Year

2009

Degree Name

Ph.D.

Department

School of Engineering Physics

Abstract

Brachytherapy is a common treatment modality used for treating prostate cancer. The radiation is emitted from within the prostate, which focuses the damage on the tumour rather than the surrounding healthy tissue. However, due to the close proximity of the rectum to the prostate, there is a possibility that the rectum will receive too much radiation dose during prostate treatment. This may lead to post-treatment rectal complications that range in severity from general rectal discomfort and bleeding, to the development of a rectal fistula that may require surgical intervention. Currently, there is no real-time quality assurance tool used to verify that the rectum does not receive too much radiation dose.

The Centre for Medical Radiation Physics (CMRP) at the University of Wollongong, Australia, has developed a new MOSFET dosimeter called the MOSkin with a unique packaging design that should demonstrate some benefits over other MOSFET dosimeters. The MOSkin can potentially serve as a real-time rectal dosimeter for use during a PDR or HDR brachytherapy treatment.

The general aim of this research is to characterise the performance of the new MOSkin dosimeter, and to determine whether the unique design of the MOSkin demonstrates dosimetric advantages over other dosimeters for brachytherapy, and in certain other applications. The performance characteristics of the MOSkin were explored by exposing it to a 6 MV x-ray field delivered by a linear accelerator (LINAC), and also with an Ir-192 PDR brachytherapy source.

The MOSkin was irradiated with a 6 MV, 1010 cm3 photon beam with the gantry set to 100 cm source-to-surface distance, and its dose response was compared to the response of a CC13 compact ionization chamber, and a couple of fiber optic dosimeters.

The fiber optic dosimeters had either a 0.5 or 1 mm diameter scintillating crystal attached to one end of the optical fiber. The MOSkin’s ability to measure the skin dose to a depth of 0.07 mm, which corresponds to the nominal depth of the basal cell layer of the epidermis, was compared to Attix chamber and fiber optic dosimeter measurements, along with skin dose measurements reported in other studies found in the literature. My research was primarily concerned with dosimetry at shallow depths in the phantom because it was believed that the MOSkin’s packaging design would prove advantageous over other dosimeters at such depths.

With regards to brachytherapy, the dose response of the MOSkin was compared to the response of a RADFET dosimeter within a rectal phantom. Brachytherapy treatment planning systems (TPS) calculate the dose by assuming that the human body is a large, homogeneous water-equivalent material. The effect that a hollow, air-filled rectal cavity has on the anterior rectal dose was investigated by measuring the dose delivered to the inner wall of a rectal phantom while it was empty, and comparing dose measurement to the dose measured within a homogeneous rectal phantom, and the dose calculated by the TPS. The results were also corroborated by Monte Carlo simulations written with the Geant4 toolkit.

The results of an early stage, Phase II clinical trial at St. George Cancer Care Centre are discussed. The MOSkin was used to measure the dose delivered to the anterior wall of the patient’s rectum during PDR brachytherapy patients during treatment, and the results are compared to the dose calculated by the TPS. The results were also compared to the observations made during the phantom study.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.