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Measured, Calculated and Monte Carlo Simulated Dose Distributions in a Unique Gold Applicator Loaded with I-125 Seeds

C Trauernicht1*, E Hering2, F Du Plessis3, G Maree4, D Van Eeden3, (1) Tygerberg Hospital and Stellenbosch University, Cape Town, ZA, (2) University Of Cape Town, Cape Town, ZA, (3) University Of The Free State, Bloemfontein, ZA, (4) Groote Schuur Hospital, Cape Town, ZA,

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The “Claws” is a unique gold applicator for whole-eye radiotherapy that is used to treat retinoblastoma. A pericorneal ring is attached to the four extraocular muscles and four legs, each loaded with I-125 seeds, are inserted in-between each pair of muscles and attached anteriorly to the ring. The aim of this study was to compare measured and calculated dose distributions in and around the applicator.


Methods: Gafchromic film and thermoluminescent dosimeters were used for dose distribution measurements in solid water eye phantoms. Dose distributions were also calculated on two treatment planning systems (TheraPlan Plus Version 3.8 Build 500 and Varian BrachyVision Version 15.6.05) and egs_brachy was used to run a 64.000.000.000 histories Monte Carlo simulation to determine the dose distribution in and around the “Claws”.


Results: Measured and Monte Carlo simulated dose distributions matched each other very well, but planned data showed significant differences of up to 20 % to measured and simulated data near the periphery of the eye between adjacent legs. The reason for this is that the gold shielding of the applicator is not taken into account in both treatment planning systems. The impact of the gold is clear when doing measurements and confirmed by the Monte Carlo simulation, which does take the gold into account.


Conclusion: A dose reduction near the periphery of the eye may have potential clinical implications, because a tumour may potentially be under-dosed. The “Claws” could potentially be redesigned with a shallower recess for the I-125 seeds in the legs for better coverage in the underdosed regions of the eye.

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Keywords

Brachytherapy, Eye Plaques, Monte Carlo

Taxonomy

TH- Brachytherapy: Low Energy Seed Brachytherapy

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