10(-15) cm(2). The total effective cross section and the effective range for destruction of supercoiled DNA extend from 3.4 to 4.4 x 10(-15) cm(2) and 12 to 14 nm, respectively, over the range 10-50 eV. The variation of the effective cross sections with electron energy is discussed in terms of the electron's inelastic mean free path, penetration depth, and dissociation mechanisms, including resonant electron capture; the latter is found to dominate the effective cross sections for single- and double-strand breaks at 10 eV. The most striking observations are that (1) supercoiled DNA is approximately one order of magnitude more sensitive to the formation of double-strand breaks by low-energy electrons than is relaxed circular DNA, and (2) the dependence of the effective cross sections on the incident electron energy is unrelated to the corresponding ionization cross sections. This finding suggests that the traditional notion that radiobiological damage is related to the number of ionization events would not apply at very low energies.     

Cross sections for low-energy (1-100 eV) electron elastic and inelastic scattering in amorphous ice

We report the integral cross sections per scatterer (i.e. elastic collision, phonon excitations, vibrational excitations, electronic excitations and ionization) for 1-100 eV electron scattering in an amorphous film of ice condensed at a temperature of 14 K. The integral cross sections are determined relative to the total from a two-stream multiple-scattering analysis of the electron energy distribution backscattered from the film. Their energy dependence is obtained from both the analysis of the elastic electron reflectivity as a function of the film thickness and the vibrational electron energy-loss spectra measured for several incident energies and large film thickness. The magnitude and various features found in the energy dependence of the cross sections are discussed, whenever possible, by comparison with data and with scattering mechanisms available in the gas phase. Microcospic effects, which are implicitly included in cross sections determined in this way, are discussed in terms of interference and coherent multiple-scattering contributions among the scattering sites as well as interactions of the scattering sites with their neighbors in the condensed phase.     

Impact of photon cross section uncertainties on Monte Carlo-determined depth-dose distributions

This work studies the impact of systematic uncertainties associated to interaction cross sections on depth dose curves determined by Monte Carlo simulations. The corresponding sensitivity factors are quantified by changing cross sections by a given amount and determining the variation in the dose. The influence of total and partial photon cross sections is addressed. Partial cross sections for Compton and Rayleigh scattering, photo-electric effect, and pair production have been accounted for. The PENELOPE code was used in all simulations. It was found that photon cross section sensitivity factors depend on depth. In addition, they are positive and negative for depths below and above an equilibrium depth, respectively. At this depth, sensitivity factors are null. The equilibrium depths found in this work agree very well with the mean free path of the corresponding incident photon energy. Using the sensitivity factors reported here, it is possible to estimate the impact of photon cross section uncertainties on the uncertainty of Monte Carlo-determined depth dose curves.     

Heavy ion-induced DNA double-strand breaks in yeast

Induction of DSBs in the diploid yeast, Saccharomyces cerevisiae, was measured by pulsed-field gel electrophoresis (PFGE) after the cells had been exposed on membrane filters to a variety of energetic heavy ions with values of linear energy transfer (LET) ranging from about 2 to 11,500 keV/microm, (241)Am alpha particles, and 80 keV X rays. After irradiation, the cells were lysed, and the chromosomes were separated by PFGE. The gels were stained with ethidium bromide, placed on a UV transilluminator, and analyzed using a computer-coupled camera. The fluorescence intensities of the larger bands were found to decrease exponentially with dose or particle fluence. The slope of this line corresponds to the cross section for at least one double-strand break (DSB), but closely spaced multiple breaks cannot be discriminated. Based on the known size of the native DNA molecules, breakage cross sections per base pair were calculated. They increased with LET until they reached a transient plateau value of about 6 x 10(-7) microm(2) at about 300-2000 keV/microm; they then rose for the higher LETs, probably reflecting the influence of delta electrons. The relative biological effectiveness for DNA breakage displays a maximum of about 2.5 around 100-200 keV/microm and falls below unity for LET values above 10(3) keV/microm. For these yeast cells, comparison of the derived breakage cross sections with the corresponding cross section for inactivation derived from the terminal slope of the survival curves shows a strong linear relationship between these cross sections, extending over several orders of magnitude.     

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.     

The inactivation of bacteriophage by ionizing radiation

The loss of energy by a fast, charged particle in dry material of atomic constitution approximating a protein is described and the important features tabulated. The inactivation of T-1, T-3, and T-7 bacteriophages in the dry state by deuterons is described and follows a simple logarithmic survival curve. The cross section per deuteron is much less than the electron microscope cross section in all three cases and indicates some kind of structure to the viruses. The variation of cross section with ionization density has been measured for T-1. It is linear and the kind of internal-structure which could account for this is discussed.     

Attenuation in very narrow photon beams

When one measures the half-value layer (HVL) or the attenuation coefficient (mu) in a high-energy photon beam, it is necessary to use a narrow beam to eliminate the scattered photons produced in the attenuator. However, lateral electron equilibrium will be compromised if the beam is too small. If the HVL and mu are based on measurements of absorbed dose, the results will then depend on field size for a polyenergetic photon spectrum. The measured values also become sensitive to detector properties. This has been examined by experiments and Monte Carlo calculations. The field size should be sufficient for lateral electron equilibrium to prevent ambiguities in the resulting HVL or mu, which are of the order of 10% for 6-MV X rays.     

Induction of incomplete and complex chromosome aberrations by 30 kVp X rays

X rays of 26-30 kVp are routinely used for mammography screening. For radioprotection purposes, a quality factor (Q) of 1 is assumed for all photon energies, but it is thought that the relative biological effectiveness (RBE) increases as the photon energy decreases. The analysis of radiation-induced chromosome aberrations is one of the most widely used methods to study the interaction between radiation and DNA. Here we present a FISH study on metaphases from peripheral blood samples irradiated with three different X-ray energies (30, 80 and 120 kVp). The study comprises two FISH approaches: one using pantelomeric and pancentromeric probes to evaluate the induction of incomplete chromosome aberrations and the other using mFISH to evaluate the induction of complex chromosome aberrations. The results indicate that exposure to 30 kVp X rays resulted in a modest increase in the induction of incomplete elements and complex aberrations compared to 80 and 120 kVp X rays.     

Differential and integral W-values for ionization in gaseous water under electron and proton irradiation: consistency of inelastic collision cross sections.

Differential and integral W-values for ionization in gaseous water for electron and proton irradiation have been analyzed from the theoretical point of view for consistency between ionization and total inelastic collision cross sections. For low-energy electrons, which are ubiquitous for all primary radiations, the experimental or compiled cross sections from different sources are sometimes not consistent with one another. A practical, self-consistent procedure is outlined in such cases. The high-energy asymptotic W-values for differential and integral ionization are calculated to be 33.7 and 34.7 eV, respectively, for electron irradiation and 34.6 and 32.5 eV, respectively, for proton irradiation. The computed variations of the W-values with energy are generally in good agreement with experiment. Integral primary W-values due only to the interactions between the incident particle and the water vapor are calculated to be 43.5 and 45.0 eV for electrons and protons, respectively, in the high-energy asymptotic limit.     

Effective cross sections for production of single-strand breaks in plasmid DNA by 0.1 to 4.7 eV electrons

We determined effective cross sections for production of single-strand breaks (SSBs) in plasmid DNA [pGEM 3Zf(-)] by electrons of 10 eV and energies between 0.1 and 4.7 eV. After purification and lyophilization on a chemically clean tantalum foil, dry plasmid DNA samples were transferred into a high-vacuum chamber and bombarded by a monoenergetic electron beam. The amount of the circular relaxed DNA in the samples was separated from undamaged molecules and quantified using agarose gel electrophoresis. The effective cross sections were derived from the slope of the yield as a function of exposure and had values in the range of 10(-15)- 10(-14) cm2, giving an effective cross section of the order of 10(-18) cm2 per nucleotide. Their strong variation with incident electron energy and the resonant enhancement at 1 eV suggest that considerable damage is inflicted by very low-energy electrons to DNA, and it indicates the important role of pi* shape resonances in the bond-breaking process. Furthermore, the fact that the energy threshold for SSB production is practically zero implies that the sensitivity of DNA to electron impact is universal and is not limited to any particular energy range.     

Charged-particle mutagenesis. 1. Cytotoxic and mutagenic effects of high-LET charged iron particles on human skin fibroblasts.

Cytotoxic and mutagenic effects of high-LET charged iron (56Fe) particles were measured quantitatively using primary cultures of human skin fibroblasts. Argon and lanthanum particles and gamma rays were used in comparative studies. The span of LETs selected was from 150 keV/microns (330 MeV/u) to 920 keV/microns (600 MeV/u). Mutations were scored at the hypoxanthine guanine phosphoribosyl transferase (HPRT) locus using 6-thio-guanine (6-TG) for selection. Exposure to these high-LET charged particles resulted in exponential survival curves. Mutation induction, however, was fitted by the linear model. The relative biological effectiveness (RBE) for cell killing ranged from 3.7 to 1.3, while that for mutation induction ranged from 5.7 to 0.5. Both the RBE for cell killing and the RBE for mutagenesis decreased with increasing LET over the range of 1.50 to 920 keV/microns. The inactivation cross section (sigma i) and the action cross section for mutation induction (sigma m) ranged from 32.9 to 92.0 microns2 and 1.45 to 5.56 X 10(-3) microns2; the maximum values were obtained by 56Fe with an LET of 200 keV/microns. The mutagenicity (sigma m/sigma i) ranged from 2.05 to 7.99 X 10(-5) with an inverse relationship to LET.