Comparisons of calculations with PARTRAC and NOREC: transport of electrons in liquid water.

Monte Carlo computer models that simulate the detailed, event-by-event transport of electrons in liquid water are valuable for the interpretation and understanding of findings in radiation chemistry and radiation biology. Because of the paucity of experimental data, such efforts must rely on theoretical principles and considerable judgment in their development. Experimental verification of numerical input is possible to only a limited extent. Indirect support for model validity can be gained from a comparison of details between two independently developed computer codes as well as the observable results calculated with them. In this study, we compare the transport properties of electrons in liquid water using two such models, PARTRAC and NOREC. Both use interaction cross sections based on plane-wave Born approximations and a numerical parameterization of the complex dielectric response function for the liquid. The models are described and compared, and their similarities and differences are highlighted. Recent developments in the field are discussed and taken into account. The calculated stopping powers, W values, and slab penetration characteristics are in good agreement with one another and with other independent sources.

Spatial distributions of inelastic events produced by electrons in gaseous and liquid water.

The spatial distributions of ionizations and other inelastic events in charged-particle tracks are important quantities that influence the final outcome of radiation interaction. Calculations of such distributions are presented for the tracks of electrons in the energy range 100 eV to 10 keV in liquid water and water vapor, and the results are compared. The distributions include the frequency of nearest-neighbor distances for all inelastic events, the mean nearest-neighbor distances for ionizations and for all inelastic events as a function of electron energy, the frequency of distances between all ionizations and all inelastic events, and the farthest distances between all inelastic events in electron tracks. The physical differences between liquid water and water vapor are discussed in terms of the respective inverse mean free paths, the collision spectra, and the nonlocalization of energy losses that are likely to occur in the liquid.

Radiotoxicity of platinum-195m-labeled trans-platinum (II) in mammalian cells.

The chemotoxicity and radiotoxicity of trans-dichlorodiammineplatinum (II) labeled with 195mPt (trans-195mPt) are investigated to ascertain the potential of radioplatinum coordination complexes as antineoplastic agents. Platinum-195m, with a half-life of about 4 days, is a prolific emitter of low-energy Auger electrons because of the high probability of internal conversion in its isomeric transitions. The kinetics of cellular uptake and retention after incubation and the radiotoxicity of this Auger electron emitter in the form of trans-195mPt is investigated using cells of the Chinese hamster V79 cell line. The cellular uptake of 195mPt reaches a plateau in about 3 to 5 h of incubation and varies nonlinearly with the extracellular concentration of radioactivity. The radioactivity is eliminated from the cells after incubation with an effective half-life of 24 h. Cell survival data, when corrected for the chemical toxicity of nonradiolabeled trans-platinum, give a cell survival curve typical for radiations with high linear energy transfer. At 37% survival, the mean lethal cellular uptake is about 1.0 mBq/cell. Dosimetric considerations, based on subcellular distribution of the radionuclide, yield a value of 4.8 for the relative biological effectiveness when compared with 250 kVp X rays. Theoretical Monte Carlo track-structure calculations indicate that the density of radical species produced in liquid water in the immediate vicinity of a 195mPt decay site is substantially greater than the density of species along the track of a 5.3 MeV alpha particle. This explains qualitatively the efficacy of 195mPt in causing high-LET radiation type biological effects. The extreme radiotoxicity of intranuclearly localized 195mPt, in conjunction with the proclivity of platinum chemotherapy agents to bind to DNA in the cell nucleus, suggests that the combination of chemical effects and the effects of Auger electrons that can be obtained with radioplatinum coordination complexes may have potential in the treatment of cancer.

Monte Carlo calculations of free ammonia production in deoxygenated solutions of glycylglycine irradiated by X rays and 60Co gamma rays.

Detailed-history Monte Carlo computer codes were used to simulate the formation, diffusion, and chemical reaction of free-radical species within deoxygenated aqueous solutions of glycylglycine irradiated by 250-kVp X rays and by 60Co gamma rays. In one reaction, hydrated electrons react with the glycylglycine solute to produce unbound, or free, ammonia. This reaction is complete by 10(-6) s within individual electron tracks for glycylglycine concentrations greater than or equal to 0.025 M. For solute concentrations from 0.025 to 1.2 M, calculated G values of free ammonia are in excellent agreement with measured values. In addition, the computer model predicts a statistically significant difference between the G value of free ammonia produced under X irradiation and that produced under 60Co gamma irradiation.

Effects of a bolus and inhomogeneities on pion stopping distributions.

In radiotherapy treatments with negative pion beams, an external bolus is often used to compensate for inhomogeneities within the body in order that the pions will have the proper stopping distribution within a tumor. However, angular beam divergence, multiple Coulomb scattering, and elastic and inelastic nuclear scattering limit the degree to which the pion stopping region can be controlled. We have used the Monte Carlo computer code PION-I to calculate pion stopping distributions for several idealized cases in order to show explicitly the effects of a number of factors on stopping distributions. Calculations have also been made for the same geometrical configurations used in measurements of pion stopping distributions in the biomedical beam at the Los Alamos Meson Physics Facility, and the calculated results are compared with experiment.

Modelling DNA damage induced by different energy photons and tritium beta-particles.

PURPOSE: To model the production of single- and double-strand breaks (ssb and dsb) in DNA by ionizing radiations. To compare the predicted effectiveness of different energy photon radiations and tritium beta-particles. MATERIALS AND METHODS: Modelling is carried out by Monte Carlo and includes consideration of direct energy depositions in DNA molecules, the production of species, their diffusion and interactions with each other and DNA. Computer-generated electron tracks in liquid water are used to model energy deposition and to derive the initial positions of chemical species. Atomistic representation of the DNA in B form with a first hydration shell is used. Photon radiations in the energy range 70keV-1MeV and tritium beta-particles are considered. RESULTS: A tentative increase for dsb yield has been predicted for 70 keV photons and tritium compared with 137Cs. This increase is more pronounced for complex dsb. Double-strand breaks are much more prone compared with ssb to combine with additional strand breaks and base damage, which contributes to break complexity. At least half of DNA breaks are hydroxyl radical mediated. CONCLUSIONS: The developed model makes predictions compatible with features of available experimental data. Break complexity has to be addressed in biophysical modelling when the relative effectiveness of radiations in DNA damage is studied. Obtained data strongly argue against the dominance of direct radiation action in DNA damage in the cellular environment predicted by some theoretical studies.

Influence of beam characteristics and detector size in negative-pion dose studies.

Dose calculations were performed for a tissue phantom irradiated by uniform circular beams of negative pions with an assumed gaussian momentum distribution. The mean momentum of the pions was varied from 104.4 to 171.5 MeV/c (mean range 5-20 cm in unit-density tissue) and the momentum spread from 0 to 5% of the mean. Depth-dose curves are shown for different mean momenta and momentum spreads. The radial distribution of dose as a function of distance from the beam axis was computed at different depths for a beam with a mean momentum of 153.4 MeV/c and spread of 2%. The responses of detectors of different sizes used to measure centre-line dose for this beam were shown by calculating depth-dose curves for detectors of radii 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0 cm. Calculations were also performed for beams having radii of 1, 2 and 3 cm. Depending on particular conditions, it appears that considerable care may often be needed to infer the absorbed dose at a given location in a phantom irradiated by a negative-pion beam.

Calculated yields and slowing-down spectra for electrons in liquid water: implications for electron and photon RBE.

Detailed Monte Carlo calculations have been carried out of slowing-down spectra and yields for a number of end-points for electrons in liquid water. These investigations were made to study differences in physical effects of different low-LET radiations and implications for RBE. Initial electron energies from 1 keV to 1 MeV were used, and all secondary electrons were followed in the computations unitl their energies fell below 10 eV. Though there are substantial differences in the slowing-down spectra at energies near and above the K-shell ionisation potential of oxygen, the energy spectrum of electrons at lower energies is found to be essentially independent of the initial energy of the primary electron. The number of events per unit energy deposited is also essentially independent of the primary electron energy. Based on these calculations, there appears to be little basis for ascribing differences in RBE for low-LET radiations to differences in physical effects produced by secondary electrons of low energy (less than or equal to 1 keV).

Effects of elastic and inelastic scattering in giving electrons tortuous paths in matter.

Heavy charged particles travel in essentially straight lines in matter, while electrons travel in tortuous paths. Frequent multiple elastic Coulomb scattering by atomic nuclei is often cited as the reason for this electron behavior. Heavy charged particles also undergo multiple Coulomb scattering. However, because they are massive, significant deflections occur only in rare, close encounters with nuclei. In contrast to heavy particles, the inelastic interaction of an electron with an atomic electron represents a collision with a particle of equal mass. In principle, therefore, repeated inelastic scattering of an electron can also produce large-angle deflections and thus contribute to the tortuous nature of an electron's track. To investigate the relative importance of elastic and inelastic scattering on determining the appearance of electron tracks, detailed Monte Carlo transport computations have been carried out for monoenergetic pencil beams of electrons normally incident on a water slab with initial energies from 1 keV to 1 MeV. The calculations have been performed with deflections due to (1) inelastic scattering only, (2) elastic scattering only, and (3) both types of scattering. Results are presented to show the spreading of the pencil beams with depth in the slab, the transmission through slabs of different thicknesses, and back-scattering from the slab. The results show that elastic nuclear scattering is indeed the principal physical process that causes electron paths to be tortuous; however, the smaller effect of inelastic electronic scattering is far from negligible.

A Monte Carlo primer for health physicists.

The basic ideas and principles of Monte Carlo calculations are presented in the form of a "primer" for health physicists. A simple integral with a known answer is evaluated by two different Monte Carlo approaches. Random numbers, which underlie Monte Carlo work, are discussed, and a sample table of random numbers generated by a hand calculator is presented. Monte Carlo calculations of dose and linear energy transfer (LET) from 100-keV neutrons incident on a tissue slab are discussed. The random-number table is used in a hand calculation of the initial sequence of events for a 100-keV neutron entering the slab. Some pitfalls in Monte Carlo work are described. While this primer addresses mainly the "bare bones" of Monte Carlo, a final section briefly describes some of the more sophisticated techniques used in practice to reduce variance and computing time.

Monte Carlo calculations of initial energies of electrons in water irradiated by photons with energies up to 1GeV.

Previous calculations of the initial energies of electrons produced in water irradiated by photons are extended to 1 GeV by including pair and triplet production. Calculations were performed with the Monte Carlo computer code PHOEL-3, which replaces the earlier code, PHOEL-2. Tables of initial electron energies are presented for single interactions of monoenergetic photons at a number of energies from 10 keV to 1 GeV. These tables can be used to compute kerma in water irradiated by photons with arbitrary energy spectra to 1 GeV. In addition, separate tables of Compton-and pair-electron spectra are given over this energy range. The code PHOEL-3 is available from the Radiation Shielding Information Center, Oak Ridge National Laboratory, Oak Ridge, TN 37830.

Effects of the tissue-air interface in calculations of beta-particle skin dose at a depth of 70 microns.

The effects that the tissue-air interface has on the basal-layer dose at a depth of 70 microns from beta emitters on the skin surface are studied using Monte Carlo calculations. The dose is decreased at small lateral distances from a point source but is increased at large distances.

A method of obtaining neutron dose and dose equivalent from digital measurements and analysis of recoil-particle tracks.

The feasibility of a digital approach to neutron dosimetry has been investigated. Such an approach uses an ionization detector capable of measuring the numbers of electrons produced within various subvolumes of a chamber gas along a charged-particle track. In addition, a computer algorithm is used to infer absorbed dose, LET, and dose equivalent given this digital track-structure information for each event. This paper describes one detector design capable of providing digital track-structure information and discusses examples of proton and C-ion tracks calculated from a Monte Carlo charged-particle transport code. The associated computer algorithm is presented next with its verification accomplished by running a variety of recoil particles through a simulated detector volume and comparing the resulting average energy deposition and dose equivalent to those unfolded by the algorithm.

Calculations for beta dosimetry using Monte Carlo code (OREC) for electron transport in water.

A Monte Carlo computer code (OREC) for calculating the detailed transport and energy deposition for primary electrons and all of their secondaries in liquid water has been investigated for use in beta-ray dosimetry. Some modifications have been made in the original code for its application to tissue and tissue-equivalent materials. The code gives reasonably good agreement with beta spectral data and depth-dose curves measured in tissue-equivalent plastics for several calibrated beta sources. The calculations permit a direct evaluation of the skin dose equivalent, i.e., the dose equivalent, Hs (0.07), at a depth of 0.070 mm in tissue. Calculations are presented for monoenergetic electrons, showing the distributions in the maximum depth of penetration and in the total pathlength traveled. Direct comparisons are made between depth-dose curves calculated for 99Tc and 147Pm plaque sources and measurements made with extrapolation chambers. The energy spectrum of beta particles emerging from a thick 99Tc plaque source also is calculated, and the angular distribution is found to be almost independent of the energy. The pulse-height spectrum in a tissue-equivalent plastic scintillator calculated for this source shows good agreement with the measured spectrum. The calculations also provide the Hs(0.07) dose equivalent for the 99Tc source, which is found to be consistent with that inferred independently from the spectral measurements. A calculated curve for converting spectrometer measurements to Hs(0.07) dose equivalent is in good agreement with a semi-empirical curve that was developed independently. It appears that calculations made with the electron transport code for water can provide useful information for beta dosimetry.