Investigation of optic disc dose reduction in ocular brachytherapy using 125I notched COMS plaques

Abstract

Background: Notched plaques were devised to reduce radiation dose to the optic disc in ocular brachytherapy. Although notched plaques have been widely used in clinic, optic disc dose reduction by the use of notched plaques has not been discussed in the literature. Therefore, this study investigated optic disc dose reduction in ocular brachytherapy using 125I notched Collaborative Ocular Melanoma Study (COMS) plaques based on our institutional practice. Methods: Using our in-house brachytherapy dose calculation program in which the American Association of Physicists in Medicine Task Group-43 Update (AAPM TG-43U1) dosimetry formalism with a line source approximation in a homogeneous water medium and COMS seed coordinates in the AAPM TG 129 were incorporated, optic disc doses for standard COMS plaques (from 12 mm to 22 mm in diameter in 2 mm increments) and notched COMS plaques with one seed removed (Case #1, from 12 mm to 22 mm in diameter in 2 mm increments) and two seeds removed (Case #2, from 14 mm to 22 mm in diameter) were calculated as a function of tumor margin-to-optic disc distance (DT) for various tumor basal dimensions (BDs) for prescription depths from 1 mm to 10 mm in 1 mm intervals. A prescribed dose was 85 Gy for an irradiation time of 168 hours to each prescription depth. Absolute and relative optic disc dose reduction by notched COMS plaques (Cases #1 and #2) for all prescription depths was calculated from optic disc dose differences between standard and notched COMS plaques. Results: Optic disc dose reduction by notched COMS plaques (Cases #1 and #2) had five unique trends which are related to maximum optic disc dose reduction and corresponding optimal DT for each BD in each plaque. Optic disc dose reduction increased with increasing prescription depth. Conclusions: The results presented in this study (magnitude of optic disc dose reduction and its trends) would enable the clinicians (both ophthalmologist and radiation oncologist) to choose an adequate plaque type among standard 125I COMS plaques and notched 125I COMS plaques (Cases #1 and #2) and a prescription depth to minimize optic disc dose for a given clinical case.