Over the last few decades, radiation therapy techniques have been greatly improved, with more complex radiation delivery systems such as intensity-modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT) being used to treat complex tumour shapes. This has greatly increased the demand for more accurate dosimetry systems to achieve better targeting of the treatment area while sparing the surrounding normal tissues. Ge-doped Si02 telecommunication optical fibres are a potential form of thermoluminescence (TL) dosimeter that offers excellent TL yield as well as high spatial resolution, the size of the optical fibre (typically ~ 100 )lm) making it particularly suitable for use in a variety of interface dosimetry situations and also for small radiotherapy fields. Using the Ge-doped Si02 optical fibres as a dosimeter, the work in this thesis aims to present measurement of the dose enhancement that can be obtained close to high atomic number media. For present purposes iodine and gold have been investigated as generators of photoelectrons using 250 kVp photons produced by a Gulmay orthovoltage x-ray machine. Additional investigation is made of the dosimetry of small photon field sizes :S 4 x 4 cm for 6 MV energy produced by a Varian linear accelerator. Results are presented for Ge-doped Si02 telecommunication optical fibres with different core diameters. These have first been characterised in terms of linearity, energy dependence and glow curve, good linearity to dose being obtained, the fibre with a 50 )lm core dopant diameter providing the greatest TL yield. Consequently, it was used for dose enhancement investigation with iodine and different thicknesses of gold' coated to the fibre i.e. 20, 40, 60, 80, and 100 nm thickness. The experimental results were favourably compared against Monte Carlo simulations with FLUKA and DOSRZnrc. The 50 )lm core dopant diameter Ge-doped Si02 optical fibres were also used for dosimetric study of field sizes of 1.0 cm x 1.0 cm, 2.0 cm x 2.0 cm, 3.0 cm x 3.0 cm and 4.0 cmx 4.0 cm at 1.5 and 5.0 cm depths in a solidwater ™ phantom. The measurements have been then compared with GafChromic film, a 2D array ion chamber as well as with MC simulations with DOSXYZnrc and FLUKA.

There have been developments in radiation therapy treatment techiques, which lead to an increase in the complexity of these treatments. The aim is to deliver highly conformal three-dimensional (3D) dose distributions, such as stereotactic radiosurgery (SRS). Polymer gel dosimetry offers three-dimensional (3D) dosimetry techniques for dose verification of dose distributions. Nisopropyl- acrylamide (NIP AM) polymer gel was the latest to develop and can be prepared under a normal atmospheric environment and has lower toxicity compared with the highly toxic polymer gels used earlier. NIPAM polymer gel using X-ray computed tomography (CT) was experimentally investigated in terms of its X-ray CT dose response, sensitivity and dose resolution. The effect of radiation beam type, radiation beam energy and radiation beam dose rate on X-ray CT dose response have also been studied. The temporal stability of NIP AM polymer gel has been examined over several days post-irradiation. The change in the polymer gel dosimeter's physical and electron densities as a function of absorbed dose was also investigated. In ,this study two new prototype phantoms were designed and constructed for imaging and irradiation of polymer gel dosimeters to provide simplicity and practicality for clinical dosimetry. The dosimetric and water equivalence properties of four NIP AM based polymer gel dosimeter formulations have been studied by examining their physical properties, interaction probability, radiation transport parameters and performing Monte Carlo modelling of depth doses. NIP AM polymer gel dosimeter irradiated at different doses using 6 Me V photon beam and 400 MU min-1 dose rate were found to have higher CT dose response (up to 37.8% at 10 Gy dose point) than results reported in the literature for NIP AM gel using similar concentration. The CT dose sensitivity of NIPAM polymer gel was found to be 0.405±0.014 H Gi1 , which is 26.2% higher than the reported sensitivity of 0.32l±0.008 H Gy-l with similar NIPAM gel concentration. The maximum change in physical density as a function of absorbed dose for polymer gel dosimeters was found to be up to ~1.0% for an absorbed dose of 20 Gy. 111

The use of Volumetric Modulated Arc Therapy (VMAT) is expected to increase, largely due to development of helical tomotherapy. VMAT minimises the occurence of hotspots and inadiation of critical organs, providing more uniform dose while sparing critical organs. Two important characteristics of VMA T are its dynamic nature and dosimetric variability in radiation delivery. These present considerable challenge for clinical physicists as the implementation of the process contains a number of sources of uncertainty and thus require robust QA.

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