The Auger effect in physical and biological research.

PURPOSE: The paper reports on progress in physics of radiationless transitions and new Auger spectra of (125)I and (124)I. We report progress in Monte Carlo track structure simulation of low energy electrons comprising majority electrons released in decay most Auger emitters. MATERIALS AND METHODS: The input data for electron capture (EC) and internal conversion(IC) were obtained from various physics data libraries. Monte Carlo technique was used for the simulation of Auger electron spectra. Similarly, electron tracks were generated using Monte Carlo track structure methods. RESULTS: Data are presented for the EC, IC and binding energy (BE) of radionuclides (124)I and (125)I. For each of the radionuclides (125)I and (124)I some examples of electron spectra of individual decays are given. Because most Auger electrons are low energy and short range, data and a short discussion are presented on recent Monte Carlo track structure development in condensed media and their accuracy. CONCLUSIONS: Accuracy of electron spectra calculated in the decay of electron shower by Auger emitting radionuclides depends on availability of accurate physics data. There are many gaps in these libraries and there is a need for detailed comparison between analytical method and Monte Carlo calculations to refine the method of calculations. On simulation of electron tracks, although improved models for sub-keV electron interaction cross sections for liquid water are now available, more experimental data are needed for benchmarking. In addition, it is desirable to make data and programs for calculations of Auger spectra available online for use by students and researchers.

Auger electrons--a nanoprobe for structural, molecular and cellular processes.

This paper provides a brief review of recently published work on biophysical and biological aspects of Auger processes. Three specific questions have been considered. (1) Does charge neutralisation contribute to molecular damage such as DNA strand breaks? (2) How many DNA double strand breaks are produced by a single decay of DNA bound (125)I? (3) What is the correlation between number of gammaH2AX foci and number of double strand breaks (DSB)? The paper also gives preliminary reports on two new calculations: (a) calculation of the spectrum of Auger electrons released during decay of (124)I and (b) the use of Auger electrons in the decay of (125)I as a probing agent of novel DNA structures.

Monte Carlo code for microdosimetry of inhaled alpha emitters.

A Monte Carlo code has been developed to calculate the local energy deposited by alpha emitters deposited on the inner surface in the lung airway. Developed to deal further with airway bifurcations, this code has been as a first step validated in a cylindrical airway configuration by comparison with well-established analytical codes in the case of contamination of bronchiolar airways with actinides. The code has then been applied to the study of uniform and non-uniform contamination of cylindrical bronchial airways by radon progeny in indoor and mine exposure conditions. In addition to the microdosimetric spectra, the average microdosimetric parameters (zp, n, z) have been evaluated. The work currently in progress consists in adapting this developed Monte Carlo code to the configuration of an airway bifurcation with realistic particles deposition.

Monte Carlo modeling of the effects of injected short-, medium- and longer-range alpha-particle emitters on human marrow at various ages.

The effects of injected short-, medium- and longer-range alpha-particle emitters ((149)Tb, (211)At/(211)Po and (213)Bi/(213)Po, respectively) on the total hemopoietic stem cell population of active normal bone marrow in humans of various ages has been estimated using Monte Carlo modeling. The fraction of the normal hemopoietic stem cells that are hit and survive has been calculated as a first step toward estimating the risk of development of therapy-induced leukemia. The fraction was lowest for the shorter-range alpha-particle emitter ((149)Tb) and highest for the longer-range alpha-particle emitter ((213)Bi/(213)Po), with the value for the medium-range alpha-particle emitter (211)At/(211)Po being intermediate between these. There was little variation in the data with the age of the subject within each alpha-particle emitter. This lack of age dependence provides reassurance that the fraction of cells hit in any subject of any age with normal marrow can be estimated by modeling newborn marrow (which requires little computing time) despite age-related differences in microarchitecture.

Response of LNCaP spheroids after treatment with an alpha-particle emitter (213Bi)-labeled anti-prostate-specific membrane antigen antibody (J591).

A theoretical drawback to alpha-particle therapy with 213Bi is the short range of the particle track coupled with the short half-life of the radionuclide, thereby potentially limiting effective cytotoxicity to rapidly accessible, disseminated individual tumor cells (e.g., as in leukemia). In this work, a prostate carcinoma spheroid model was used to evaluate the feasibility of targeting micrometastatic clusters of tumor cells using 213Bi-labeled anti-prostate-specific membrane antigen (PSMA) antibody, J591. In prostate cancer, vascular dissemination of tumor cells or tumor cell clusters to the marrow constitutes an important step in the progression of this disease to widespread skeletal involvement, an incurable state. Such prevascularized clusters are ideal targets for radiolabeled antibodies because the barriers to antibody penetration that are associated with the capillary basal lamina have not yet formed. Beta- and gamma-emitting radionuclides such as 131I, which are widely used in radioimmunotherapy, are not expected to be effective when targeting single cells or small cell clusters. This is because the range of the emissions is one to two orders of magnitude greater than the target size, and the energy deposited per traversal is insufficient to produce any significant radiobiological effect. Spheroids of the prostate cancer cell line, LNCaP-LN3, were used as a model of prevascularized micrometastases; their response to an anti-PSMA antibody, J591, radiolabeled with the alpha-particle emitter 213Bi (T(1/2), 45.6 min.) has been measured. The time course of spheroid volume reductions was found to be sensitive to the initial spheroid volume. J591 labeled with 0.9 MBq/ml 213Bi resulted in a 3-log reduction in spheroid volume on day 33, relative to control, for spheroids with an initial diameter of 130 microm; 1.8 MBq/ml were required to achieve a similar response for spheroids with an initial diameter of 180 microm. Equivalent spheroid responses were observed after 12 Gy of acute external beam photon irradiation. Monte Carlo-based microdosimetric analyses of the 213Bi decay distribution in individual spheroids of 130-microm diameter yielded an average alpha-particle dose of 3.7 Gy to the spheroids, resulting in a relative biological effectiveness factor of 3.2 over photon irradiation. The activity concentrations used in the experiments were clinically relevant, and this work supports the possibility of using 213Bi-labeled antibodies not only for disseminated single tumor cells, as found in patients with leukemia, but also for micrometastatic tumor deposits up to 180 microm in diameter (1200 cells).

Radiation effects in spheroids of cells exposed to alpha emitters.

PURPOSE: To simulate spheroids of cells and use the spheroid models to establish methods of calculating cell survival following exposure to alpha irradiation from decays of 211At and 213Bi. METHODS: Using a Monte Carlo technique, two models of spheroids of cells were generated in which cells either did not or were allowed to overlap. At 40% packing of the volume it is shown that the two models are equivalent for longer-ranged alpha particles. The overlapping model was used to examine the effects of spheroid size and cell packing on cell survival. Cell survival was also calculated for different distributions of alpha decays such as uniform distribution within the spheroid, external to the spheroid and as a shell on the periphery of the spheroid. RESULTS: Three examples of the cell structure of the spheroids are shown. Detailed calculations show that cell survival decreases from 57% to 37% as the spheroid diameter increases from 75 microm to 225 microm for an average of one 211At decay per cell and 50% packing. Increasing the packing of the cells in the spheroid from 40% to 70% reduces survival from 46% to 26% for 200 microm diameter spheroids for one 211At or 213Bi decay per cell. The presence of small regions of unlabelled cells within the spheroids does not significantly change cell survival. CONCLUSIONS: Monte Carlo programmes generating spheroids of cells and their subsequent use to score cell survival for alpha irradiation should be useful in the design and interpretation of work on spheroids of cells in vitro and the application of such modelling to the study of very small tumours in vivo. Copies of the programmes are available from the author on request.

The survival of monolayers of cells growing in clusters irradiated by 211At appended to the cell surfaces.

A model of cell survival is described for the case of closely packed clusters of cells growing in monolayers. For alpha-particle decays on the cell surfaces, it is shown that cross-firing between cells produces nonuniform dose distributions within the cluster and that cells in larger clusters are exposed on average to greater doses. The model is used to simulate the survival of SK-MEL-28 human melanoma cells labeled with different radiolabeled monoclonal antibodies. The survival data suggest that this cell line is more sensitive to high-LET radiation than previously thought.

Theoretical treatment of human haemopoietic stem cell survival following irradiation by alpha particles.

PURPOSE: To calculate survival of human haemopoietic stem cells irradiated by alpha particles from 149Tb (initial energy 3.97 MeV) and 211At (average initial energy 6.87 MeV). METHODS: Following as closely as possible radiobiological data, Monte Carlo methods were used to calculate passages of alpha particles (originating in three geometries) through the nuclei of haemopoietic stem cells. Survival of stem cell populations was calculated from the probability of surviving each passage (a function of LET). RESULTS: For decays targeted to the surface of individual cells 37% survival was found at 1.3 passages per nucleus for 149Tb and 6.5 for 211At/Po decays. For decays distributed in a large volume the D0 doses were 0.81 and 0.87 for 149Tb and 211At respectively. When 36% of the marrow is occupied by fat cells alphas from 149Tb are more effective with a D0 of 0.68Gy compared to 0.82Gy for the 211At/Po combination. CONCLUSIONS: When the isotopes are targeted to the cell, in terms of passages, 149Tb is five times more effective at cell killing than 211At, which when expressed in terms of dose, increases to a factor of 9. When the isotopes are broadly distributed (such as in marrow or in vitro) the differences are considerably reduced.

The minimum Do for cell killing for alpha-particle emitters uniformly distributed in an extended medium.

For cells irradiated by alpha particles in suspension, there is presently no simple test to show that the cell killing can be attributed to alpha-particle passages through the nuclei. In this communication, for a uniform distribution of alpha-particle sources and spherical nuclei, a D0 is calculated such that at least 63% of the cell nuclei have at least one alpha-particle passage. For a uniform distribution of alpha-particle sources and spherical nuclei, it is shown that the average dose for 63% of the cell nuclei to be hit (37% not hit) is equal to the average dose per hit. For this condition an energy deposition of any size would result in cell death and the average dose is the minimum D0 possible. Minimum D0 values are calculated using a Monte Carlo treatment for nuclear diameters from 4 to 10 microm and initial alpha-particle energies between 3.18 and 8.38 MeV.

Monte Carlo/numerical treatment of alpha-particle spectra from sources buried in bone and resultant doses to surface cells.

PURPOSE: To calculate the depth distribution of radioactive decays in bone by fitting a calculated alpha-particle energy spectrum to a measured spectrum. The dose to bone surface cells is then calculated from the depth distribution. MATERIALS AND METHODS: The spectra are calculated using Monte Carlo methods and a numerical depth distribution obtained by trial and error. RESULTS: Alpha spectra for 210Po and 226Ra (plus decay products) in bone were calculated and fitted to experimental spectra. The dose to bone surface cells was calculated using the numerical technique and compared to previously published data. CONCLUSIONS: Alpha-particle spectra can be successful fitted using this method, which gives a more realistic distribution of decays with depth than simple surface and volume models.

Microdosimetry of haemopoietic stem cells irradiated by alpha particles from the short-lived products of 222Rn decays in fat cells and haemopoietic tissue.

The Monte Carlo method is used to model fat cells and the nuclei of stem cells in haemopoietic tissue where 222Rn is dissolved in different amounts in the fat and tissue. Calculations are performed for fat cells of diameters 50 and 100 microns and for stem cell nuclei of 8 and 16 microns diameters for various fractions of fat filling the volume. Average doses (and their distributions) to stem cell nuclei from single passages of alpha particles are presented. In addition to dose, the relationship between LET and dose is obtained, illustrating the importance of 'stoppers' in the calculations. The annual average dose equivalent for a concentration of 1 Bq/m3 in air agrees well with other authors at 12 mu Sv/year. The method also allows the calculation of the fraction of stem cell nuclei hit annually. Here for 1 Bq/m3, stem cell nuclei of diameter 8 microns and 100% fat filing 15 x 10(-7) of the stem cell nuclei are hit.

Modelling of Auger-induced DNA damage by incorporated 125I.

We have analyzed a newly available high resolution and precision repeat of the original Martin and Haseltine experiment which includes the influence of DMSO on the results. The new model includes the production and diffusion of radical species and .OH radical attack on DNA as well as the direct hits. Calculations of single-strand breaks use individual Auger electron along with the tracks of electrons and radical species superimposed on an atomistic model of B-DNA. Comparison of the preliminary calculations with the experiment supports the earlier choice of data for the amount of energy required to produce a single-strand break, i.e. 17.5 eV. In a separate simulation we found that an average of less than two ionizations inducing a single-strand break gave the best fit to experimental data. Direct hits were found to be predominantly occurring at short range while the damage by .OH radicals was mainly of the long-range type.

Double-strand breaks from 125I incorporated in the DNA and cell death.

Track structure calculations of the local energy deposition by electrons emitted during the decay of 125I are used to demonstrate that the range of high energy deposition is small (< 10 nm) and restricted to the DNA and its immediate environment. An experiment in which 125I is incorporated into the DNA of synchronized CHO cells during a pulse and decays are allowed to accumulate a given time after the incorporation is described. Here it is shown that damage from 125I decays in newly replicated DNA (cells frozen for decay accumulation within 1 h after labelling) are relatively non-toxic whereas decays in mature DNA (cells frozen 5 h after labelling) are highly lethal. It is suggested that during DNA maturation the labelled DNA becomes associated with (or reorganized into) a radiosensitive nuclear structure and that damage to this structure is the primary cause of radiation-induced cell death.

Monte Carlo track structure studies of energy deposition and calculation of initial DSB and RBE.

Estimation of exposure due to environmental and other sources of radiations of high-LET and low-LET is of interest in radiobiology and radiation protection for risk assessment. To account for the differences in effectiveness of different types of radiations various parameters have been used. However, the relative inadequacy of the commonly used parameters, including dose, fluence, linear energy transfer, lineal energy, specific energy and quality factor, has been made manifest by the biological importance of the microscopic track structure and primary modes of interaction. Monte Carlo track structure simulations have been used to calculate the frequency of energy deposition by radiations of high- and low-LET in target sizes similar to DNA and higher order genomic structure. Tracks of monoenergetic heavy ions and electrons were constructed by following the molecular interaction-by-interaction histories of the particles down to 10 eV. Subsequently, geometrical models of these assumed biological targets were randomly exposed to the radiation tracks and the frequency of energy depositions obtained were normalized to unit dose in unit density liquid water (l0(3) kg m-3). From these data and a more sophisticated model of the DNA, absolute yields of both single- and double-strand breaks expressed in number of breaks per dalton per Gray were obtained and compared with the measured yields. The relative biological effectiveness (RBE) for energy depositions in cylindrical targets has been calculated using 100 keV electrons as the reference radiation assuming the electron track-ends contribution is similar to that in 250 kV X-ray or Co60 gamma-ray irradiations.

Monte Carlo calculations of ion passages through brain endothelial nuclei during boron neutron capture therapy.

The Monte Carlo technique is used to examine the passage of ions from the (n, p) reaction in nitrogen and the (n, alpha) reaction in 10B through the nuclei of endothelial capillary wall cells of normal white matter. Contributions to the dose from different regions are evaluated, as is the distribution of doses to the nuclei for mean doses from 1 to 100 Gy. Calculation of mean dose to the nuclei is generalized for any neutron fluence and element concentration. The dose-averaged LETs for both reactions are found giving (for the particular conditions of the calculation); 60 keV/microns for the (n, p) reaction and 216 keV/micron for the (n, alpha) reaction in boron limited to the lumen, and 224 keV/micron for a uniform distribution of boron. From a consideration of 'minimal survival' and hit and dose distributions, evidence is presented which suggests that the RBE for the (n, alpha) reaction depends upon the distribution of the boron in the tissue, with the uniform distribution being less effective at cell killing than the boron restricted to the lumen for the same mean dose to the cell nuclei.

The paradoxical nature of DNA damage and cell death induced by 125I decay.

Chinese hamster ovary cells were synchronized at the G1/S-phase boundary of the cell cycle and pulse-labeled for 10 min with 125I-iododeoxyuridine 30 min after entering the S phase. Cell samples were harvested for freezing and 125I-decay accumulation at intervals ranging from 15 to 480 min after termination of labeling. The survival data showed a marked shift from cell killing characteristic of low-LET radiation to that more characteristic of killing by high-LET radiation with increasing intervals between DNA pulse-labeling and decay accumulation. Cells harvested and frozen within 1 h after pulse-labeling yielded a low-LET radiation survival response with a pronounced shoulder and a large D0 of up to 0.9 Gy. With longer chase periods the shoulder and the D0 decreased progressively, and cells harvested 5 h after pulse-labeling or later exhibited a high-LET survival response (D0: 0.13 Gy). Two interpretations for these findings are discussed. (1) If DNA is the sole target for radiation death, the results indicate that DNA maturation increases radiation damage to DNA or reduces damage repair. (2) If radiation cell death involves damage to higher-order structures in the cell nucleus, the findings suggest that newly replicated DNA is not attached to these structures during the initial low-LET period, but 125I starts to induce high-LET radiation effects as labeled DNA segments become associated with the target structure(s). On balance, or data favor the latter interpretation.

Energy deposition in small cylindrical targets by monoenergetic electrons.

Calculations of energy deposition in cylindrical target volumes of diameter and height 1-100 nm, including those similar to the dimensions of biological molecules and structures such as DNA, nucleosomes and chromatin fibre, have been made. The calculations used the Monte Carlo track structure program MOCA8B for electrons of initial energy 0.1-100 keV. Details of the calculation are presented, as well as a selection of results. The frequency distributions of energy deposition events per gray per target, placed at random in a homogeneous aqueous medium, are given for uniform irradiation with monoenergetic electrons of various energies. The frequency distributions have been used to predict the initial biophysical parameters such as relative effectiveness for initial damage. These suggest that the final biological effects which depend on complex local damage may show substantial variations in biological effectiveness for different low linear energy transfer radiations, whereas those that depend on simple local damage may not.