Current electron beam treatment planning algorithms are inadequate to calculate dose distributions in heterogeneous phantoms. Fast Monte Carlo algorithms are accurate in general but their clinical implementation needs validation. Calculations of electron beam dose distributions performed using the fast Monte Carlo system XVMC and the well-benchmarked general-purpose Monte Carlo code EGSnrc were compared with measurements. Irradiations were performed using the 9 MeV and 15 MeV beams from the Clinac 18 accelerator with standard conditions. Percent depth doses and lateral profiles were measured with thermoluminescent dosimeter and electron diode respectively. The accelerator was modelled using EGS4/BEAM, and using an experiment-based beam model. All measurements were corrected by EGSnrc calculated stopping power ratios. Overall, the agreement between measurement and calculation is excellent. Small remaining discrepancies can be attributed to the non-equivalence between physical and simulated lung material, precision in energy tuning, beam model parameters optimisation and detector fluence perturbation effects.

A study is performed to assess the utility of a computer numerically controlled (CNC) mill to produce missing tissue compensating filters and for the delivery of intensity-modulated beams for inverse treatment planning. A computer aided machining (CAM) software is used to assist in the design and construction of such filters. Geometric measurements of stepped and wedged surfaces are made to examine the accuracy of surface milling. Results show that the deviation of the filter surfaces from design does not exceed 1.5%. Effective attenuation coefficients are measured for CadFree and Cerrobend in a 6 MV photon beam. The ability of the CNC mill to accurately produce surfaces is further verified with dose profile measurements in a 6 MV photon beam. Dose profiles, measured beneath the test phantoms and beneath a flat phantom are compared to those produced by a commercial treatment planning system. Agreement between measured and predicted profiles is within 2%, indicating the viability of the system for filter production.

The dosimetric characteristics of a standard Varian 52-leaf multileaf collimator (MLC) and BrainLAB m3 micro-multileaf collimator (micro-MLC) have been investigated for square, rectangular, and irregular fields for 6 MV and 18 MV photon beams provided by a Varian Clinac 2300 C/D linear accelerator (linac). The percentage depth dose data and the conventional collimator factor are unaffected by the addition of MLC or micro-MLC shaped field unless, in the latter case, the tertiary field is much less than the jaw setting. However, relative dose factors for a given MLC or micro-MLC field size depend on the jaw setting. The penumbra is generally sharpest for fields defined by the micro-MLC and the least sharp for fields defined by the MLC. Average transmission values were found to be between 1.5% and 2.5%. Comparison and evaluation of two treatments, one delivered using the MLC and the other using the micro-MLC, for the same radiosurgical target volume are described.

The treatment head of the Clinac-18 medical linear accelerator was modelled using-the EGS4 Monte Carlo simulation package. Photon-energy spectra for fields ranging from 2 x 2 cm$ sp2$ to 20 x 20 cm$ sp2$ in size were generated and the primary and scatter spectra were analyzed separately. The generated x-ray spectra were used in the calculation of the percent depth dose (PDD) distributions for flattened and unflattened 10 MV x-ray beams in a water phantom at a source-surface distance of 100 cm for the various field sizes. The agreement between calculated and measured depth doses is excellent. / Measurements of the dose in the build-up region show that the depth of dose maximum (d$ sb{max}$) increases with increasing field size for fields up to 5 x 5 cm$ sp2$ for both the flattened and unflattened beams. As the field size is increased beyond 5 x 5 cm$ sp2,$ d$ sb{max}$ decreases with increasing field size for the flattened x-ray beam while remaining nearly constant for the unflattened beam. Additionally, the surface dose of the flattened beam is found to approach that of the unflattened beam for large field sizes. Calculations show that the decrease in d$ sb{max}$ as the field size is increased above 5 x 5 cm$ sp2,$ and the rapid increase in the surface dose for the flattened x-ray beam with increasing field size, are due to the degradation of the flattened-beam parameters caused by low-energy photons produced in the flattening filter.

No description available.

Attenuation and scatter are limiting factors in image quality and quantitation of organ function by single photon emission computed tomography (SPECT). To correct brain images for attenuation an effective water/tissue attenuation coefficient of 0.12 cm$ sp{-1}$ (at 140 keV) or larger has been recommended in order to compensate for the additional bone (skull) attenuation. / It has been determined that the reconstructed images are overcorrected in the centre by 5%, and the optimum correction occurs for a reduced coefficient of 0.09 cm$ sp{-1}$. The overcorrection is due to increased attenuation at the edges of all projections where the path length through the bone is greater, although the bone also increases the scatter at the projection edges. / A correction scheme which uses effective bone and water coefficients was developed to compensate for the attenuation. Alternatively, prior to attenuation correction, a common scatter correction was found to be effective in explicitly removing the bone and water scatter.

In this thesis, the use of the Fricke-gelatin dosimeter in NMR-based radiation dosimetry is investigated. The relationship between the proton spin-lattice relaxation rate and the absorbed dose for the Fricke-gelatin dosimeter is determined using pulsed NMR at 25 MHz. This relationship is used to calculate the NMR dose sensitivity. Practical considerations of Fricke-gelatin dosimetry, such as the spontaneous conversion of ferrous to ferric ions and the effects of oxygen and sodium chloride on the dose response, are determined. Sensitization of the dosimeter by bubbling with a nitrous oxide/oxygen gas mixture is investigated. The system is modelled assuming a multi-site fast exchange between water bound to gelatin, water bound to ions, and water in the bulk. Preliminary aspects of the model are investigated and compared with experimentally determined data. Finally, the utility of the dosimeter is demonstrated by magnetic resonance imaging a Fricke-gel irradiated with an $ sp{192}$Ir radioactive seed.

Commercial positron emission tomography scanners that use block detectors have additional blurring on spatial resolution, referred to as block effect. We studied the origin of the block effect, using experiments in which all other blurring effects were minimized and precisely determined. Bismuth germanate crystals (1 mm width) and a small (1 mm) 68Ge source were used to probe the spatial resolution of a CTI HR+ block detector and two precise translation stages to move detectors. Coincidence aperture functions for crystals in the block and for single crystals were compared. The central crystals in the block showed an additional blurring of 0.8 mm whereas the edge ones showed no additional blurring. The apparent centroids of the crystals in the block are not located at the geometric centers, which gives errors in the reconstruction algorithm assumed uniform sampling. Our results suggest that the additional blurring in scanners with block detectors is not only due to the use of block detectors.

Stereotactic radiosurgery with several static conformal beams shaped by a micro multileaf collimator (muMLC) is used for treating small irregularly shaped brain lesions. Specific requirements for this technique are a precise localization and positioning of the target (1mm) and a precise (1mm) and numerically accurate (+/-5%) dose delivery. In this work, a pencil beam algorithm based treatment planning software BrainScan 5.2 (Brainlab, Germany) is validated against measurements (diode, radiographic films) and Monte Carlo simulations (BEAMnrc and XVMC codes). The latter is required because of difficulties in obtaining precise and accurate dose measurements for small fields. A dedicated muMLC component module for the BEAMnrc code was developed as part of this project. Results show that Monte Carlo calculations agree with measured dose distributions to within 2% and/or 1 mm except for field sizes smaller than 1.2 cm where agreement is within 5% due to uncertainties in measured output factors. Comparison with the pencil beam algorithm calculations were performed for square and irregularly shaped fields at different incidence angles on rectangular and humanoid homogeneous phantoms. Results show that the pencil beam algorithm is suitable for radiosurgery although some differences were found in the comparison of interleaf leakage and beam profile penumbras.

In positron emission mammography, the use of planar detector limits the angular coverage and introduces more noise than conventional positron emission tomography. / We first studied the sampling artifacts introduced from the use of discrete crystals. The images are reconstructed by back-projecting lines of response from and to the centroid of interaction within the crystal. We postulate that the sampling artifact should be reduced by allowing the lines of response to shift away from the centroid towards the next most probable crystal element. / We then studied noise in the peripheral region of the images. The solid angle function is an image uniformity correction function. The solid angle function is the last thing applied before the images are displayed. We postulate that image quality should improve by re-ordering the solid angle function and the smoothing algorithm. / These two techniques have shown an improvement in contrast, resolution, and noise. An ROC curve analysis showed an improvement of 9.5 % in accuracy.