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UBC Theses and Dissertations

Characterization of small high energy proton beams in homogenous and heterogenous media Charland, Paule

Abstract

This thesis advances the study of small high energetic photon fields in radiotherapy. Small photon field irradiation is aimed at delivering a uniform dose to a well defined target while minimizing the dose to the surrounding normal tissue. The dosimetry of small x-ray fields is complicated by two factors: the relationship between detector size and field dimensions and the lack of equilibrium in lateral charged particles. Additionally, a longitudinal charged particle disequilibrium is present when materials with different atomic composition and density than water are introduced in a water-like phantom. , Small radiation dosimeters such as diamond, diodes film and a mini-ion chamber have a better spatial resolution to detect the steep dose fall-off at the edge of small photon fields than the large Markus chamber. The line spread function (LSF) of the film densitometer can be estimated by simple measurement of a slit image. Deconvolution of the measured, beam profile from a linear accelerator (linac) with the LSF of a detector yields an estimate of the true inherent beam profile of the linac. Conversely, the LSF of any detector can be estimated by deconvolution from measured data once the inherent profile is known. Similarly, a blurring function representing the finite source size effect of the head of the linac which is missing in a Monte Carlo simulation can be obtained. Because the deconvolution process is highly sensitive to noise, the Total Least Squares (TLS) approach offers a reasonable means to overcome this problem. To deal with inhomogeneous media, the density scaling theorem has been modified to incorporate the effect of a change in atomic number of a material. This modified scaling found an application in the convolution-superposition dose model and provided better agreement with the Monte Carlo generated data. The idea of electronic disequilibrium has been taken into account in our simple depth dose model. A prototype second order differential equation allowed energy to be carried away, analogous to the notion of electron range, and hence we were able to simulate a build-up region for the depth dose curve as well as inhomogeneities.

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