Optic Disc Dose Comparison Between 125I and 103Pd Collaborative Ocular Melanoma Study (COMS) Plaques Based on Current Clinical Practice

Purpose The purpose of this study is to compare optic disc dose (ODD) between I-125 and Pd-103 Collaborative Ocular Melanoma Study (COMS) plaques in ocular brachytherapy. Methods A previously validated in-house brachytherapy dose calculation program was used for ODD calculations. ODD was calculated as a function of tumor margin-to-optic disc distance (DT) up to 5 mm for various tumor basal dimensions (BDs), for a prescription depth of 5 mm, and for standard and notched COMS plaques loaded with I-125 (model: IAI-125A) and Pd-103 (model: IAPd-103A) seeds. ODD calculations were repeated for prescription depths from 2 mm to 10 mm in 1 mm intervals. A prescribed dose of 85 Gy (irradiation time: 120 hours) was normalized to each prescription depth. Dose conversion factors (DCFs) for each prescription depth were calculated by taking a ratio of [total reference air kerma (TRAK) per seed](prescription depth) to [TRAK per seed](5 mm). ODD reduction by notched COMS plaques was calculated for each prescription depth by subtracting ODD for notched COMS plaques from ODD for standard COMS plaques. Results Trends of ODD as a function of DT for various BDs are similar between the two seed types in both standard and notched COMS plaques. However, due to the energy difference, there exists a transition distance (d(t)) for each BD in each plaque at which ODD for I-125 COMS plaques equals that for Pd-103 COMS plaques. For small BDs, at DT <d(t), ODD for Pd-103 COMS plaques is higher than that for I-125 COMS plaques while at DT >d(t), the opposite is observed. For the largest 1-3 BD(s), contrarily, d(t) occurs within the tumor, and thus, ODD for I-125 COMS plaques is always higher. Trends of ODD reduction by notched COMS plaques as a function of DT for various BDs are the same for the two seed types except that maximum ODD reduction by Pd-103 COMS notched plaques is larger. DCF increases with increasing prescription depth for both seed types. Conclusions There exist ODD differences between I-125 and Pd-103 COMS plaques and the differences depend on DT, BD, plaque size, and prescription depth.