Feasibility of Intensity Modulated High Dose Rate Ocular Brachytherapy Using Middle Energy Isotopes Coupled with a Partially Shielded Applicator

Dupere, Justine M.

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Feasibility of Intensity Modulated High Dose Rate Ocular Brachytherapy Using Middle Energy Isotopes Coupled with a Partially Shielded Applicator

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The purpose of this dissertation is to investigate a new approach of using middle-energy high dose rate brachytherapy sources, Yb-169 and Se-75, coupled with a gold shielded ring applicator to treat ocular melanoma. The feasibility of this approach is computationally tested using MCNP6 and validated against an I-125 COMS plaque, which is currently the standard treatment modality. The proposed ring source consists of an assembly of discrete sources delivered into an applicator with a conical collimated opening that is tangent to the outside of the source tube. Using MCNP6, the dosimetric output of a ring of Yb-169 and Se-75 sources was simulated within the collimator at various conical diameters and angles to demonstrate the dosimetric distribution for various prescription dose depths and target sizes. Using various angles of collimation, the prescription dose was delivered to a target apex of 3.5-8.0 mm depths into the eye covering target sizes ranging from 10.0-15.5 mm in diameter. This proposed device reduced the maximum absorbed dose to critical structures relative to I-125 by 5.2% to the posterior lens, 9.3% to the iris, 13.8% to the optic nerve, and 1.3% to the sclera, when using Yb-169. The percent difference in dose to the critical structures within the eye was about 30% higher when using Se-75 compared to Yb-169 however it is still within acceptable limits. Greater conformal dose distributions were achieved from the conical opening and gold shielding on the applicator. This design also permits the use of a remote afterloader, either manual or automated, which eliminates dose to the surgeon’s hands. While Yb-169 is the optimal isotope for this treatment, the longer half-life of Se-75 (120 days) may make this treatment option more practical.

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