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J. P. Stokes; Convergent Well Brachytherapy Plaque Design - Demonstration of Concept Utilizing Large Scale Models and Visible Light Photometry. Invest. Ophthalmol. Vis. Sci. 2008;49(13):44. doi: https://doi.org/.
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To design and test models for modified brachytherapy plaques that will allow adequate irradiation of choroidal melanoma while protecting the optic nerve, macula, iris, and adjacent retina from radiation damage.
A large model eye and several models of proposed plaques utilizing convergent fins in various confugurations were constructed. Visible light sources were used to model gamma radiation sources in configuration and photon flux distribution. The confounding effects from reflection and diffraction of visible light were minified by coating all illuminated surfaces with deep black, minimally reflective material with high surface irregularity. A visible light luxometer was used to measure light intensity and compare radiation intensity delivered to the apex of the model tumor versus that delivered to the macula, optic nerve head, adjacent and contralateral retina, and various points within the model tumor body itself.
With a model of a medium to large choroidal melanoma immediately adjacent to the macula, the ratio of apical tumor electromagnetic irradiation to macular or optic nerve irradiation was significantly reduced compared to that predicted in a similar case using a standard Collaborative Ocular Melanoma Study style plaque. The degree of homogeneity vs. variability and achievement of adequate irradiation within the model melanoma body were dependent upon the configuration of the model's converging fins, their individual thicknesses, and, in particular, the number of fins used to divide the electromagnetic radiation sources. No improvement in the ratio of apical tumor irradiation to contralateral retina irradiation was observed.
This study suggests that gamma radiation brachytherapy plaques using convergent-well style configuratons of metallic fins may allow irradiation of posterior choroidal melanomas with decreased risk of radiation damage to the macula and optic nerve compared to that observed with standard brachytherapy plaques.
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