The spin-lattice relaxation rate R$ sb1$ of irradiated Fricke solution was studied as a function of the absorbed dose D. The R$ sb1$ increases linearly with D up to a dose of $ sim$250 Gy after which the response saturates. A model describing the R$ sb1$ of a solution of either ferrous (Fe$ sp{2+})$ or ferric (Fe$ sp{3+})$ ions is presented; it is based on fast exchange between protons on water molecules in the bulk and protons on water molecules in the coordination shell of the ions. All inherent relaxation parameters of the different proton groups are determined. An extension of the model is made to describe the spin-lattice relaxation behaviour of irradiated Fricke solution. Good agreement between model predictions and experimental results is observed. The model relates the spin-lattice relaxation rate of a Fricke dosimeter to the chemical yield of ferric ion, thus creating an absolute dosimetry technique. Various practical aspects of the NMR-Fricke system are described.

No description available.

Biological effects and prostate motion were studied for prostate cancer patients treated with external beam radiotherapy. The prostate motion was determined using an ultrasound-based patient positioning system (BAT: B-mode Acquisition and Targeting) just after conventional patient setup. The changes in planned biological effects due to prostate motion were calculated for the prostate target organ and for the rectum and bladder normal tissues using TCP (tumor control probability) and NTCP (normal tissue complication probability) calculations, respectively. The prostate TCP calculations were carried out for both prostate PTV and GTV structures, whereas the rectum and bladder wall structures were used for the NTCP calculations.

In a large multi-center study where many groups collaborate to produce data, the conditions under which the data are collected can vary significantly. In the case of the NIH funded MRI study of normal brain development (NIHPD), 6 centers across the US collect MRI scans of the developing child brain. In all, four different models of scanners from 2 manufacturers are used. The subject of this thesis is to quantify differences in acquired data that are due to scanner differences and thus enable improved biological study. / The aim of this study is to analyze the scanner-induced differences across sites using a specialized phantom recommended by the American College of Radiology (ACR). Together with the 7 tests listed by the ACR MRI accreditation manual, phantom data from each site can be analyzed for parameters that are characteristic of image quality. The measurements are done automatically using programs written at the MNI, that use MINC tools and the MNI autoreg package to compute the necessary parameters. / The results demonstrate that, in general, all data obtained complied with the 7 tests of the ACR MRI manual. Statistically significant variations over time and between sites were found in the data. However, the magnitude of the variations is of the order of 1%-2% indicating stable data acquisitions and comparability of data between sites.

Radiosurgery is characterized by high radiation doses, delivered via small diameter radiation beams in a single session, placing stringent requirements on the numerical and spatial accuracy of dose delivery to the target volume within the brain. In this thesis, physical and clinical aspects of radiosurgery are discussed, including a method for the production of cylindrical dose distributions with rectangular beams using cylindrical dynamic rotation. / The measurements of radiosurgical x-ray beam parameters are presented. Monte Carlo simulations determine that a measured increase depth of dose maximum with increasing field size is a result of primary dose deposition in phantom for small diameter beams. / An analytical representation based on a curve-fitting process is developed to parametrize radiosurgical x-ray beam percentage depth doses as a function of depth in phantom, field diameter and beam energy using bi-exponential and polynomial functions. / Measurements of dose in the build-up region of x-ray beams ranging from 1 x 1 cm$ sp2$ to 30 x 30 cm$ sp2$ show that the depth of dose maximum increases rapidly with increasing field size at small fields, reaches a maximum around 5 x 5 cm$ sp2$ and then gradually decreases with increasing field size for large fields. Monte Carlo simulations attribute effect observed at large fields to the scatter contamination of the primary beam from the linac head. This scatter contamination is measured by a half-block technique and further experiments show that it consists of electrons originating in the flattening filter of the linac.

An interactive 3D target localisation and delineation tool has been developed for radiosurgery planning of arteriovenous malformations (AVMs). With this system, magnetic resonance (MR), MR angiography (MRA) and computed tomography (CT) volumes can be fused in stereotactic space. Stereotactic angiography (SA) images can be linked to the MRA volume by recovering the SA acquisition geometry. The MRA and SA images can be correlated (1) by ray-tracing through the MRA volume with the recovered SA acquisition geometry and overlaying the images onto the SA images and (2) by localising the AVM onto a volume rendered representation of the MRA with a 3D cursor and projecting its position onto the SA images. Target contours can then be drawn on the MRA/MR/CT images and simultaneously projected onto the SA images. / The plans of patients who had previously undergone radiosurgery at our institution employing SA images for localisation and MR images for delineation were investigated. MRA datasets were also acquired at the time of MR scanning employing the 3D TOF technique. Some ray-traced MRA images correlated well visually with the SA images, others presented inconsistencies which suggest that MRA should be used only as complement to SA images. / The role of the different modalities (M-RA, MR and SA) in the definition of target volumes is investigated by defining the target contours with different combinations of modalities within the interactive system. The target volumes drawn with different modalities were compared to a reference volume, drawn using MRA, MR and SA images, and presented underestimation and overestimation of target volumes ranging from 20% to 92% and from 3% to 40%. / The dosimetric implications of image fusion for target delineation are investigated by retrospective evaluation of the dose coverage of the reference target volume by the original treatment plan. Target coverage inferior to 60% of the reference target volumes by the original treatment plans was obtained for the patients. Treatment plan optimisation was performed to evaluate the possible dosimetric improvements resulting from image fusion for AVM target delineation. The plans were improved by employing three, eight and four isocentres for the 3 patients, and resulted in target coverage equal or superior to 98% for all three patients.

Electron beams are used to treat superficial lesions in radiation oncology by taking advantage of the sharp dose fall-off and the limited range of the particles in tissue. The irregular shape of individual tumors, however, often requires custom made shielding in order to geometrically shape the radiation field to the target, while minimizing the dose to surrounding tissues. In many institutions, low melting alloy or lead cutouts are used for electron beam shaping. In this work, electron dosimetry beam parameters such as percentage depth dose (PDD), outputs, and beam profiles, were measured with ten different electron beams from two linear accelerators. The dependence of beam characteristics on field size and shape, particularly for small cutouts, was investigated. In addition, this project examined different methods for measuring electron PDDs, including film densitometry, ion chambers, and diode dosimetry. / The work presented here demonstrates that the depth dose effect is significant when one of the field dimensions of the cutout is less than R p, the practical range of electrons. For these cutouts, it was observed that both PDD and outputs vary significantly due to the lack of lateral electronic equilibrium. As the cutout becomes smaller, the depth of dose maximum (dmax) shifts towards the surface, while the output at dmax decreases. Therefore, it is crucial that PDDs and outputs are either measured or calculated for small field electron cutouts used in the clinical setting.

Intensity modulated photon beam radiation therapy often results in dynamically delivered beams with small field sizes and steep dose gradients. This defines a need for an integrating, tissue-equivalent, high resolution dosimeter. 3D ferrous sulfate gel based dosimetry involves the use of magnetic resonance (MR) images of radiosensitive paramagnetic gels. The goal of this work is to create a patient specific quality assurance (QA) procedure that links measured dosimetnc information to clinical goals. / The gel dosimeter system is tested through a set of simple experiments which characterize and confirm the system as a valid QA tool for conformal and intensity modulated radiation therapy. / For this work, dynamic photon beams are created on a commercially available inverse treatment planning system and the treatment is delivered to a gel filled acrylic mold. Software has been developed to quantify dose from the QA MR images, and to register this information to the planning computed tomography (CT) scan. The software displays the measured dose on the planning CT, and calculates dose-volume histograms for the registered measured data and contoured patient structures. This work reveals good agreement between planned and measured dose distributions, with less than 5% difference in the mean doses of the contoured patient structures.

During the past ten years, radiosurgery has moved from an obscure radiation treatment modality practiced in only a few specialized centers in the world, to a mainstream radiotherapeutic technique practiced in most major radiotherapy centers. Currently, the main thrust of development in radiosurgery is aimed at conformal dose delivery to irregular intracranial targets. This thesis deals with theoretical and practical aspects of the use of static, non-coplanar, conformal fields in radiosurgery. / For a typical radiosurgical case involving an irregular target, a comparison was made between treatment plans using the dynamic technique with one and two isocenters and a treatment plan using 7 fixed, non-coplanar, irregularly shaped beams. The static conformal fields plan achieved a target-dose conformation similar to the 2-isocenter dynamic plan, treating 2 to 3 times less healthy tissue to intermediate and high doses that did the 1-isocenter dynamic plan, while delivering a much more uniform dose to the target volume. / A comparison was also made between treatment plans using a varying number of static conformal fields. While the degree of tissue sparing and target-dose homogeneity were both shown to increase with the number of static fields, this increase was found to become smaller and smaller as the number of fields was successively raised from 5 to 7, from 7 to 9 and, ultimately, from 9 to 11. A conclusion is reached that a number of fields between 7 and 9 represents a reasonable compromise between the degree of tissue sparing and target-dose homogeneity achieved, and the ease with which the radiosurgical procedure is planned and delivered.

Following a departmental expansion, an isocentric cobalt-60 external beam teletheraphy unit was modified to produce a large fixed field for total body irradiation. The sourcehead was separated from the gantry and installed at a distance of 251.2 cm from the floor. The collimator was removed and replaced with a custom built secondary collimator projecting a 277 x 132.6 cm 2 radiation field at floor level. The work presented in this thesis describes the measurements performed to bring the unit into clinical use for total body irradiation. A custom flattening filter was placed below the secondary collimator to flatten the beam to within +/-3% of the central axis dose as measured at 10 cm in water. Percent depth dose, tissue-phantom-ratios, surface dose and absolute output were measured in the radiation field. The effects of inhomogeneities were studied and the thickness of lead used for lung attenuators was determined. Verification of treatment planning and delivery was performed with an Alderson-Rando anthropomorphic phantom and showed dose uniformity within +/-10% of the prescribed dose when a lead attenuator was used over the lung.