Monte Carlo Electron Track Structure Calculations in Liquid Water Using a New Model Dielectric Response Function.

Monte Carlo track structure codes provide valuable information for understanding radiation effects down to the DNA level, where experimental measurements are most difficult or unavailable. It is well recognized that the performance of such codes, especially at low energies and/or subcellular level, critically depends on the reliability of the interaction cross sections that are used as input in the simulation. For biological media such as liquid water, one of the most challenging issues is the role of condensed-phase effects. For inelastic scattering, such effects can be conveniently accounted for through the complex dielectric response function of the media. However, for this function to be useful it must fulfill some important sum rules and have a simple analytic form for arbitrary energy- and momentum-transfer. The Emfietzoglou-Cucinotta-Nikjoo (ECN) model offers a practical, self-consistent and fully analytic parameterization of the dielectric function of liquid water based on the best available experimental data. An important feature of the ECN model is that it includes, in a phenomenological manner, exchange and correlation effects among the screening electrons, thus, going beyond the random-phase approximation implicit in earlier models. In this work, inelastic cross sections beyond the plane wave Born approximation are calculated for low-energy electrons (10 eV-10 keV) based on the ECN model, and used for Monte Carlo track structure simulations of physical quantities relevant to the microdosimetry of low-energy electrons in liquid water. Important new developments in the physics of inelastic scattering are discussed and their effect on electron track structure is investigated by a comparison against simulations (under otherwise identical conditions) using the Born approximation and a simpler form of the dielectric function based on the Oak Ridge National Laboratory model. The results reveal that both the dielectric function and the corrections to the Born approximation may have a sizeable effect on track structure calculations at the nanometer scale (DNA level), where the details of inelastic scattering and the role of low-energy electrons are most critical.

Calculation of cellular S-values using Geant4-DNA: The effect of cell geometry.

PURPOSE: Geant4-DNA is used to calculate S-values for different subcellular distributions of low-energy electron sources in various cell geometries. METHOD: Calculations of cellular S-values for monoenergetic electron sources with energy from 1 to 100 keV and the Auger-electron emitting radionuclides Tc-99m, In-111, and I-125 have been made using the Geant4 Monte Carlo toolkit. The Geant4-DNA low-energy extension is employed for simulating collision-by-collision the complete slowing-down of electron tracks (down to 8 eV) in liquid water, used as a surrogate of human cells. The effect of cell geometry on S-values is examined by simulating electron tracks within different cell geometries, namely, a spherical, two ellipsoidal, and an irregular shape, all having equal cellular and nuclear volumes. Algorithms for randomly sampling the volume of the nucleus, cytoplasm, surface, and whole cell for each cell phantom are presented. RESULTS: Differences between Geant4-DNA and MIRD database up to 50% were found, although, for the present radionuclides, they mostly remain below 10%. For most source-target combinations the S-values for the spherical cell geometry were found to be within 20% of those for the ellipsoidal cell geometries, with a maximum deviation of 32%. Differences between the spherical and irregular geometries are generally larger reaching 100-300%. Most sensitive to the cell geometry is the absorbed dose to the nucleus when the source is localized on the cell surface. Interestingly, two published AAPM spectra for I-125 yield noticeable differences (up to 19%) in cellular S-values. CONCLUSION: Monte Carlo simulations of cellular S-values with Geant4-DNA reveal that, for the examined radionuclides, the widely used approximation of spherical cells is reasonably accurate (within 20-30%) even for ellipsoidal geometries. For irregular cell geometries the spherical approximation should be used with caution because, as in the present example, it may lead to erroneous results for the nuclear dose for the commonly encountered situation where the source is localized to the cell surface.

Disability status, disease parameters, defense styles, and ego strength associated with psychiatric complications of multiple sclerosis.

OBJECTIVE: The aim of the present study was to identify disease parameters, defensive styles and ego strength measurements associated with various forms of psychiatric complications in patients with multiple sclerosis (MS). METHODS: Seventy-nine patients with MS participated in the study and 158 healthy subjects matched for age and sex served as controls. A wide range of clinical information was collected and the following self-report instruments were used: General Health Questionnaire, Symptom Distress Check List, Defense Style Questionnaire, MMPI Ego Strength Scale and Hostility and Direction of Hostility Questionnaire. RESULTS: The odds of being assessed with a psychiatric diagnosis upon interview were 6.7 times greater among patients compared to controls and 9.3 times greater among patients with recent-onset MS compared to patients with long-term disease. Psychiatric complications of MS were closely associated with age of the disease onset and the degree of disability due to MS. Additionally, higher rates of introverted hostility, adoption of maladaptive ego defenses and weakened ego strength were also closely associated with several forms of psychological distress, especially depressive symptoms. CONCLUSIONS: MS patients experience elevated symptoms of psychological distress, especially depressive symptoms, which are most closely associated with disease parameters. However, the crucial role of various personality traits such as ego defenses and hostility features in the psychiatric symptom formation also appear to contribute to the development of depressive symptoms. Clinicians involved in the clinical management of patients with MS should identify and modify treatment if these specific personality markers that indicate the exhaustion of the patient's resources to cope with the physical and psychological stress of the illness are present.