Computational methods in radiation oncology
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis examines computer technology in radiation oncology and the intimate role that it must now play in the dosimetry practices of this discipline. Aspects of the dosimetry and implementation of five radiation therapies are examined, namely total-body irradiation (TBI), total-skin electron therapy (TSET), electron therapy, superficial therapy and ophthalmic brachytherapy. Computational techniques, in particular, Monte Carlo and several other numerical methods are used. The Monte Carlo platform, EGS4, and the treatment planning system, GRATIS, have been implemented on SUN and Silicon Graphics workstations. Monte Carlo methods are used in the investigation of electron therapy planning and superficial and brachytherapy dosimetry practices. Monte Carlo techniques are used for radiation protection calculations of linear accelerator bunker design and for the optimisation of an in vivoX-ray fluorescence (XRF) technique used to measure platinum uptake associated with cisplatin chemotherapy. Inverse Monte Carlo methods have been examined and implemented. Inverse methods, applied to in-phantom dose measurements, are used to determine phase-space information. such as spectra, for an incident electron beam. Analogous methods are examined for megavoltage and superficial X-rays in particular, source parameterisation with attenuation and photoactivation techniques. Two linear accelerators, a Varian 2100C and a Varian 600C, provide Megavoltage X-rays or electrons. The TBI therapy uses a 6 MV X-ray beam. The TSET technique uses 6 MeV electrons which degraded to a lower energy by a screen placed in front of the patient at an extended source distance. The 9 Me V and the 20 Me V electron modalities are also closely examined. Two Philips superficial therapy units, RT100 and RT50 provide 10 through 100 kVp X-rays. 1251 seeds are used for the investigation of ophthalmic brachytherapy dosimetry. Methods of dosimetry incorporated in this work include in-phantom, ionisation chamber and diode measurements. Thermoluminescent dosimeters (TLDs), Silverhalide and radio chromic films are used. Measurements have been performed in water, solid water. polymethyl-methacrylate (PMMA), and polystyrene phantoms. Fricke, ferrous based gels are investigated as a method of dosimetry in a uniform medium. Three-dimensional dose distributions are examined for several radiation modalities. The concentration of radiation-induced ferric ions and hence dose is determined using magnetic resonance imaging (MRI). A high-purity germanium detector and a thalium doped sodium iodide detector are employed for the measurement of source spectra and for fluorescing and activated materials.