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Model-Based Dose Calculations for Eye Plaque Brachytherapy

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M Lesperance

M Lesperance*, M Martinov, M Inglis-Whalen, R Thomson, Carleton University, Ottawa, ON

WE-C-108-10 Wednesday 10:30AM - 12:30PM Room: 108

Purpose: To develop a comprehensive understanding of dosimetry for ocular brachytherapy through model-based dose calculations involving a full human eye model for various plaque models and three different photon-emitting radionuclides (¹²⁵I, ¹⁰³Pd and ¹³¹Cs).

Methods: A representative computational model of the human eye and surrounding tissues was developed based on ocular geometry and compositions from the literature. Mass energy absorption and attenuation coefficients for all media were calculated. Monte Carlo simulations using the EGSnrc user-code BrachyDose were performed for a variety of photon-emitting plaques covering the range of plaque models encountered in clinical practice, including the widely-used COMS plaque. Maximum, minimum and average doses to ocular structures, as well as dose volume histograms (DVH) for the full eye model were compared to those for a homogeneous water phantom, both including and excluding (TG-43) inter-seed and eye plaque effects.

Results: Mass energy absorption and attenuation coefficients for ocular media differ from those for water by up to 10% for photon energies in the 10-30 keV range. For the COMS plaque in the full eye model, average doses to important structures such as the tumour and lens differ from those for the plaque in water by roughly 8-10% and 13-14%, respectively, and from TG-43 calculation by 2-17% and 29-34%, respectively. Maximum, minimum and average doses differ considerably from doses to points of interest (as they are currently reported); the lens has a larger average dose by 7-9% and optic nerve has a larger maximum dose by 16-22%, than at the center of the lens and the optic disk, respectively.

Conclusion: Dose distributions for the full eye model simulations deviate significantly from TG-43 and from simulations that include the plaque but assume the eye to be water equivalent, highlighting the need for accurate model-based dose calculations in ocular brachytherapy.

Funding Support, Disclosures, and Conflict of Interest: Funding support: NSERC, OGS

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