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.

In radiation therapy it is very important to accurately measure the amount of radiation delivered. The effectiveness of the treatment depends on delivering the dose with an accuracy of 5% or better. The dosimetry in different clinics must also be consistent. For these reasons national and international calibration protocols have been developed. In the US, the American Association of Physicists in Medicine (AAPM) has published several national dosimetry protocols for the calibration of high-energy photon and electron beams. In this study the absorbed dose-to-water determined according to TG-21 and TG-51 protocols, developed by Task Group 21 and Task Group 51 of the Radiation Therapy of AAPM, are compared. The older protocol, TG-21, is based on exposure calibrated ionization chambers using a ⁶⁰Co beam. Many standards laboratories have started to replace exposure standards with those involving absorbed dose-to-water. The new protocol, TG-51, is based on absorbed dose-to-water calibrated ionization chambers using a ⁶⁰Co beam. Also, there are some differences between the beam quality specifiers and data proposed by the two protocols. A comparison between TG-21 and TG-51 protocols was done by determining the radiation dose rate at a designated distance for 6 and 18 MV photon beams, and 16 and 20 MeV electron beams, generated by Clinac a 2100 C linear accelerator. The cylindrical ionization chambers used in this study were Capintec PR-06G and PR-05. The results of the study show a discrepancy between the absorbed dose-to-water determined according to TG-21 and TG-51 protocols of about 1.4% and 1.7% for 6 and 18 MV photon beams, respectively. Absorbed dose-to-water determined according to TG-21 and TG-51 protocols for 16 MeV energy electron beams agree within 1.8%, while the ones of 20 MeV energy beams agree to within 2.4%. The change from exposure to absorbed dose-to-water calibrated ionization chambers has the largest impact on the differences between TG-21 and TG-51 absorbed dose-to-water, while the change in beam quality specifier and stopping power ratios have only a very small effect on these differences. The TG-51 protocol is very simple, minimizing the chance of mistakes, because it starts with absorbed dose-to-water calibration, while the TG-21 is very complex, starting with the calibration for exposure, which is different from the absorbed dose-to-water, the clinical quantity of interest. The TG-51 protocol allows the determination of a more accurate absorbed dose in a ⁶⁰Co beam than the TG-21 protocol since it uses an absorbed dose-to-water calibration factor directly measured, while the exposure based dosimetry system is dependent on external physical data which are not measured in clinics. / Graduation date: 2003

No description available.

Recent events have underscored the need for the United States government to provide streamlined emergency response procedures and subsequent dose estimations for personnel responding to incidents involving radioactive material. Indeed, the National Council on Radiation Protection and Measurements Report No. 138 (NCRP 2001) indicates that exposures received by first responders will be important for a number of reasons, including planning for the appropriate use of key personnel in an extended emergency situation. In response, the Department of Homeland Security has published Protective Action Guides (DHS 2006) to help minimize these exposures and associated risks. This research attempts to provide some additional radiological exposure knowledge so that an Incident Commander, with limited or no information, can make more informed decisions about evacuation, sheltering-in-place, relocation of the public, turn-back levels, defining radiation hazard boundaries, and in-field radiological dose assessments of the radiation workers, responders, and members of the public. A method to provide such insight begins with providing a model that describes the physics of radiation interactions, radiation source and geometry, collection of field measurements, and interpretation of the collected data. A Monte Carlo simulation of the model is performed so that calculated results can be compared to measured values. The results of this investigation indicate that measured organ absorbed doses inside a tissue equivalent phantom compared favorably to the derived organ absorbed doses measured by the Panasonic thermoluminescence dosimeters and with Monte Carlo ‘N’ Particle modeled results. Additionally, a Victoreen 450P pressurized ion chamber measured the integrated dose and these results compared well with the Panasonic right lateral TLD. This comparison indicates that the Victoreen 450P ionization chamber could potentially serve as an estimator of real-time effective dose and organ absorbed dose, if energy and angular dependence corrections could be taken into account. Finally, the data obtained in this investigation indicate that the MCNP model provided a reasonable method to determine organ absorbed dose and effective dose of a simulated Radiological Dispersal Device in an Inferior-Superior geometry with Na99mTcO4 as the source of radioactive material.

Recent events have underscored the need for the United States government to provide streamlined emergency response procedures and subsequent dose estimations for personnel responding to incidents involving radioactive material. Indeed, the National Council on Radiation Protection and Measurements Report No. 138 (NCRP 2001) indicates that exposures received by first responders will be important for a number of reasons, including planning for the appropriate use of key personnel in an extended emergency situation. In response, the Department of Homeland Security has published Protective Action Guides (DHS 2006) to help minimize these exposures and associated risks. This research attempts to provide some additional radiological exposure knowledge so that an Incident Commander, with limited or no information, can make more informed decisions about evacuation, sheltering-in-place, relocation of the public, turn-back levels, defining radiation hazard boundaries, and in-field radiological dose assessments of the radiation workers, responders, and members of the public. A method to provide such insight begins with providing a model that describes the physics of radiation interactions, radiation source and geometry, collection of field measurements, and interpretation of the collected data. A Monte Carlo simulation of the model is performed so that calculated results can be compared to measured values. The results of this investigation indicate that measured organ absorbed doses inside a tissue equivalent phantom compared favorably to the derived organ absorbed doses measured by the Panasonic thermoluminescence dosimeters and with Monte Carlo ‘N’ Particle modeled results. Additionally, a Victoreen 450P pressurized ion chamber measured the integrated dose and these results compared well with the Panasonic right lateral TLD. This comparison indicates that the Victoreen 450P ionization chamber could potentially serve as an estimator of real-time effective dose and organ absorbed dose, if energy and angular dependence corrections could be taken into account. Finally, the data obtained in this investigation indicate that the MCNP model provided a reasonable method to determine organ absorbed dose and effective dose of a simulated Radiological Dispersal Device in an Inferior-Superior geometry with Na99mTcO4 as the source of radioactive material.

Diamond has long been the material of interest for radiotherapy dosimetry due to its high sensitivity, radiation hardness and near tissue equivalency. However natural diamond detector has not become a popular choice because of variability among detectors, high cost and response dependence on dose rate. The recent success in synthesizing single crystal diamond has reignited the interest. Synthetic diamond is highly reproducible in purity and electrical properties, combined with small size, it is a suitable candidate for small field dosimetry. A newly available synthetic single crystal diamond detector is being studied to determine the basic dosimetric characteristic and applicability in small field dosimetry. A series of measurements were made in comparison with a 0.125c.c ionization chamber, and two diode detectors. Response of the diamond detector is independent on dose, dose rate and energy. The output factors of small fields determined by the diamond detector is lower than that of the diode detectors and higher than that of the ionization chamber which are known to over response and under response respectively. In percentage depth dose and beam profile measurements, the diamond detector performs similarly with the two diode detectors. It is found that the diamond detector is suitable for small field relative dosimetry. Further investigation is required to study the spatial resolution of the diamond detector in different measurement geometry and the suitability in determining percentage depth dose in the buildup region. / published_or_final_version / Diagnostic Radiology / Master / Master of Medical Sciences

No description available.

No description available.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1983. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 128-139).

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 86-87). Also available on the World Wide Web.