Thesis ... of the University of Pennsylvania ... by Ben Leon Glascock.

Thesis (MSc (Medical Physics))--University of Limpopo (Medunsa Campus),2010. / Key words: Monte Carlo simulation, MCNP5 code, Beta irradiation, Teflon-encased eye applicator, Dosimetry, Strontium-90 (Sr-90) Introduction: The treatment of various superficial lesions of the eye and skin has been conducted for many years, using Strontium-90 ophthalmic applicators. The dosimetry of the Sr-90 eye applicator is necessary, since it helps to determine a precise dose distribution within the eye globe. This also aids in optimizing the dose to be delivered to the target tissue of the eye without harming normal tissues, through surface dose rate determination. Thus, the surface dose rates are used to determine the lens and sclera dose, and also to specify the effectiveness of the applicator. These eye applicators are no longer manufactured and are commercially unavailable, because they have gone out of fashion. Those available are more than 20 years old. Due to recurrence in pterygium, glaucoma surgery enhancement and treatment of conjunctivae, the resurgence of the Sr-90 eye applicator is clinically needed. Hence, the Department of Medical Physics (University of Limpopo, MEDUNSA) proposed a new model of the Sr-90 ophthalmic applicator called the Teflon-encased eye applicator. Aim: To determine the radiation depth dose rate distributions of the Teflon-encased eye applicator, and to compare the calculated dose rates with that of the standard eye applicator (SIA. 8975) previously used and studied in MEDUNSA. Material and method: MCNP5 version 1.20 based Monte Carlo code was used. The first step involves verification of strontium-90 (Sr-90) and Yttrium-90 (Y-90) spectra. Second step, a new applicator model was designed. The third step, applicator was setup with water phantom, to determine dose distribution in water. Surface dose rate and central axis depth dose rate distributions were calculated. These were obtained in three different phases by varying the thickness of Teflon, different sources and changing the surface source distance (SSD) in order to determine their effects on central axis depth dose rates 2 and surface dose rates. The relationship of results was verified by correlation and ANOVA F- tests. Results and discussion: All spectra were demonstrated to be as reliable and accurate with relative errors ranging up to 7.9%, and correspond well to published available spectra. A Teflon thickness of 0.1 cm was sufficient to filter out and suppresses Sr-90 beta particles, and gave maximum beta penetration of 0.8 cm. No betas reached the back side of the applicator shaft. Only about 90% of the initial source dose escaped Teflonencased eye applicator. The surface dose rate increased exponentially with a decrease in Teflon thickness with regression coefficient of 97%. It also decreased linearly with increase in SSD and source thickness with a variation correlation of 99% and 99%, respectively. The source thicknesses of 0.03 cm, 0.04 cm, 0.045 cm and 0.05 cm gave closest results of 38.32 cGy/s ± 2.7%, 36.45 cGy/s ± 2.8%, 34.90 cGy/s ± 2.8% and 32.75 cGy/s ± 1.5% respectively, to the standard eye applicator having 36.55 cGy/s ± 2.5%. The depth dose results have a strong correlation and significance of 99%. An increased of Teflon thickness from 0.1 cm to 0.125 cm lead to a 27% decrease in central axis depth dose rate. All ten statistical checks from MCNP were passed with average relative error of ±3%, at one standard deviation. The accuracy of calculated central axis depth dose rates was within 5%. Conclusion: The central axis depth dose rate of the Teflon-encased eye applicator can only be calculated at a distance less than 0.5 cm depth of water, due to the applicator’s geometry. The geometry, materials, applicator shape, source size, and distance between source and phantom, input spectra and MCNP code used caused differences in results. However it was possible to minimise the differences. The surface dose rate can only be defined at a depth of 0.01 cm in a water phantom in order to accurately estimate the dose to lens and sclera. The dosimetry of the Teflon-encased eye applicator is similar to that of a standard eye applicator. Also, this newly modeled applicator is effective and it can be manufactured for clinical treatment purposes. Key words: Monte Carlo simulation, MCNP5 code, Beta irradiation, Teflon-encased eye applicator, Dosimetry, Strontium-90 (Sr-90) Introduction: The treatment of various superficial lesions of the eye and skin has been conducted for many years, using Strontium-90 ophthalmic applicators. The dosimetry of the Sr-90 eye applicator is necessary, since it helps to determine a precise dose distribution within the eye globe. This also aids in optimizing the dose to be delivered to the target tissue of the eye without harming normal tissues, through surface dose rate determination. Thus, the surface dose rates are used to determine the lens and sclera dose, and also to specify the effectiveness of the applicator. These eye applicators are no longer manufactured and are commercially unavailable, because they have gone out of fashion. Those available are more than 20 years old. Due to recurrence in pterygium, glaucoma surgery enhancement and treatment of conjunctivae, the resurgence of the Sr-90 eye applicator is clinically needed. Hence, the Department of Medical Physics (University of Limpopo, MEDUNSA) proposed a new model of the Sr-90 ophthalmic applicator called the Teflon-encased eye applicator. Aim: To determine the radiation depth dose rate distributions of the Teflon-encased eye applicator, and to compare the calculated dose rates with that of the standard eye applicator (SIA. 8975) previously used and studied in MEDUNSA. Material and method: MCNP5 version 1.20 based Monte Carlo code was used. The first step involves verification of strontium-90 (Sr-90) and Yttrium-90 (Y-90) spectra. Second step, a new applicator model was designed. The third step, applicator was setup with water phantom, to determine dose distribution in water. Surface dose rate and central axis depth dose rate distributions were calculated. These were obtained in three different phases by varying the thickness of Teflon, different sources and changing the surface source distance (SSD) in order to determine their effects on central axis depth dose rates 2 and surface dose rates. The relationship of results was verified by correlation and ANOVA F- tests. Results and discussion: All spectra were demonstrated to be as reliable and accurate with relative errors ranging up to 7.9%, and correspond well to published available spectra. A Teflon thickness of 0.1 cm was sufficient to filter out and suppresses Sr-90 beta particles, and gave maximum beta penetration of 0.8 cm. No betas reached the back side of the applicator shaft. Only about 90% of the initial source dose escaped Teflonencased eye applicator. The surface dose rate increased exponentially with a decrease in Teflon thickness with regression coefficient of 97%. It also decreased linearly with increase in SSD and source thickness with a variation correlation of 99% and 99%, respectively. The source thicknesses of 0.03 cm, 0.04 cm, 0.045 cm and 0.05 cm gave closest results of 38.32 cGy/s ± 2.7%, 36.45 cGy/s ± 2.8%, 34.90 cGy/s ± 2.8% and 32.75 cGy/s ± 1.5% respectively, to the standard eye applicator having 36.55 cGy/s ± 2.5%. The depth dose results have a strong correlation and significance of 99%. An increased of Teflon thickness from 0.1 cm to 0.125 cm lead to a 27% decrease in central axis depth dose rate. All ten statistical checks from MCNP were passed with average relative error of ±3%, at one standard deviation. The accuracy of calculated central axis depth dose rates was within 5%. Conclusion: The central axis depth dose rate of the Teflon-encased eye applicator can only be calculated at a distance less than 0.5 cm depth of water, due to the applicator’s geometry. The geometry, materials, applicator shape, source size, and distance between source and phantom, input spectra and MCNP code used caused differences in results. However it was possible to minimise the differences. The surface dose rate can only be defined at a depth of 0.01 cm in a water phantom in order to accurately estimate the dose to lens and sclera. The dosimetry of the Teflon-encased eye applicator is similar to that of a standard eye applicator. Also, this newly modeled applicator is effective and it can be manufactured for clinical treatment purposes.

Reproduction de : Thèse de doctorat : Génie des procédés et de l'environnement : Toulouse, INPT : 2007. / Titre provenant de l'écran-titre. Bibliogr. 46 réf.

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Thesis (M.A.)--University of Wisconsin--Madison. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves [80-88]).

The kinetics of the reaction of solid strontium chloride with fluorine gas have been studied by gravimetric, thermometric, and microphotographic methods. ESR and X-ray crystallography were used to study the products. The reaction commenced after an induction period of 1 to 10 minutes. On single crystal specimens studied microscopically, formation and growth of nuclei of SrF₂ thereafter occupied several minutes before the nuclei coalesced to form a continuous SrF₂ layer. By thermometric studies on a polycrystalline boule of reactant on a thermocouple, the extent of reaction during the nucleation period was found to be proportional to t⁴ , suggesting nuclei formed proportional to t² and subsequent two-dimensional growth at constant linear rate. On single crystal specimens, microphotography showed a t² law for number of nuclei only for one specimen with a rough surface. For smooth surfaces, number of nuclei was generally constant, but linear growth was confirmed in many cases. Two growth rates were measured, an initial rate of 6.4 x 10ˉ⁴ mm sec ˉ¹ and a less reproducible rate to which a transition sometimes occurred in later stages of 1.7 x 10ˉ³mm secˉ¹. The nucleation was found to be non-activated and the change in rate was ascribed, together with an increase in the number of nuclei late in the nucleation period, to effects of mechanical strain. The development of the main reaction after establishment of a continuous reaction interface was followed gravimetrically, and found to obey the Ginstling-Brounshtein equation for diffusion through a spherical shell of solid reaction product, having a sharp interface with the reactant. A lower limit of 2 x 10ˉ⁵ cm² secˉ¹ was found for the diffusion coefficient, suggesting that the process is gaseous diffusion in cracks in the product layer. The crystallinity of the product depended on the rate of reaction. For rapid, high-temperature reaction, the product gave a powder diffraction pattern, but for a sample reacted more slowly with a controlled supply of F₂, the product was found to be essentially a single crystal (diffuse diffraction spots indicating ranges of disorientation of no more than about 5°) with the same crystallographic orientation as the reactant. Attempts to locate the ESR signal found in earlier work were only partially successful, but suggest that the signal is largely in the product phase, and that it represents a byproduct rather than a reaction intermediate. / Science, Faculty of / Chemistry, Department of / Graduate

Strontium is a rare earth element, present in products such as pyrotechnics, medications, glass and certain pigments. Exposure of humans to strontium mainly comes through dietary means, through the consumption of food and water. While high levels of strontium have been shown to be toxic in animal studies, low levels may be beneficial, such as for the treatment of osteoporosis. Some women in Canada choose to self-supplement with strontium with the intention of preventing this bone disease. At present, there is no clinical tool to monitor strontium levels in these women. A technology that could montior women would be useful as it would allow the determination of whether the self-supplementation is indeed beneficial. To measure strontium in humans, a non-invasive, non-destructive technique called X-ray fluorescence (XRF) is used. This thesis describes work to develop improved technology for in vivo measurements of strontium in bone using XRF. A new XRF system for measuring strontium in bone was designed around a VITUS H150 Silicon Drift Detector (SDD) from KETEK GmbH, and used a 109Cd source in a 180º backscatter geometry. The system was calibrated against a series of anthropomorphic finger phantoms which were 3D printed with a strontium doped hydroxyapatite core and varying polylactic acid (PLA) thicknesses to simulate different thicknesses of soft tissue. Phantoms with a range range of strontium concentrations were created to test the system. It was determined that the new system was able to perform as well as previously tested radioisotope-based in vivo strontium XRF systems, with the system having the potential to perform significantly better if a significantly more active source could be employed. Calibration using the 3D printed phantoms was also found to perform extremely well, indicating that this phantom methodology is a viable way to make more anatomically correct calibration phantoms in the future. A Monte Carlo model was created in the EGS 5 code of the experimental geometry and the model performance was benchmarked against experimental data. This model was then used to test two separate issues. First, the model was used to determine the validity of coherent normalization for in vivo strontium measurements in the finger. Second, the model was used to determine if there was a radioisotope source that could result in better performance of the system. The coherent normalization was shown to not be valid in terms of correction for soft tissue attenuation, but may be valid as a correction method for errors in positioning or patient motion. In combination with a new Compton correlation method that can estimate the thickness of overlying soft tissue, the implementation of coherent normalization would reduce variability in the system’s measurements of the strontium signal. Finally, through the testing of alternative radioisotope sources, 103Pd was identified as a promising alternative source of fluorescing photons and it is recommended that an experimental XRF system employing this source be tested to verify this result. / Dissertation / Doctor of Philosophy (PhD)

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