A Monte Carlo investigation of the dosimetric properties of monoenergetic neutron beams for neutron capture therapy

A Monte Carlo simulation study has been carried out to investigate the suitability of neutron beams of various energies for therapeutic efficacy in boron neutron capture therapy. The dosimetric properties of unidirectional, monoenergetic neutron beams of varying diameters in two different phantoms (a right-circular cylinder and an ellipsoid) made of brain-equivalent material were examined. The source diameter was varied from 0.0 to 20.0 cm; neutron energies ranged from 0.025 eV up to 800 keV, the maximum neutron energy generated by a tandem cascade accelerator using 2.5-MeV protons in a 7Li(p,n)7Be reaction. Such a device is currently under investigation for use as a neutron source for boron neutron capture therapy. The simulation studies indicate that the maximum effective treatment depth (advantage depth) in the brain is 10.0 cm and is obtainable with a 10-keV neutron beam. A useful range of energies, defined as those neutron energies capable of effectively treating to a depth of 7 cm in brain tissue, is found to be 4.0 eV to 40.0 keV. Beam size is shown not to affect advantage depth as long as the entire phantom volume is used in determining this depth. Dose distribution in directions parallel to and perpendicular to the beam direction are shown to illustrate this phenomenon graphically as well as to illustrate the differences in advantage depth and advantage ratio and the contribution of individual dose components to tumor dose caused by the geometric differences in phantom shape.     

A Monte Carlo study of the chlorophyll fluorescence emission and its effect on the leaf spectral reflectance and transmittance under various conditions.

Spectral hemispherical reflectance R(lambda) and transmittance T(lambda) are affected by chlorophyll (Chl) fluorescence which may complicate the evaluation of optical parameters of leaves. Measured Chl a fluorescence spectral emission F(lambda) is itself affected by several distortion effects on the leaf level (fluorescence reabsorption, secondary fluorescence, inner filter, surface and subsurface reflections etc.). In this work we propose a Monte Carlo photon transport (MCPT) model capable for treating a variety of optical distortion effects on the leaf level. In the forward mode the model decouples R(lambda), T(lambda) and their fluorescence contributions FR(T)(lambda). To obtain the absorption and scattering spectra of the leaf, utilized in the forward modeling, we have suggested an inversion procedure employing the experimental R(lambda), T(lambda). The attention was paid on the correction of the leaf absorption and scattering spectra caused by the optical effects on the sample level including Chl fluorescence contribution to measured R(lambda), T(lambda).     

A Monte Carlo simulation of chemical reactions

A computer-based algorithm to solve complex chemical rate equations is introduced. A simple Monte Carlo sampling method is used to generate chemical reactions in numbers proportional to reaction probabilities, and a second-order Runge-Kutta method is used to calculate time. The method is compared with a closed form mathematical solution for a simple chemical system, and it is compared with a numerical integration of the rate equations for a more complicated system.     

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).     

A Monte Carlo study of the self-assembly of bacteriorhodopsin

Bacteriorhodopsin (BR) and specific lipid molecules self-assemble into a quasi two-dimensional lattice structure known as the purple membrane (PM). In the PM, BR molecules exist in a trimeric form with lipid molecules present in the space enclosed by each trimeric unit and in the inter-trimer space. These trimeric units, which have a roughly circular cross-section, are arranged in hexagonal patterns with long-ranged crystalline order. In this work, we investigate the self-assembly of BR in the PM via Monte Carlo simulations of a two-dimensional model of the membrane and proteins. The protein molecules are modeled as 120 degrees sectors of a circle and the lipid molecules enter into the model through effective protein-protein interactions. The sectors cannot overlap with each other, and in addition to this excluded volume interaction there are site-site attractive interactions between specific points of the proteins to mimic interactions between helices on the proteins and lipid-induced interactions. At low values of the attractive well depth, the proteins are found in the monomeric form at all concentrations. At moderate and high values of the attractive well depth, trimers are formed as the concentration increases, and with a further increase in concentration the trimers organize into a hexagonal lattice. The interactions between the proteins and those induced by the intra-trimer lipids play an equally important role in the formation of trimers and the lattice. The lipids in the inter-trimer space cause the trimers to orient in a specific direction in the hexagonal crystal lattice.     

A Monte Carlo study of the amylosic chain conformation

Monte Carlo studies of the unperturbed amylosic chain conformation have been carried out in the approximation of separable chain configuration energies. Sample chains of arbitrary chain length have been generated so as to be distributed consistent with refined estimates of the configuration energy and thus suitable for evaluation of averages of the desired configuration-dependent properties. Perspective drawings of representative chains from the Monte Carlo sample have been made for comparison with standard idealizations of amylosic chain conformation. He molecular model employed generates a randomly coiling chain possessing perceptible regions of left-handed pseudohelical backbone trajectory. Distribution functions for the end-to-end distance of short amylosic chains disclose some propensity for the chain to suffer self-intersections at sort range in the chain sequence, which may vitiate the usual amylosic chain models based on the assumed independence of sets of glycosidic linkage torsion angles. The amylosic persistence vector and persistence length have been calculated as a function of chain length for the chain model employed.     

Monte Carlo investigation of electron specific energy distribution in a single cell model

Radiation and Environmental Biophysics - Knowledge of microdosimetric quantities of certain radionuclides is important in radio immune cancer therapies. Specific energy distribution of...