Year

1998

Degree Name

Doctor of Philosophy

Department

Department of Engineering Physics

Abstract

The near surface build up characteristics of photon and electron beams used for radiotherapy treatment of cancer patients have been studied. For the measurement of skin dose various detectors were developed. The Attix parallel plate ionisation chamber was used as the benchmark chamber for in phantom dose build up measurements. Extrapolation of thermoluminescent dosimeters readings were employed to produce invivo results, within the first 0.1cm of tissue. A metal oxide semiconductor field effect transistor (MOSFET) system provided on line integral dosimetry. Radiochromic film was employed as an off-axis skin dose measurement.

Neodymium Iron Boron rare earth lanthanide magnets were tested for their ability remove electron contamination from photon beams produced by a linear accelerator. Results show a reduced surface and build up dose. For a 20cm x 20cm field size at 100cm source surface distance (SSD) using a 0.6cm perspex block tray, percentage surface dose is 18% and 32% with and without the magnetic field applied respectively at 6MV . A helium/air replacement system has been developed which enhances the skin sparing properties of megavoltage photon beams by replacing 25cm of air above the phantom with helium. By combining the magnetic and helium devices the percentage surface dose is 9% of maximum. This is due to the elimination of low energy electron contamination normally generated in the air column.

Monte Carlo simulations were performed in the build up region. Using experimental and Monte Carlo generated data as input, a model has been developed to predict central axis build up dose for 6MV photons. This is performed by separating dose components produced by i) photons scattered by the phantom and ii) electron contamination. Dose deposition changes depend on field size, source surface distance (SSD), angle of incidence, beam modifying devices (block trays, blocks and wedges) and patient geometry. The model accounts for these variables.

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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.