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The increasing use of intensity modulated radiation therapy (IMRT) to deliver conformal radiation treatment has prompted the search for a faster and more cost effective quality assurance (QA) system. The standard technique relies on the use of film for two-dimensional dose distribution verification. Although film is considered the gold standard and is widely used for this purpose, the procedures involved are relatively lengthy, labour intensive and costly for a multiple field IMRT verification. In this study, we investigate the use of an amorphous silicon electronic portal imaging device (a-Si EPID) to complement the film. The dosimetric behaviour of the device is studied both experimentally and numerically using the EGSnrc Monte Carlo simulation routine. The intrinsic build-up of the flat panel EPID was found to be 1.1 cm of water equivalent material. The response of the flat panel EPID was found to be linear between 0 and 300 cGy. To calibrate the flat panel EPID for two dimensional dose measurements, the deconvolution method was chosen. The scatter dose kernel required for this calibration method was calculated and characterized by varying the energy, spectrum and phantom material using a 6MV pencil beam. We found that flat panel EPID scatter kernel has as much as 80% more scattering power than the water scatter kernel in the region 1 cm away from the center of a 6MV pencil beam. This confirms that a flat panel EPID behaves significantly differently from water dosimetrically and requires an accurate dose scatter kernel for calibration. A 1.0 cm wide picket fence test pattern was used to test the accuracy of the kernel. Using the deconvolution method with the calculated dose kernels, the measurements from the flat panel EPID show improved agreement with the films.

Polymer gel dosimeters are a three dimensional (3D) dosimetry system that may be conveniently applied for verifying highly conformal radiation therapies where standard dosimetry techniques are insufficient. Polymer gel dosimetry with optical computed tomography (OptCT) can be used to measure spatial dose distributions with high resolution. While long experience with MRI has yielded many studies reporting on experiments involving validation of clinical deliveries using polymer gel dosimeters, there are very few studies of this type where OptCT is used. OptCT is a relatively new technique and consequently has not yet been adopted into the clinical environment. As a result, methods and software tools for integrating OptCT measurements into clinical systems are not available. Previous studies from the Medical Physics research group at the Cancer Centre of Southeastern Ontario (CCSEO) and Queen’s University have therefore been limited to simple deliveries and two dimensional (2D) comparisons. In this thesis various software tools and calibration techniques have been developed to allow comparative analysis between OptCT measurements with dose distributions calculated by treatment planning software. Further, a modification of the γ-evaluation (Low et al. 1998) is presented whereby the vector components of γ are used to identify the sources of disagreement between compared dose distributions. Test simulations of the new γ-tool revealed that individual vector components of γ, as well as the resulting vector field can be used to identify certain types of disagreements between dose distributions: especially spatial misalignments caused by geometric misses. The polymer gel dosimetry tools and analysis software were applied to a clinical validation mimicking a prostate conformal treatment with patient setup correction using image guidance. In one experiment greater than 90 % agreement was found between dose distributions in 4%T 50%C NIPAM/Bis dosimeters (Senden et al. 2006) measured with the Vista OptCT unit and dose distributions calculated by Eclipse treatment planning software. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-12-18 15:46:33.78

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