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Monte Carlo dosimetry of total body irradiation Burns, Levi

Abstract

The total body irradiation (TBI) technique at the Vancouver Cancer Centre uses a sweeping Cobalt-60 beam with patient-specific lung compensators and a stationary flattening filter, with the patient lying in supine and prone positions during each fraction. The dose calculations for this technique are limited to the dose delivered to a point at patient mid-separation at the level of the umbilicus, and the mean dose to the body must fall within a 10% tolerance of prescription. While the current technique is effective, it has been in place for over 23 years, and detailed dose data to critical organs is necessary before the technique can be upgraded to a more conformal technique that would offer an improved patient experience. For this purpose, a Monte Carlo simulation technique has been developed and applied to collect organ dose data from TBI treatments based on retrospective data of patients recently treated with 12 Gy in 6 fractions. 20 patients, including adults and pediatric patients, are simulated by constructing Monte Carlo phantoms in each of the supine and prone positions based on planning CT images. The supine and prone dose distributions are summed with a deformable registration tool and the doses to the lungs, kidneys, thyroid, and liver are analyzed as well as the dose delivered to the body. It is determined that while all doses fall within prescription, there is a trend where smaller patients receive lower mean body doses and viceversa (mean body dose range: 10.93-12.01 Gy). For most patients, the lungs and liver consistently receive doses below the mean body dose, and the thyroid and kidneys consistently receive higher doses than the mean body dose. This thesis presents an overview of the background physics and biology of TBI as well as a comprehensive survey of different techniques described in the literature. The Monte Carlo simulation technique used for the retrospective study is described with calibration, validation, and optimization details, and the organ dose results of the retrospective study are shown.

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