| Summary: | Introduction: The “Claws” is a unique gold applicator for whole-eye radiotherapy that was designed at Groote Schuur Hospital. It is used to treat retinoblastoma. Under general anaesthesia, a pericorneal ring is attached to the four extraocular muscles, and four legs, each loaded with I125 seeds, are inserted beneath the conjunctiva in-between each pair of muscles and attached anteriorly to the ring. The four legs that are now sutured onto the ring give it a claw-like appearance, hence the name for the applicator. The applicator was designed in such a way that the dose is directed towards the middle of the eye, while sparing surrounding tissues. The dose to the organs at risk could never be determined accurately, because the treatment planning system (TPS) is not able to take into account the gold shielding. Additionally, the TPS approximates each seed as a point source and not as a line source, therefore not taking any anisotropy into account. Aims: The first aim of this project was to accurately determine various dosimetric and physical characteristics of a single I-125 seed and to then compare these to published data. Spectral measurements of the OncoSeed 6711 using various detectors were also done. The next aim was to formalize the model of the “Claws” so that the applicator can potentially also be manufactured elsewhere. The next aim was to describe the “Claws” dosimetrically. This was done - Using thermoluminescent dosimeters in a solid water phantom - Using gafchromic film in a solid water phantom - Using treatment planning systems TheraPlan Plus and BrachyVision - Using Monte Carlo simulations – egs_brachy The final aim of the thesis was the comparison of measured and calculated data. The Monte Carlo simulations take into account the seed anisotropy as well as the gold shielding; therefore the relative dose to critical structures can be estimated more reliably. Method and Materials: Gafchromic film and thermoluminescent dosimeters (TLDs) were used for measurements in various specially designed phantoms to determine the seed parameters, as well as dose distributions in the eye. Dose distributions were calculated on two treatment planning systems. A CAD drawing of the “Claws” was created and used to create the input file for Monte Carlo simulations using egs_brachy. The final Monte Carlo calculation simulated 64.000.000.000 particle histories at voxel sizes of 0.1 mm x 0.1 mm x 0.1 mm. Results: Measured seed data matched published seed data. Significant dose distribution changes were found when comparing measured and Monte Carlo data to planned data, especially near the periphery of the eye between adjacent legs. The Monte Carlo calculated dose to the optic nerve is 64.8 % of the central dose in the eye, while the planned dose is 93.7 %. The Monte Carlo lens dose varies from 72.0 % - 86.1 %, while the planned dose varies from 73.0 % - 84.3 %. Monte Carlo calculated dose to the bony orbit is 11.3 %, while the planned dose is 54.7 %. Conclusion: Measured seed data matched published seed data. The “Claws” were formalized with CAD drawings. Measured and Monte Carlo simulated dose distributions matched well, while planned dose distributions showed discrepancies in certain regions of the eye and outside of the eye. This clearly indicates that the gold shielding of the applicator walls must be taken into account during dose calculations. It can be concluded that the “Claws” were extensively described and characterized in this work.
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