New radiation therapy (RT) techniques for treating cancer are continually under development. Our ability to demonstrate the safe and accurate implementation of new RT treatment techniques is dependent on the information provided by current dosimetric tools. Advanced dosimetric tools will become increasingly necessary as treatments become more complex. This work examines the readout of an advanced dosimeter --- the polyacrylamide, gelatin, and tetrakis (hydroxymethyl) phosphonium chloride (PAGAT) dosimeter --- using a prototype fan-beam optical computed tomography (CT) scanner. A number of developments sought to improve the performance of the optical CT device. A new fan-creation method (laser diode module) and new matching tank were introduced. Artefact removal techniques were developed to remove flask seam artefacts and ring artefacts via sinogram space. A flask registration technique was established to achieve reproducible placement of flasks in the optical CT scanner. A timing-correction technique was implemented to allow for the scanning of continuously rotating samples. A number of experiments examined factors related to the PAGAT dosimeter. Comparisons of post-irradiation scans to pre-irradiation scans improved dosimeter readout quality. Changes to the PAGAT dosimeter cooling/scanning routine provided further improvements to dosimeter readout. Evaluations of calibration curves showed that a linear calibration curve was less capable of describing PAGAT dose response than a quadratic calibration curve. Intra-gel calibration using another dose distribution was shown to be no less accurate than self calibration, but inter-gel calibrations saw a statistically significant increase in absolute readout errors. A set of investigations examined how optical CT scanning protocols affected readout quality for PAGAT dosimeters. Doubling the dose delivered to the dosimeter doubled the signal-to-noise ratio. Acquiring and averaging additional light profiles at each projection angle provided only slight reductions in readout noise. Sampling a higher number of projection angles provided substantial reductions in readout noise. Those reductions in readout noise were not lost when sinograms with many projections were encapsulated into sinograms of fewer projection angles. Detector element binning (sinogram space) and pixel binning (image space) also provided substantial reductions in readout noise. None of these elements of the scanning protocol had statistically significant effects on readout errors. Finally, distinct imaging artefacts seen throughout this work were shown to be caused by radiation-induced refractive index changes in PAGAT dosimeters. Radiation-induced refraction (RIR) artefacts result when dose gradients caused the refraction of fan-beam raylines towards high dose regions. A filtering technique was developed to remove RIR artefacts in sinogram space, but this technique caused substantial blurring to the measured dose distribution. / Graduate / 0760 / 0756 / 0752 / warreng1983@gmail.com

Dosimetry is a vital component of treatment planning in radiation therapy. Methods of radiation dosimetry currently include the use of: ionization chambers, thermoluminescent dosimeters (TLDs), solid-state detectors and radiographic film. However, these methods are inherently either 1D or 2D and their use involves the perturbation of the radiation beam. Although the dose distribution within tissues following radiation therapy treatments can be modeled using computerized treatment planning systems, a need exists for a dosimeter that can accurately measure dose distributions directly and produce 3D dose maps. Some radiation therapy and brachytherapy treatments require mapping the dose distributions in high-resolution (typically < 1 mm). A dosimetry technique that is capable of producing high resolution 3D dose maps of the absorbed dose distribution within tissues is required. Gel dosimetry is inherently a 3D integrating dosimeter that offers high spatial resolution, precision and accuracy. Polymer gel dosimetry is founded on the basis that monomers dissolved in the gel matrix polymerize due to the presence of free radicals produced by the radiolysis of water molecules. The amount of polymerization that occurs within a polymer gel dosimeter can be correlated to the absorbed dose. The gel matrix maintains the spatial integrity of the polymers and hence a dose distribution can be determined by imaging the irradiated polymer gel dosimeter using an imaging modality such as MRI, x-ray computed tomography (CT), ultrasound, optical CT or vibrational spectroscopy. Polymer gel dosimeters, however, suffer from oxygen contamination. Oxygen inhibits the polymerization reaction and hence polymer gel dosimeters must be manufactured, irradiated and scanned in hypoxic environments. Normoxic polymer gel dosimeters incorporate an anti-oxidant into the formulation that binds the oxygen present in the gel and allows the dosimeter to be made under normal atmospheric conditions. The first part of this study was to provide a comprehensive investigation into various formulations of polymer and normoxic polymer gel dosimeters. Several parameters were used to characterize and assess the performance of each formulation of polymer gel dosimeter including: spatial resolution and stability, temporal stability of the R2-dose response, optimal R2-dose response for changes in concentration of constituents and the effects of oxygen infiltration. This work enabled optimal formulations to be determined that would provide greater dose sensitivity. Further work was done to investigate the chemical kinetics that take place within normoxic polymer gel dosimeters from manufacture to post-irradiation. This study explored the functions that each of the constituent chemicals plays in a polymer gel dosimeter. Although normoxic polymer gel dosimeters exhibit very similar characteristics to polyacrylamide polymer gel dosimeters, one important difference between them was found to be a decrease in R2-dose sensitivity over time in the normoxic polymer gel dosimeter compared to an increase in the polyacrylamide polymer gel dosimeters. From an investigation into the function of anti-oxidants in normoxic polymer gel dosimeters, alternatives were proposed. Several alternative anti-oxidants were explored in this study that found that whilst some were reasonably effective, tetrakis (hydroxymethyl) phosphonium chloride (THPC) had the highest reaction rate. THPC was found not only to be an aggressive scavenger of oxygen, but also to increase the dose sensitivity of the gel. Hence, a formulation of normoxic polymer gel dosimeter was proposed, called MAGAT, that comprised: methacrylic acid, gelatin, hydroquinone and THPC. This formulation was examined in a similar fashion to the studies of the other formulations of polymer and normoxic polymer gel dosiemeters. The gel was found to exhibit spatial and temporal stability and an optimal formulation was proposed based on the R2-dose response. Applications such as IVBT require high-resolution dosimetry. Combined with high-resolution MRI, polymer gel dosimetry has potential as a high-resolution 3D integrated dosimeter. Thus, the second component of this study was to commission a micro-imaging MR spectrometer for use with normoxic polymer gel dosimeters and investigate artifacts related to imaging in high-resolutions. Using high-resolution MRI requires high gradient strengths that, combined with the Brownian motion of water molecules, was found to produce an attenuation of the MR signal and hence lead to a variation in the measured R2. The variation in measured R2 was found to be dependent on both the timing and amplitude of pulses in the pulse sequence used during scanning. Software was designed and coded that could accurately determine the amount of variation in measured R2 based on the pulse sequence applied to a phantom. Using this software, it is possible to correct for differences between scans using different imaging parameters or pulse sequences. A normoxic polymer gel dosimeter was irradiated using typical brachytherapy delivery and the resulting dose distributions compared with dose points predicted by the computerized treatment planning system.The R2-dose response was determined and used to convert the R2 maps of the phantoms to dose maps. The phantoms and calibration vials were imaged with an in-plane resolution of 0.1055 mm/pixel and a slice thickness of 2 mm. With such a relatively large slice thickness compared to the in-plane resolution, partial volume effects were significant, especially in the region immediately adjacent the source where high dose gradients typically exist. Estimates of the partial volume effects at various distances within the phantom were determined using a mathematical model based on dose points from the treatment planning system. The normalized and adjusted dose profiles showed very good agreement with the dose points predicted by the treatment planning system.

A dynamic mathematical model was developed to simulate the response of polyacrylamide gel (PAG) dosimeters to a single spherical radioactive brachytherapy seed. Simulations were conducted for a high dose-rate (HDR) seed using 192Ir and a low dose-rate (LDR) seed using 125I. The model is able to predict the amount of polymer formed, the crosslink density, and the volume fraction of aqueous phase as a function of radial distance and time. Results show that PAG dosimeters can provide accurate HDR brachytherapy dosimetry at distances larger than 4 mm from the centre of the seed but will give poor results for LDR due to monomer diffusion. Experiments were conducted to evaluate the potential for using pentacosa-10,12-diynoic acid (PCDA) as the reporter molecule in micelle gel dosimeters for optical computed tomography (CT) readout. Several gels containing PCDA that was solubilized using sodium dodecyl sulfate (SDS) responded to radiation by changing from colourless to blue. Unfortunately, all phantoms that showed colour changes were turbid, making them unsuitable for optical CT scanning. Several techniques were used to produce transparent gels containing PCDA but none of these gels responded noticeably to radiation. Only turbid gels with precipitated PCDA responded, indicating that the colour change was due to oligomerization within PCDA crystals and that PCDA molecules solubilized in micelles did not undergo oligomerization. As a result, PCDA is not suitable for use in radiochromic micelle gel dosimeters. A new recipe for a radiochromic leuco crystal violet (LCV) micelle gel dosimeters with enhanced dose sensitivity was developed for optical CT readout. The recipe contains LCV, trichloro acetic acid (TCAA), Cetyl Trimethyl Ammonium Bromide (CTAB), 2,2,2-Trichloroethanol (TCE), and gelatin. Experiments were conducted to improve understanding about interactions between the different components of LCV micelle gel, highlighting the importance of pH on dose sensitivity and transparency. Results also showed the effectiveness of chlorinated compounds in improving dose sensitivity. Statistical techniques were used to build empirical models that were used to optimize the gel recipe. Additional testing in larger phantoms will be required to assess the effectiveness of the proposed gel for clinical dosimetry. / Thesis (Ph.D, Chemical Engineering) -- Queen's University, 2014-03-27 11:11:47.655