A practical method of estimating medium-heterogeneity corrected dose without a Monte Carlo calculation in ocular brachytherapy using 125I COMS plaques

Abstract

To provide a practical method of estimating medium-heterogeneity corrected dose without a Monte Carlo (MC) calculation in ocular brachytherapy using 125I Collaborative Ocular Melanoma Study (COMS) plaques. Using egs_brachy, MC simulations (1) under task group-43 assumptions with fully loaded seed configurations in water (HOMO) and (2) with effects of plaque backing, insert and inter-seed interactions (HETERO) were performed for seven 125I COMS plaques (10 mm-22 mm in diameter), and homogeneous dose (DHOMO) and heterogeneous dose (DHETERO) for central-axis and off-axis points were determined. For DHOMO, 85 Gy was normalized to a depth of 5 mm. Central-axis heterogeneity correction factors (HCFs) from a depth of 0 mm (inner sclera) to 22 mm (opposite retina) were derived from a ratio of DHETERO to DHOMO. Off-axis HCFs for optic disc/macula and lens as a function of distance from optic disc/macula (DT/MT) for various basal dimensions of tumor (BD/BM) were derived from DHETERO/DHOMO. Central-axis HCF varied with a dose reduction of 10.3-19.8% by heterogeneity. Off-axis HCF for optic disc/macula varied significantly depending on DT/MT and BD/BM with a dose reduction of 11.3-38.3%. Off-axis HCF for lens had a dependence on MT and BM with its variation of 11.0-19.0%. A clinical example of using HCFs to estimate DHETERO was presented. The practical method of using depth-dependent central-axis HCF and DT/MT- and BD/BM-dependent off-axis HCF provided in this study will facilitate a heterogeneous dose estimate for 125I COMS plaques without MC calculations.