A practical approach to estimating optic disc dose and macula dose without treatment planning in ocular brachytherapy using 125I COMS plaques

BackgroundIt has been reported that proximity of the tumor to the optic disc and macula, and radiation dose to the critical structures are substantial risk factors for vision loss following plaque brachytherapy. However, there is little dosimetry data published on this. In this study, therefore, the relationship between distance from tumor margin and radiation dose to the optic disc and macula in ocular brachytherapy using I-125 Collaborative Ocular Melanoma Study (COMS) plaques was comprehensively investigated. From the information, this study aimed to allow for estimation of optic disc dose and macula dose without treatment planning.MethodsAn in-house brachytherapy dose calculation program utilizing the American Association of Physicists in Medicine Task Group-43U1 formalism with a line source approximation in a homogenous water phantom was developed and validated against three commercial treatment planning systems (TPS). Then optic disc dose and macula dose were calculated as a function of distance from tumor margin for various tumor basal dimensions for seven COMS plaques (from 10mm to 22mm in 2mm increments) loaded with commercially available I-125 seeds models (IAI-125A, 2301 and I25.S16). A prescribed dose of 85Gy for an irradiation time of 168h was normalized to a central-axis depth of 5mm. Dose conversion factors for each seed model were obtained by taking ratios of total reference air kerma per seed at various prescription depths (from 1mm to 10mm in 1mm intervals) to that at 5mm.ResultsThe in-house program demonstrated relatively similar accuracy to commercial TPS. Optic disc dose and macula dose decreased as distance from tumor margin and tumor basal dimension increased. Dose conversion factors increased with increasing prescription depth. There existed dose variations (<8%) among three I-125 seed models. Optic disc dose and macula dose for each COMS plaque and for each seed model are presented in a figure format. Dose conversion factors for each seed model are presented in a tabular format.ConclusionsThe data provided in this study would enable clinicians in any clinic using I-125 COMS plaques to estimate optic disc dose and macula dose without dose calculations.