X-ray computed tomography (CT) polymer gel dosimetry (PGD) is an attractive tool for three-dimensional (3D) radiation therapy (RT) treatment verification due to the availability of CT scanners in RT clinics. Nevertheless, wide-spread use of the technique has been hindered by low signal-to-noise CT images largely resulting from gel formulations with low radiation sensitivity. However, a new gel recipe with enhanced dose sensitivity was recently introduced that shows great promise for use with CT readout. This dissertation describes development of an CT PGD system for 3D verification of RT treatments using the new gel formulation. The work is divided into three studies: gel characterization, commissioning of a multislice CT scanner and investigation of a dose rate dependence observed during gel characterization.
The first component of this work examines the dosimetric properties of the new gel formulation. The response of the gel is found to be stable between 15 - 36 hours post-irradiation and excellent batch reproducibility is seen for doses between 0 - 28 Gy. A dose rate dependence is found for gels irradiated between 100 - 600 MU/min, indicating machine dose rate must be consistent for calibration and test irradiations to avoid dosimetric error. An example clinical application is also presented using an IMRT treatment verification that demonstrates the potential of the system for use in modern RT.
The second component of this work focuses on commissioning a multislice CT scanner for CT PGD. A new slice-by-slice background subtraction technique is introduced to account for the anode heel effect. Additional investigations show recommendations for optimizing image quality in CT PGD using a single slice machine also apply to multislice scanners. In addition, the consistency of CT numbers across the multislice detector array is found to be excellent for all slice thicknesses. Further work is performed to assess the tube load characteristics of the scanner and develop a scanning protocol for imaging large gel volumes. Finally, images acquired throughout the volume of an unirradiated active gel show variations in CT data across each image on the order of 7 HU. However, these variations are not expected to greatly influence gel measurements as they are consistent throughout the gel volume.
The third component of this work examines the dose rate dependence found during gel characterization. Studies using gel vials and 1 L cylinders indicate the response of the gel does not depend on changes in mean dose rate on the order of seconds to minutes. However, the machine dose rate remains, indicating variations in dose rate on the order of milliseconds influence the response of the gel. An attempt is made to mitigate the effect by increasing the concentration of antioxidant in the gel system but results in reduced overall response. Further work is performed to determine if self-crosslinking of one of the gel components contributes to the observed machine dose rate dependence.
In summary, this dissertation has significantly advanced the field of gel dosimetry by providing a prototype CT PGD system with enhanced dose resolution for complex RT treatment verification. / Graduate / 0992 / 0495 / 0756 / holly.johnston@utsouthwestern.edu
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/4939 |
| Date | 20 September 2013 |
| Creators | Johnston, Holly A. |
| Contributors | Hilts, Michelle Louise, Jirasek, Andrew |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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