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R. H. Mcglynn, Jr., P. T. Finger; Seed Position (Anisotropy) Can Affect Macular Dose During Ophthalmic Plaque Radiation Therapy. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5140.
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© ARVO (1962-2015); The Authors (2016-present)
To determine if radioactive seed orientation in standard COMS-type gold plaques affects macular radiation exposure.
Bebig Plaque Simulator X 5.3.6 was used to determine foveal radiation exposure in eyes with 12, 16 and 20 mm gold COMS-type plaques, over tumors of varying widths, thicknesses and intraocular locations. Overall, 288 eye plaque dose simulations were performed using traditional seed distributions and those with seeds oriented radially towards the fovea (as to exploit the anisotropy effect). Anisotropy is the directionally dependent aspect of radiation distribution of a seed source which accounts for decreased emission along the long axis of a seed. In this study tumor thicknesses were 2.5, 5 and 7.5mm. Locations were tumors centered on the ciliary body, equator, fovea and half way between the equator and the fovea. Plaques were tested with gold seed guides and silastic inserts and with 103Pd and 125I seeds. Plaques delivered 85 Gy to the tumor apex over a 7 day period.
When a 12 mm plaque was loaded with 125I seeds in a radial seed distribution for treatment of a 5-mm thick tumor (located at the ciliary body), there was a decrease in foveal radiation exposure of 1.3% compared to the standard seed orientations. At the equatorial position the decrease was 3.6%, the half equator position 15% and at the fovea 0.5%. When the plaque was loaded with 103Pd seeds the magnitude of decrease at these four locations was 2.3%, 8.7%, 24% and 4.3%, respectively. Similar degrees of decrease were found with tumor thicknesses of 2.5 and 7.5 mm as well as when the plaques were fitted with silastic and gold seed-guide inserts.
Seed orientation within the plaque affects dose to normal ocular structures. Orienting radioactive seeds radially towards the fovea produces the greatest effect when tumors are located near but not directly under the macula. This is important because plaque irradiation of tumors in this location is associated with a high risk for radiation maculopathy. Utilizing anisotropy may allow for a 15 -25% reduction in radiation to the macula.
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