GEANT4 is a Monte Carlo code developed in an object-oriented environment. It provides three models of electromagnetic physics, namely Standard, Low-energy, and Penelope. In this work, we examined the accuracy of the photon and electron interactions, and developed a user code for external beam radiotherapy simulations. The code has the capabilities to collect phase space data, perform variance reduction schemes, and calculate dose distributions in a phantom. For all three models, the dose distributions of monoenergetic and clinical photon beams in water are in good agreement with the EGSnrc simulations. For electron beams, the results depend on the maximum electron step size. Significant step size artifacts are also found in the transport of electrons through a thin aluminum slab. Our studies of interface perturbation effects and Fano cavity response illustrated potential problems with the condensed history algorithm. Overall, GEANT4 can be used in applications where electron transport is not critical.

The aim of stereotactic radiosurgery is to deliver a high and uniform radiation dose to the target volume and a minimized dose to the surrounding healthy tissue. Various linac-based radiosurgical techniques are used clinically: multiple non-coplanar converging arcs, dynamic arc rotation, and conical rotation. The techniques differ in their beam distribution over the patient's head. / A study of the beam distribution characteristics for the clinical linac-based radiosurgical techniques is presented. Two spiral linac-based radiosurgical techniques are developed: the uniform dose-rate spiral irradiation and the dose-rate-weighted spiral irradiation. Both exhibit the same spiraling beam entry trace over the patient's head; however, they differ in their beam distribution along the spiral. The dose-rate-weighted spiral irradiation provides a uniform beam distribution over the 2pi solid angle available in radiosurgery. / The currently existing techniques and the spiral techniques are then compared using the cumulative dose-volume histogram (CDVH) tools available with the McGill Treatment Planning System (MPS). The dose-rate-weighted spiral technique leads to lower dose inhomogeneities within the target volume and better dose conformity within the target. Moreover, it also encompasses smaller volumes of tissue at all isodose levels with larger differences at low isodose levels. A conclusion is reached that the dose-rate-weighted spiral irradiation technique offers interesting advantages over the currently used clinical linac-based techniques.

Polymer gel dosimetry, in conjunction with x-ray computed tomography (x-ray CT) imaging, is a three-dimensional dosimetric tool that shows promise in the verification of complex radiation therapy treatments. Previous studies have shown that x-ray CT imaging of gel dosimeters is robust, easy-to-use, and has wide clinical accessibility. The effects of x-ray CT dose imparted to the gel dosimeter, during imaging of the delivered therapy dose distributions, is not well understood. This thesis quantifies the effects of CT dose on normoxic polyacrylamide gel (nPAG) dosimeters. The investigation is comprised of four parts. First, quantification of the x-ray CT dose given during CT imaging of nPAG gels was measured using ion chamber measurements and filmed dose profiles for a range of typical gel dosimetry imaging protocols (200 mAs (current-time), 120-140 kVp (peak potential energy of photons), 2-10 mm slice thickness). It was found that CT doses ranged from 0.007 Gy/slice (120 kVp, 2 mm) to 0.021 Gy/slice (140 kVp, 10 mm) for volumetric phantoms. Second, Raman spectroscopy was used to determine the effect of photon energy on the dose response of nPAG dosimeters exposed to photon energies from a CT scanner (140 kVp photons) and from a Linac (6 MV photons). A weaker response was exhibited within the gels irradiated with kV photons than MV photons. Thirdly, the measurements of the given x-ray CT dose as established in the first study and the dose response of the polymer gel to different photon energies in the second study were correlated to estimate the induced changes of the nPAG CT number ("NCT ), caused by x-ray CT imaging of the polymer gel. (CT number is defined to be the measured attenuation coefficient normalized to water.) For typical gel imaging protocols (as above with 16-32 image averages), it was found that "NCT <0.2 H is induced in active nPAG gel dosimeters. This "NCT is below the current threshold of detectability of CT nPAG gel dosimetry. Finally, the traditional method of chemically fixing the dose response mechanism of nPAG gels by passive oxygenation of the gel, is investigated to determine if oxygenation would mitigate the changes caused by x-ray CT imaging of the gels. It was determined that oxygen diffusion was too slow to cause fixation of nPAG dosimeters, as the diffusion constant was 1.2 ± 0.2 × 10−6cm2/s, or 25% of the diffusion constant for anoxic PAG gel dosimeters. In conclusion, it was found that x-ray CT dose in polymer gel dosimeters is not a concern for standard gel imaging protocols. X-ray CT dose can potentially be a concern when large numbers of image averages (e.g. >60 image averages) are utilized, as in gel imaging protocols for high-resolution scans.

No description available.

No description available.

Silicates analogous to cosmic dust were synthesized, modified, and analyzed utilizing ion-beam techniques with Rutherford backscattering spectrometry (RBS) and x-ray diffraction (XRD). Silicate dust is a common constituent in interstellar space, with an estimated 50% of dust produced in the stellar winds of M class Asymptotic Giant Branch (AGB) stars. Silicate dust acts as a surface upon which other chemicals may form (water ice for example), increasing significance in the cosmochemistry field, as well as laboratory astrophysics. Silicate formation in the stellar winds of AGB stars was simulated in the laboratory environment. Three sequential ion implantations of Fe-, MgH2-, and O- with thermal annealing were used to synthesize a mixture appropriate to silicate dust in the surface layers of a p-type Si substrate. Post implantation He+ irradiation was shown to preferentially induce crystalline formation in the analogue prior to thermal annealing. This effect is believed to originate in the ion-electron interaction in the Si substrate. The effects of ionization and ion energy loss due to electronic stopping forces is believed to precipitate nucleation in the amorphous media. For annealing temperatures of 1273 K, predominant quartz formation was found in the substrate, whereas lower annealing temperatures of 1000 K formed enstatite without post-implantation He+ irradiation, and olivine with He+ irradiation. Post annealed crystalline phase modification was investigated via x-ray diffraction and elemental compositions were investigated utilizing RBS. Finally, the interdiffusion of Fe and Mg at temperatures of 900-1100 K was investigated with RBS, and activation energies for interdiffusion were extracted for the transition from amorphous to crystalline phase in the silicate analogues. Fe had an interdiffusion energy of 1.8 eV and Mg 1.5eV. The produced analogues have similar properties to those inferred from infrared spectroscopy of the stellar winds of M-class AGB stars with an oxygen-rich outflow. This work established a method of silicate production using ion beam modifications, explored He+ irradiation effects in the annealed structures, and derived interdiffusion activation energies for Fe and Mg in the amorphous structure. Grain sizes were <100 nm with the observed formation of quartz, enstatite, and olivine.

<p>Research using mice and rats has gained interest because they are robust test beds for clinical drug development and are used to elucidate disease etiologies. Blood vessel visualization and blood flow measurements are important anatomic and physiologic indicators to drug/disease stimuli or genetic modification. Cardio-pulmonary blood flow is an important indicator of heart and lung performance. Small animal functional imaging provides a way to measure physiologic changes minimally-invasively while the animal is alive, thereby allowing for multiple measurements in the same animal with little physiologic perturbation. Current methods of measuring cardio-pulmonary blood flow suffer from some or all of these limitations-they produce relative measurements, are limited to global or whole animal or organ regions, do not provide vasculature visualization, limited to a few or singular samples per animal, are not able to measure acute changes, or are very invasive or requires animal sacrifice. The focus of this work was the development of a small animal x-ray imaging system capable of minimally invasive real-time, high resolution vascular visualization, and cardio-pulmonary blood flow measurements in the live animal. The x-ray technique used was digital subtraction angiography (DSA). This technique is a particularly appealing approach because it is easy to use, can capture rapid physiological changes on a heart beat-to-beat basis, and provides anatomical and functional vasculature information. This DSA system is special because it was designed and implemented from the ground up to be optimized for small animal imaging and functional measurements. This system can perform: 1) minimally invasive in vivo blood flow measurements, 2) multiple measurements in the same animal in a rapid succession (every 30 seconds-a substantial improvement over singular measurements that require minutes to acquire by the Fick method), 3) very high resolution (up to 46 micron) vascular visualization, 4) quantitative blood flow measurements in absolute metrics (mL/min instead of arbitrary units or velocity) and relative blood volume dynamics from discrete ROIs, and 5) relative mean transit time dynamics on a pixel-by-pixel basis (100 µm x 100 µm). The end results are 1) anatomical vessel time course images showing the contrast agent flowing through the vasculature, 2) blood flow information of the live rat cardio-pulmonary system in absolute units and relative blood volume information at discrete ROIs of enhanced blood vessels, and 3) colormaps of relative transit time dynamics. This small animal optimized imaging system can be a useful tool in future studies to measure drug or disease modulated blood flow dynamics in the small animal.</p> / Dissertation

No description available.

Les néphropaties (maladie des tissus rénaux) postradiques constituent l'un des facteurs limitants pour l'élaboration des plans de traitement lors des radiothérapies abdominales. Le processus actuel, qui consiste à évaluer la fonctionnalité relative des reins grâce à une scintigraphie gamma deux dimensions, ne permet pas d'identifier les portions fonctionnelles qui pourraient être évitées lors de l' élaboration des plans de traitement. Une méthode permettant de cartographier la fonctionnalité rénale en trois dimensions et d'extraire un contour fonctionnel utilisable lors de la planification a été développée à partir de CT double énergie injectés à l'iode. La concentration en produit de contraste est considérée reliée à la fonctionnalité rénale. La technique utilisée repose sur la décomposition à trois matériaux permettant de reconstruire des images en concentration d'iode. Un algorithme de segmentation semi-automatisé basé sur la déformation hiérarchique et anamorphique de surfaces permet ensuite d'extraire le contour fonctionnel des reins. Les premiers résultats obtenus avec des images patient démontrent qu'une utilisation en clinique est envisageable et pourra être bénéfique. / Post-radic nephroaties (illness of kidney tissues) are one of the limitating factors when realising treatment planning for abdominal radiotherapies. The present process which consists in evaluating the Differential Kidney Function with a two-dimensional gamma scintigraphy, does not allow any identification of functional parts that could be taken into account during planification. The method presented in this document gives a three dimensional functional map of the kidney and a functional contour usable in planification, using double energy CT images with iodine contrast agent.The concentration in contrast agent is assumed to be related to kidney function. The proposed method is based on a three-materials decomposition allowing to reconstruct images of iodine concentration. The functionnal contour is extracted through a semi-automatic algorithm based on hierarchic and anamorphic surface deformation. First results obtained with patient images demonstrate the possibility of several future clinical applications.

Durant la dernière décennie, les développements technologiques en radiothérapie ont transformé considérablement les techniques de traitement. Les nouveaux faisceaux non standard améliorent la conformité de la dose aux volumes cibles, mais également complexifient les procédures dosimétriques. Puisque des études récentes ont démontré l’invalidité de ces protocoles actuels avec les faisceaux non standard, un nouveau protocole applicable à la dosimétrie de référence de ces faisceaux est en préparation par l’IAEA-AAPM. Le but premier de cette étude est de caractériser les facteurs responsables des corrections non unitaires en dosimétrie des faisceaux non standard, et ainsi fournir des solutions conceptuelles afin de minimiser l’ordre de grandeur des corrections proposées dans le nouveau formalisme de l’IAEA-AAPM. Le deuxième but de l’étude est de construire des méthodes servant à estimer les incertitudes d’une manière exacte en dosimétrie non standard, et d’évaluer les niveaux d’incertitudes réalistes pouvant être obtenus dans des situations cliniques. Les résultats de l’étude démontrent que de rapporter la dose au volume sensible de la chambre remplie d’eau réduit la correction d’environ la moitié sous de hauts gradients de dose. Une relation théorique entre le facteur de correction de champs non standard idéaux et le facteur de gradient du champ de référence est obtenue. En dosimétrie par film radiochromique, des niveaux d’incertitude de l’ordre de 0.3% sont obtenus par l’application d’une procédure stricte, ce qui démontre un intérêt potentiel pour les mesures de faisceaux non standard. Les résultats suggèrent également que les incertitudes expérimentales des faisceaux non standard doivent être considérées sérieusement, que ce soit durant les procédures quotidiennes de vérification ou durant les procédures de calibration. De plus, ces incertitudes pourraient être un facteur limitatif dans la nouvelle génération de protocoles. / During the past decade, technological developments in radiation therapy have considerably transformed treatment techniques. Novel nonstandard beams improve target dose conformity, but increase the complexity of dosimetry procedures. As recent studies demonstrated the invalidity of these protocols to nonstandard beams, a new protocol applicable to nonstandard beam reference dosimetry is in preparation by the IAEA-AAPM. The first goal of the study is to characterize the factors responsible for non-unity corrections in nonstandard beam dosimetry, and provide conceptual solutions to minimize the magnitude of the corrections. The second goal is to provide methods to estimate uncertainties accurately in nonstandard beam dosimetry, and estimate uncertainty levels achievable in typical clinical situations. Results of this study show that reporting dose to the sensitive volume of the chamber filled with water reduces the correction factor approximately by half under high gradients. A theoretical expression of correction factor is obtained for ideal nonstandard reference fields. In radiochromic film dosimetry, levels of uncertainty of the order of 0.3% are achieved with strict procedures and show great potential for nonstandard beam measurements. Results also suggest that experimental uncertainties in nonstandard beam are an important issue to consider both during daily QA routine and reference dosimetry, and could be a limiting factor in the new generation of protocols.