Dosimetry of the Teflon encased strontium eye applicator

Abstract

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.