The macro response Monte Carlo method for electron transport

Abstract

The main goal of this thesis was to prove the feasibility of basing electron depth dose calculations in a phantom on first-principles single scatter physics, in an amount of time that is equal to or better than current electron Monte Carlo mcthods.The Macro Response Monte Carlo (MRMC) method achieves run times that are on the order of conventional electron transport methods such as condensed history, with the potential to be much fa,ster. This is possible because MRMC is a Local-toGlobal method, meaning the problem is broken down into two separate transport calculations. The first stage is a local, in this case, single scatter calculation, which generates probability distribution functions (PDFs) to describe the electron's energy, position and trajectory after leaving the local geometry, a small sphere or "kugel" A number of local kugel calculations were run for calcium and carbon, creating a library of kugel data sets over a range of incident energies and sizes (0.025 cm to 0.1 cm in radius).The second transport stage is a global calculation, where steps that conform to the size of the kugels in the library are taken through the global geometry. For each step, the appropriate PDFs from the MRMC library are sampled to determine the electron's new energy, position and trajectory. The electron is immediately advanced to the end of the step and then chooses another kugel to sample, which continues until transport is completed.The MRMC global stepping code was benchmarked as a series of subroutines inside of the Peregrine Monte Carlo code. It was compared to Peregrine's class II condensed history electron transport package, EGS4, and MCNP for depth dose in i ii simple phantoms having density inhomogeneities. Since the kugels completed in the library were of relatively small size, the zoning of the phantoms was scaled down from a clinical size, so that the energy deposition algorithms for spreading dose across 5-10 zones per kugel could be tested. Most resulting depth dose calculations were within 2-3s of well-benchmarked codes, with one excursion to 4%.This thesis shows that the concept of using single scatter-based physics in clinical calculations would not only be possible, but would likely be more efficient than current methods, provided large kugels were generated.

Monte Carlo modelling of gamma-ray stopping efficiencies of Geiger–Müller counters

“…In addition, as mentioned above and discussed in paper 1, the possible effects of the treatment of electron transport in MCNP5 and PENELOPE were to be investigated. As stated in paper 1, this could only be achieved through comparison between MCNP5 and PENELOPE simulated stopping efficiency curves and those that are obtained using a MC code that employs electron transport simulation on an event-by-event basis (also referred to as the single scatter approach [53,54]) as well as experimental stopping efficiencies. This would also enable more accurate MC modeling of the intrinsic gamma-ray stopping efficiency in cylindrical GM counters.…”

“…The inaccuracies in the electron transport when the CH technique is utilized are usually attributed to the use of (a) multiple scattering angular distributions, (b) multiple scattering energy loss distributions and (c) the straight line approximation [54]. The reader is referred to [53,54] for a comprehensive discussion on the impact of these mechanisms on the results of low energy electron transport through absorbers.…”

“…In this section, a short overview of the treatment of electron transport in GMMC will be given. It should also be mentioned that much of the electron transport modeling follows that of CREEP, a single scatter MC code written for reliable low-energy electron transport in arbitrary media [53]. Use of CREEP was, however, not considered in the work presented in this thesis due to the fact that it does not treat coupled photon/electron transport through absorbers; that CREEP is capable of only simulating electron transport in simple slab geometries as well as due to issues related to limited availability.…”

“…The two electrons emitted from the interaction site are indistinguishable and therefore, the more energetic one is considered to be the incident electron. Similar to the sampling scheme employed in CREEP [53], GMMC utilizes the ionization subshell energy spectra provided in the EEDL database to calculate the energy of the secondary knock-on electrons. Once the sampling of the knock-on electron energy is completed, energy of the incident electron is updated as:…”

What is the optimum leaf width of a multileaf collimator?

“…[4][5][6][7] In another approach an attempt was made at improving the effective resolution of a MLC and reducing the artifacts by superimposing two MLC-shaped fields with a small shift of the isocenter between them. [8][9][10][11] A related more recent approach uses a collimator rotation instead of an isocenter shift. 12,13 In still another technique the resolution is increased by placing a grid with narrow parallel slits below the MLC and again superimposing two or more fields from the same direction of incidence.…”