Cell uptake and radiotoxicity studies of an nuclear localization signal peptide-intercalator conjugate labeled with [99mTc(CO)3]+

A trifunctional bioconjugate consisting of the SV40 nuclear localization signal (NLS) peptide, an aliphatic triamine ligand, and the DNA intercalating pyrene has been synthesized and quantitatively labeled with [(99m)Tc(OH(2))(3)(CO)(3)](+). The radiotoxicity of the resulting nucleus-targeting radiopharmaceutical on B16F1 mouse melanoma cells has been investigated to evaluate the activity of Auger and Coster-Kronig electrons on the viability of cells. We found a dose-dependent significant radiotoxicity of the nucleus-targeting radiopharmaceutical clearly related to the low energy decay of (99m)Tc. These principal results imply a possible therapeutic strategy based on the use of the low-energy Auger electron-emitting (99m)Tc radionuclide attached to nucleus-targeting molecules and comprising an intercalator. Highly efficient DNA targeting vectors could complement the usual role of (99m)Tc in diagnostic applications. The Auger electrons emitted by the (99m)Tc nuclide induce DNA damage leading ultimately, through a mitotic catastrophe pathway, to necrotic cell death. Non-DNA-targeting (99m)Tc complexes display much lower radiotoxicity.     

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

We investigated the DNA damage from Auger electrons emitted from incorporated stable iodine (¹²⁷I), following photoelectric absorption of external x-rays. The effectiveness of the Auger electrons in producing DNA double-strand breaks (DSB) was determined theoretically, using Monte Carlo simulations of the radiation physics and chemistry, and was shown to be in reasonable agreement with DNA damage measured using the comet assay. The DSB yields were measured in CHO cells for ⁶⁰Co (as a non-Auger-promoting radiation) and for tungsten-filtered 100 kVp x-rays capable of producing Auger electron emission. The theoretical study showed that on average, 2.5 Auger electrons were emitted for N-shell orbital vacancies and up to 10 Auger electrons were emitted from L₁-shell vacancies. These Auger bursts produced approximately 0.03 DSB per N-shell vacancy and 0.3 DSB per K-shell or L-shell vacancy. The calculated yield of DSB from Auger cascades per unit dose (1 Gy) in water was approximately 1.7 for tungsten-filtered 100 kVp x-rays, assuming 20% IUdR substitution of thymidine. The comet assay yielded an experimental value of 3.6±1.6 per 1 Gy for the same conditions. The Monte Carlo simulations also demonstrated a high complexity of DSB produced by Auger cascades with virtually all DSB from inner shell orbitals (i.e. K, L shells) accompanied by compounded strand breakage and base damage, indicating a difficult lesion to repair. This finding agrees well with comet assay results of DNA repair, where an increase in the DSB yield in IUdR-sensitized cells was shown to persist after a time of 24 h. We conclude that Auger cascades in iodine produce a modest increase in the number of initial strand breaks of the order of 10% but the complex nature of these DSB makes them very difficult to repair or potentially prone to misrepair. The accentuated DNA damage may have major consequences for cell survival and may be exploitable in kilovoltage photon activation therapy (PAT) of tumors sensitized with iodine. Received: 23 October 2000 / Accepted: 26 March 2001     

The Auger electron dosimetry of indium-111 in mammalian cells in vitro

Most of the radionuclides used in the formulation of radiopharmaceuticals emit Auger electrons when they undergo radioactive decay. The release of these low-energy electrons at extracellular sites produces little direct damage to intracellular structures. However, many radiopharmaceuticals, or their metabolites, can be transported into the cell where the Auger electrons have the potential to damage nearby intracellular macromolecules, including DNA. In this preliminary study, chromosome damage, expressed as 60Co equivalent doses, and the effects on cell division following treatment with intracellular and extracellular 111In were measured in Chinese hamster V79 cells. The chromosome aberration yield in cells irradiated by intracellular 111In indicated that damage was induced at a rate of 7.2 X 10(-4) Gy/decay for levels of activity up to 0.075 Bq/cell and 4.5 X 10(4) and 2.9 X 10(4) Gy/decay for intermediate (0.204 Bq/cell) and high (0.389 Bq/cell) levels, respectively. Extracellular 111In-chloride produced damage at a rate of about 6.1 X 10(-12) Gy/decay. As little as 4.4 mBq/cell (about 4.4 X 10(3) Bq/ml of culture) of intracellular 111In was able to affect cell division, whereas extracellular 111In at 1.150 MBq/ml of culture had little effect. These data indicate that the Medical Internal Radiation Dose and International Committee on Radiation Units methods for organ dosimetry may underestimate the potential of intracellular Auger electron emitters to produce radiation damage.     

Efficient mutation induction by 125I and 131I decays in DNA of human cells

To examine the role of radiation energy deposition in DNA on cellular effects, we investigated the ability of 125IdUrd and 131IdUrd to kill cells and induce mutations at the hprt locus. We employed human lymphoblastoid cells proficient (TK6) or deficient (SE30) in the ability to incorporate a thymidine analog into DNA by way of the thymidine kinase (TK) scavenger pathway. Iodine-125 releases a shower of low-energy Auger electrons upon decay which deposit most of their energy within 20 nm of the decay site, whereas 131I is a high-energy beta/gamma emitter that is generally considered to emit sparsely ionizing radiation. Although 125IdUrd incorporated into cellular DNA was very effective at producing toxic and mutagenic effects in TK6 cells, virtually no effect was seen in TK-deficient cells incubated with similar levels of 125IdUrd in the extracellular medium. In response to 131IdUrd treatment, 0.45 X 10(-6) mutants were induced per centigray dose deposited within the nucleus in TK-proficient cells, whereas few mutations were induced in TK-deficient cells at doses up to 38 cGy from 131I decays occurring in the medium. The differences in biological response between TK6 and SE30 cells cannot be explained by differential radiosensitivity or IdUrd sensitization of the cell lines involved. We conclude that both 125I and 131I decays occurring while incorporated into DNA are more effective at inducing cell killing and mutations in human cells than either nonincorporated decays or low-LET radiations. These results suggest that localized energy deposition is an important factor in producing biologically important damage by both of these isotopes, and that residual lesions following the decay of DNA-incorporated radioisotopes may contribute to the toxic and mutagenic effects observed in TK-proficient cells. Furthermore, they emphasize that certain beta/gamma-emitting isotopes such as 131I may be particularly hazardous when incorporated into DNA.     

Evaluation of the cytotoxic and mutagenic potentiality of technetium-99m in Escherichi coli.

Since technetium-99m (99mTc) was introduced in medical research it has become one of the most employed radionuclides in nuclear medicine. 99mTc is ideal for routine use on the labeling of different radiopharmaceuticals due to its favorable characteristics. However, some biological effects have been described. These effects may be related to internal conversion electron and/or Auger electron emissions from 99mTc decay that present high linear energy transfer and can generate reactive oxygen species (ROS) in the medium. We evaluated in Escherichia coli K12S and Salmonella typhimurium TA102, both proficient in DNA repair, contribution of those decay emissions on the cytotoxicity induced by 99mTc, both either by generating lesions on DNA or by inducing alterations at membrane. We also studied the genotoxic and/or mutagenic potentiality of 99mTc, in Salmonella typhimurium, using the Ames test. The results showed that: i/ 99mTc is cytotoxic to the Escherichia coli K12S strains; ii/ this effect is related to the electrons (Auger and internal conversion) emissions, and iii/ the 99mTc is not mutagenic and/or genotoxic, when measured by Ames test.     

A Monte Carlo simulation of Auger cascades

The energy imparted to biological tissue after the decay of incorporated Auger emitters stems from two sources: (a) energy deposition by the Auger and Coster-Kronig electrons and (b) the charge potential which remains on the multiple ionized atom after the end of the cascade. For the numerical assessment of both the kinetic energy of the released electrons and the charge potential, a new and--for purposes of microdosimetry--precise method is presented. Based on relativistic Dirac-Fock calculations and a rigorous bookkeeping, this method provides a perfect energy balance of the considered atomic system when applied to Monte Carlo simulations of Auger cascades. By comparing the results for charge distribution for krypton and iodine with experimental data and the electron spectrum of 125I with theoretical data, it can be shown that the approach followed in this work is reasonable and appropriate for the determination of the energy deposited by incorporated Auger emitters in small volumes of condensed matter. The total energy deposited by 125I in a volume of 20-nm diameter is 2.03 keV which is made up by multiple ionization (1.07 keV) and energy deposition by the emitted Auger electrons (0.96 keV).     

Biological damage from the Auger effect,possible benefits

Summary Decay of radioactive isotopes by K-capture leads to the Auger effect and results in the loss of several orbital electrons and the emission of X-rays. Whereas radiation effects are produced from the emitted electrons, the consequences of the Auger effect are strictly localized to the site of the decaying nuclide.The paper reviews the biological consequences of the decay of¹²⁵I which produces the Auger effect. Nearly all data were obtained from DNA labeled with¹²⁵I-5-iodo-2-deoxyuridine (IUdR) in bacteria and mammalian cells. Parameters of effects were cell death, DNA strand breaks, and mutation induction. In order to recognize in a cell the contribution from the Auger effect and that of absorbed radiation, experimental data are analysed in terms of the specific energy for the nuclear volume which contains the isotope.The data indicate that decay of¹²⁵I is far more toxic than is expected on the basis of absorbed dose to the labeled nucleus. Moreover, it is emphasized that the toxicity of the¹²⁵I decay is largely determined by events immediately localized to the site of decay.Because the consequences of the Auger effect are strictly localized to the molecular site of the decay,¹²⁵I and perhaps other nuclides decaying by K-capture promise to be interesting tools in cell biology and molecular biology. First data on the Auger effect as a tool are summarized.It appears that recognizable biological damage is only observed when the Auger effect takes place in vitally important molecules, an example of which is DNA.Dedicated to Prof. Dr. H. Muth on the occasion of his 60th birthday.     

99mTc-Labeled HYNIC-DAPI Causes Plasmid DNA Damage with High Efficiency

^99mTc is the standard radionuclide used for nuclear medicine imaging. In addition to gamma irradiation, ^99mTc emits low-energy Auger and conversion electrons that deposit their energy within nanometers of the decay site. To study the potential for DNA damage, direct DNA binding is required. Plasmid DNA enables the investigation of the unprotected interactions between molecules and DNA that result in single-strand breaks (SSBs) or double-strand breaks (DSBs); the resulting DNA fragments can be separated by gel electrophoresis and quantified by fluorescent staining. This study aimed to compare the plasmid DNA damage potential of a ^99mTc-labeled HYNIC-DAPI compound with that of ^99mTc pertechnetate (^99mTcO₄ ⁻). pUC19 plasmid DNA was irradiated for 2 or 24 hours. Direct and radical-induced DNA damage were evaluated in the presence or absence of the radical scavenger DMSO. For both compounds, an increase in applied activity enhanced plasmid DNA damage, which was evidenced by an increase in the open circular and linear DNA fractions and a reduction in the supercoiled DNA fraction. The number of SSBs elicited by ^99mTc-HYNIC-DAPI (1.03) was twice that caused by ^99mTcO₄ ⁻ (0.51), and the number of DSBs increased fivefold in the ^99mTc-HYNIC-DAPI-treated sample compared with the ^99mTcO₄ ⁻ treated sample (0.02 to 0.10). In the presence of DMSO, the numbers of SSBs and DSBs decreased to 0.03 and 0.00, respectively, in the ^99mTcO₄ ^– treated samples, whereas the numbers of SSBs and DSBs were slightly reduced to 0.95 and 0.06, respectively, in the ^99mTc-HYNIC-DAPI-treated samples. These results indicated that ^99mTc-HYNIC-DAPI induced SSBs and DSBs via a direct interaction of the ^99mTc-labeled compound with DNA. In contrast to these results, ^99mTcO₄ ⁻ induced SSBs via radical formation, and DSBs were formed by two nearby SSBs. The biological effectiveness of ^99mTc-HYNIC-DAPI increased by approximately 4-fold in terms of inducing SSBs and by approximately 10-fold in terms of inducing DSBs.     

The radiation dose to cells in vitro from intracellular indium-111

Most of the radionuclides used in nuclear medicine emit low energy Auger electrons following radioactive decay. These emissions, if intracellular, could irreparably damage the radiosensitive structures of the cell. The resulting radiation dose, which is a measure of biological damage in the affected cell, could be many times the average radiation dose to the associated organ. In this series of experiments, the radiation dose to the nucleus of a chinese hamster V79 cell was determined for the intracellular radiopharmaceutical 111indium-oxine. Assuming the cell nucleus to be the radiosensitive volume, the radiation dose would be primarily due to the low energy Auger electrons. A much smaller dose would be absorbed from the penetrating X- and gamma-rays and internal conversion electrons released from other radiolabelled cells in the culture. The radiation dose to the cell from the intranuclear decay of 111In was empirically established from cell survival studies to be 3.5 mGy/decay, using cobalt-60 as a reference radiation. The average dose to V79 cells from extracellular 111In (i.e., from 111In located outside the target cell) was calculated to be 5.8 pGy/decay. This suggests that for an intracellular radiopharmaceutical, the radiation dose of consequence would be delivered by the low energy Auger electrons. In contrast, Auger electrons from an extracellular radiopharmaceutical could not directly damage the cell nucleus and therefore would not contribute to the radiation dose.     

Electron and photon spectra for three gadolinium-based cancer therapy approaches

Some recent neutron capture therapy research has focused on using compounds containing the element gadolinium, which produces internal conversion and Auger cascade electrons. The low-energy, short-range Auger electrons are absorbed locally and increase cell killing dramatically as the gadolinium compounds are introduced into the cell nucleus and bind to the DNA. Detailed electron and photon spectra are needed for biophysical modeling and Monte Carlo calculations of damage to DNA. This paper presents calculated electron and photon spectra for three cases: thermal neutron absorption by (157)Gd, the beta-particle decay of (159)Gd, and the K-shell photoelectric event in gadolinium. The Monte Carlo sampling of atomic and nuclear transitions for each of the three cases was used to calculate a large number of representative decays. The sampled decays were used to determine average emissions and energy deposited in small spheres of tissue. The kinetic energy nuclear recoil from gamma-ray and electron emissions was calculated and found to be more than 10 eV for 26% of all (157)Gd neutron capture reactions.     

Comparison of cell inactivation by Auger electrons using the two reagents 4-[123I]iodoantipyrine and [123I]NaI

The Auger electron emitter ¹²³I was examined in the form of 4-[¹²³I]iodoantipyrine and as [¹²³I]NaI for its effectiveness in killing cells of different sensitivity to photon irradiation. Micronucleus assays showed that 4-[¹²³I]iodoantipyrine is 2–3 times more effective in cell inactivation than [¹²³I]NaI. This can be attributed to the fact that antipyrine, for reason of its lipid solubility, can enter cells and can reach the nucleus, whereas [¹²³I]NaI is excluded from the cytoplasm. In the nucleus Auger decay is conceivably located on the DNA where it may invoke high-LET irradiation damage. Irradiation damage by [¹²³I]NaI is by long range Auger and internal conversion electrons and hence less densely ionising. Results of the present study demonstrate, however, that the enhancement of micronuclei frequency (MNF) seen with 4-[¹²³I]iodoantipyrine as compared to [¹²³I]NaI is similar for all cell lines and that the ratio of 4-[¹²³I]iodoantipyrine/[¹²³I]NaI MN response remains the same. Experiments with the free radical scavenger DMSO, indicated nearly identical dose reduction factors for both ¹²³I carriers. These two observations strongly suggest that the cell inactivation by 4-[¹²³I]iodoantipyrine is not by direct high-LET ionisation of DNA, but is due to an indirect effect. The indirect radiation effect of Auger decay in the nucleus could arise because 4-[¹²³I]iodoantipyrine is not incorporated into the DNA, but is only associated with chromatin where the DNA is shielded by histones. Received: 24 May 2000 / Accepted: 1 November 2000     

The question of relative biological effectiveness and quality factor for auger emitters incorporated into proliferating mammalian cells

The problem of determining RBE values for Auger emitters incorporated into proliferating mammalian cells is examined. In general, the reference radiation plays a key role in obtaining experimental RBE values. Using survival of cultured Chinese hamster V79 cells as the experimental model, new data are provided regarding selection of a reference radiation for internal Auger emitters. These data show that gamma rays delivered acutely (137Cs) are more than twice as lethal as gamma rays delivered chronically with an exponentially decreasing dose rate (99mTc). The results confirm that the reference radiation should be delivered chronically in a manner consistent with the extended exposure received by the cells in the case of incorporated radionuclides. Through a direct comparison of the radiotoxicity of Auger emitters and alpha emitters, the high RBE values reported for DNA-bound Auger emitters are confirmed. These studies reveal that the DNA binding compound [125I]iododeoxyuridine (125IdU) is about 1.6 times more effective in killing V79 cells than 5.3 MeV alpha particles from intracellularly localized 210Po-citrate. In addition, toxicity studies with the radiochemicals 125IdU and [125]-iododeoxycytidine (125IdC) establish the equivalence of the radiosensitivity of thymine and cytosine base sites in the DNA. In view of these results, and information already available, the question of establishing quality factors for Auger emitters is considered. Finally, a method for calculation of the dose equivalent for internal Auger emitters is advanced.     

Evaluation of technetium-99m decay on Escherichia coli inactivation: effects of physical or chemical agents.

Technetium-99m (99mTc) has been used in nuclear medicine and in biomedical research to label molecular and cellular structures employed as radiotracers. Here, we have evaluated, on a DNA repair proficient Escherichia coli strain, the 99mTc decay inactivation and the influence of the (i) pre-treatment with metal ion chelators or of the (ii) treatment with a free radical scavenger on the protection of the cells against the lethal effect of the 99mTc. As SnCl2 is frequently used as a reducing agent in the 99mTc-labeling process, we have also studied the capability of SnCl2 to alter the biological effects induced by the 99mTc decay. As we are exposed to either chemical or physical agents in the nature, we have decided to study a possible influence of the ultraviolet solar radiation in the biological phenomena induced by the 99mTc decay. Our data point out (i) a very important role of the Auger and/or conversion electrons in the cytotoxicity induced by the 99mTc decay; (ii) SnCl2, the metal ion chelators and the free radical scavenger protect the cells against the lethal effect of the 99mTc; and (iii) near-UV does not alter the lethal effect of the 99mTc decay.     

Induction of sperm head abnormalities by incorporated radionuclides: dependence on subcellular distribution, type of radiation, dose rate, and presence of radioprotectors

In contrast to the biological effects caused by exposure to external beams of radiation, the effects of tissue-incorporated radionuclides are highly dependent on the type of radiation emitted and on their distribution at the macroscopic, microscopic, and subcellular levels, which are in turn determined by the chemical nature of the radionuclides administered. Induction of abnormalities of sperm heads in mice is investigated in this work after the injection of a variety of radiochemicals including alpha emitters. When the initial slopes of the dose-response curves are used to compare the relative biological effectiveness (RBE) of different radiocompounds, the alpha particles emitted in the decay of 210Po are more effective than Auger electrons emitted by 125I incorporated in the DNA of the spermatogonial cells, and both emissions are more effective than X rays. It is also shown that the Auger emitters (125I, 111In) distributed in the cell nucleus are more efficient in producing abnormalities than the same radionuclides localized in the cytoplasm. These findings are consistent with our earlier observations, where spermatogonial cell survival is assayed as a function of the testicular absorbed dose. Further, chronic irradiation of testis with gamma rays from intratesticularly administered 7Be is about three times more effective in causing abnormalities than a single acute exposure to 120-kVp X rays. The resulting RBE values correlate well with our data on sperm head survival with the same radiocompounds. Finally, the radioprotector cysteamine, when administered in small, nontoxic amounts, significantly reduces the incidence of sperm abnormalities from alpha-particle radiation as well as emissions from 125I incorporated into DNA, the dose reduction factors being 10 and 14, respectively.     

HDAC inhibitors act with 5-aza-2'-deoxycytidine to inhibit cell proliferation by suppressing removal of incorporated abases in lung cancer cells

5-Aza-2'-deoxycytidine (5-aza-CdR) is used extensively as a demethylating agent and acts in concert with histone deacetylase inhibitors (HDACI) to induce apoptosis or inhibition of cell proliferation in human cancer cells. Whether the action of 5-aza-CdR in this synergistic effect results from demethylation by this agent is not yet clear. In this study we found that inhibition of cell proliferation was not observed when cells with knockdown of DNA methyltransferase 1 (DNMT1), or double knock down of DNMT1-DNMT3A or DNMT1-DNMT3B were treated with HDACI, implying that the demethylating function of 5-aza-CdR may be not involved in this synergistic effect. Further study showed that there was a causal relationship between 5-aza-CdR induced DNA damage and the amount of [(3)H]-5-aza-CdR incorporated in DNA. However, incorporated [(3)H]-5-aza-CdR gradually decreased when cells were incubated in [(3)H]-5-aza-CdR free medium, indicating that 5-aza-CdR, which is an abnormal base, may be excluded by the cell repair system. It was of interest that HDACI significantly postponed the removal of the incorporated [(3)H]-5-aza-CdR from DNA. Moreover, HDAC inhibitor showed selective synergy with nucleoside analog-induced DNA damage to inhibit cell proliferation, but showed no such effect with other DNA damage stresses such as gamma-ray and UV, etoposide or cisplatin. This study demonstrates that HDACI synergistically inhibits cell proliferation with nucleoside analogs by suppressing removal of incorporated harmful nucleotide analogs from DNA.     

Kinetics of uptake, retention, and radiotoxicity of 125IUdR in mammalian cells: implications of localized energy deposition by Auger processes

The kinetics of uptake, retention, and radiotoxicity of 125IUdR have been studied in proliferating mammalian cells in culture. The radioactivity incorporated into the DNA is directly proportional to the duration of incubation and to the extracellular concentration of 125I. The rate of proliferation of cells is related to the intracellular radioactive concentration and is markedly reduced at medium concentrations greater than or equal to 0.1 mu Ci/ml. At 37% survival the high LET type cell survival curve is characterized by an uptake of 0.035 pCi/cell, and the cumulated mean lethal dose to the cell nucleus is about 80 rad compared to 580 rad of X-ray dose for this cell line. The strong cytocidal effects of the decay of 125I correlate with localized irradiation of the DNA by the low energy Auger electrons.     

99mTc Pyrene Derivative Complex Causes Double-Strand Breaks in dsDNA Mainly through Cluster-Mediated Indirect Effect in Aqueous Solution

Radiation therapy for cancer patients works by ionizing damage to nuclear DNA, primarily by creating double-strand breaks (DSB). A major shortcoming of traditional radiation therapy is the set of side effect associated with its long-range interaction with nearby tissues. Low-energy Auger electrons have the advantage of an extremely short effective range, minimizing damage to healthy tissue. Consequently, the isotope ^99mTc, an Auger electron source, is currently being studied for its beneficial potential in cancer treatment. We examined the dose effect of a pyrene derivative ^99mTc complex on plasmid DNA by using gel electrophoresis in both aqueous and methanol solutions. In aqueous solutions, the average yield per decay for double-strand breaks is 0.011±0.005 at low dose range, decreasing to 0.0005±0.0003 in the presence of 1 M dimethyl sulfoxide (DMSO). The apparent yield per decay for single-strand breaks (SSB) is 0.04±0.02, decreasing to approximately a fifth with 1 M DMSO. In methanol, the average yield per decay of DSB is 0.54±0.06 and drops to undetectable levels in 2 M DMSO. The SSB yield per decay is 7.2±0.2, changing to 0.4±0.2 in the presence of 2 M DMSO. The 95% decrease in the yield of DSB in DMSO indicates that the main mechanism for DSB formation is through indirect effect, possibly by cooperative binding or clustering of intercalators. In the presence of non-radioactive ligands at a near saturation concentration, where radioactive Tc compounds do not form large clusters, the yield of SSB stays the same while the yield of DSB decreases to the value in DMSO. DSBs generated by ^99mTc conjugated to intercalators are primarily caused by indirect effects through clustering.     

Auger electron emitters: Insights gained from in vitro experiments

Summary This paper outlines the evolution of the current rationale for research into the biological effects of tissue-incorporated Auger electron emitters. The first section is a brief review of the research conducted by several groups in the last fifteen years. The second section describes the in vitro model used in our studies, dosimetric calculations, experimental techniques and recent findings. The third section focuses on the use of Auger electron emitters as in vitro microprobes for the investigation of the radiosensitivity of distinct subcellular components. Examination of the biological effects of the Auger electron emitter¹²⁵I located in different cellular compartments of a single cell line (V79 hamster lung fibroblast) verifies that DNA is the critical cell structure for radiation damage and that the sensitive sites are of nanometer dimensions. The data from incorporation of several Auger electron emitters at the same location within DNA suggest that there are no saturation effects from the decay of these isotopes (i.e. all the emitted energy is biologically effective) and provide some insight into which of the numerous physical mechanisms accompanying the Auger decay are most important in causing cell damage. Finally the implications of Auger electron emission for radiotherapy and radiation protection in diagnostic nuclear medicine are detailed and further research possibilities are suggested.     

Comparative microdosimetry of photoelectrons and Compton electrons: an analysis in terms of generalized proximity functions

A current discussion on mammography screening is focused on claims of high relative biological effectiveness (RBE) of mammography X rays compared to conventional 200 kV X rays. An earlier assessment in terms of the electron spectra of these radiations has led to the conclusion that the RBE is bound to be less than 2, regardless of specific model assumptions and the microdosimetric properties of electrons. The present study extends this result in terms of the microdosimetric proximity function, t(x), for electrons, which is essentially the spatial auto-correlation function of energy within particle tracks. If pairs of DNA lesions, e.g. chromosome breaks or deletions, bring about the observed damage, the value t(x) determines for a specified radiation the relative frequency of pairs of lesions a distance x apart. The effectiveness of the radiation is thus proportional to an average of the values of t(x) over the distances, x, for which lesions can combine. The analysis suggests that 15 keV electrons can have a low-dose relative biological effectiveness (RBE(M)) of 1.6 relative to 40 keV electrons if the interaction distances do not exceed about 1 micro m. An extension of the concept, the reduced proximity function, t(delta)(x), permits the inclusion of models with an energy threshold, such as delta = 100 eV, 500 eV or 2 keV, for the formation of each of the DNA lesions. This makes it possible to assess the potential impact of the Auger electrons which accompany most photoelectrons, but only a minority of the Compton electrons. It is found that the Auger electrons could make photoelectrons substantially more effective than Compton electrons at energies below 10 keV but not at energies above 15 keV. The conclusions obtained for the RBE of 15 keV electrons relative to 40 keV electrons will be roughly representative of the RBE of mammography X rays relative to conventional 200 kV X rays.     

Calculation on spectrum of direct DNA damage induced by low-energy electrons including dissociative electron attachment

In this work, direct DNA damage induced by low-energy electrons (sub-keV) is simulated using a Monte Carlo method. The characteristics of the present simulation are to consider the new mechanism of DNA damage due to dissociative electron attachment (DEA) and to allow determining damage to specific bases (i.e., adenine, thymine, guanine, or cytosine). The electron track structure in liquid water is generated, based on the dielectric response model for describing electron inelastic scattering and on a free-parameter theoretical model and the NIST database for calculating electron elastic scattering. Ionization cross sections of DNA bases are used to generate base radicals, and available DEA cross sections of DNA components are applied for determining DNA-strand breaks and base damage induced by sub-ionization electrons. The electron elastic scattering from DNA components is simulated using cross sections from different theoretical calculations. The resulting yields of various strand breaks and base damage in cellular environment are given. Especially, the contributions of sub-ionization electrons to various strand breaks and base damage are quantitatively presented, and the correlation between complex clustered DNA damage and the corresponding damaged bases is explored. This work shows that the contribution of sub-ionization electrons to strand breaks is substantial, up to about 40–70%, and this contribution is mainly focused on single-strand break. In addition, the base damage induced by sub-ionization electrons contributes to about 20–40% of the total base damage, and there is an evident correlation between single-strand break and damaged base pair A–T.