The eye is not radiologically water-equivalent, as doses from simulations of the plaque in the full eye model differ considerably from doses for the plaque in a water phantom and from simulated TG-43 calculated doses. This demonstrates the importance of model-based dose calculations for eye plaque brachytherapy, for which accurate elemental compositions of ocular media are necessary.

The combined effects of ocular and plaque media on dose are significant and vary with plaque model and radionuclide, suggesting the importance of model-based dose calculations employing accurate ocular and plaque media and geometries for eye plaque brachytherapy.

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: NSERC, OGS

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