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X-ray computed tomography for performing polymer gel dosimetry : a feasibility study

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Title: X-ray computed tomography for performing polymer gel dosimetry : a feasibility study
Author: Hilts, Michelle Louise
Degree Master of Science - MSc
Program Physics
Copyright Date: 1999
Abstract: Radiation therapy treatment of cancer is increasingly concerned with delivering dose distributions that conform to the tumour volume. For verification of treatment planning computer dose calculations, these conformal therapies demand an accurate, sensitive, high resolution three dimensional (3D) dosimeter. Polymer gels are novel, inherently 3D, tissue equivalent radiation dosimeters. Traditionally, dose distributions recorded in polymer gel are read out using magnetic resonance imaging (MRI). Presented here is a feasibility study on a new 3D dosimetry technique that uses x-ray computed tomography (CT) to read dosimetric information from polymer gels. The technique exploits a gel density change that occurs in response to ionizing radiation. This study has three main goals: 1) to develop a protocol for producing quality CT polymer gel images; 2) to evaluate the nature and reproducibility of the dosimeter's CT number (JVcr)-dose response; and 3) to compare this technique with MRI polymer gel dosimetry. A quantitative discussion of the density changes occurring in the gel in response to ionization radiation is also provided. Experiments are conducted using a PAG (polyacrylamide and gelatin) gel dosimeter irradiated with four intersecting 10 M V photon beams. The NCT -dose response is found to be linear and reproducible over the range of 200 to 1OOOcGy. At room temperature the response is (8.7 ± 0.3)xl0⁻³ NCT/cGy resulting in a limited dose resolution, - 100 cGy. Gel temperature during imaging is determined to have only a small effect, 0.3%/°C, on the dose response. Spatial resolution is 0.5 mm in the image plane and 10mm-1mm (depending on noise requirements) in the third dimension. Despite the low dose resolution, preliminary results indicate this technique provides accurate localization of high dose regions and, given the availability and speed of CT imaging, has the potential to be a valuable and practical tool for radiation therapy clinics.
URI: http://hdl.handle.net/2429/9653
Series/Report no. UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/]
Scholarly Level: Graduate

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