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F. A. Navas, J. Stokes; Refinement of Convergent Brachytherapy Plaque Design: Demonstration of Radiation Distribution Utilizing Large Scale Models and Visible Light Photometry. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3384.
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© ARVO (1962-2015); The Authors (2016-present)
To measure the radiation distribution from a refined model of a previously proposed brachytherapy plaque redesign. The goal is to allow adequate treatment for choroidal melanoma while protecting the macula, optic nerve and adjacent and contralateral retina from radiation damage.
A scale model was made of heavy cardboard stock fins cut and assembled to model the metallic fin arrangement of a proposed redesigned plaque. Cylindrical visible light sources were used to simulate the emission of gamma radiation from commercially available I-125 seeds. The thickness of the cardboard material was adequate to block passage of all visible light, modeling the blockage of gamma radiation by the metallic fins of the proposed plaque. In order to prevent reflected light from corrupting measurements, the fin surfaces were covered with black paint and a rough black material that was shown to absorb >98% of incident light. Compared to a previous model, the convergent fins were slanted anteriorly by several degrees and the arrangement of the visible light sources was modified. As with the prior redesigned plaque model, this model was designed to treat a zone comparable to a large, posterior melanoma just outside the macula. A visible light lux meter was used to measure light intensity delivered to the apex of the model tumor, within the tumor, and at its base, and to compare these to that delivered to the macula, optic nerve head, anterior segment, and adjacent and contralateral retina.
The ratio of apical or basal tumor electromagnetic irradiation to macula, optic nerve, or adjacent retina was greatly reduced compared to that predicted in a similar case using a standard COMS-style plaque. The ratio of irradiation of the tumor apex to irradiation of contralateral retina was also improved compared to the previously presented model. There was a rapid drop off of radiation intensity anterior and posterior to the treatment zone, with drastically reduced radiation to the posterior pole. The intensity of anterior segment radiation with this refined model was higher than with the previously presented redesign.
This study suggests that gamma radiation brachytherapy plaques using convergent-well style configurations of high Z-value metallic fins allow irradiation of posterior choroidal melanomas with decreased risk of radiation damage to the macula and optic nerve compared to standard brachytherapy plaques.
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