Keeping an eye on the ring: COMS plaque loading optimization for improved dose conformity and homogeneity

Abstract

PurposeTo improve tumor dose conformity and homogeneity for COMS plaque brachytherapy by investigating the dosimetric effects of varying component source ring radionuclides and source strengths.Material and methodsThe MCNP5 Monte Carlo (MC) radiation transport code was used to simulate plaque heterogeneity-corrected dose distributions for individually-activated source rings of 14, 16 and 18 mm diameter COMS plaques, populated with 103Pd, 125I and 131Cs sources. Ellipsoidal tumors were contoured for each plaque size and MATLAB programming was developed to generate tumor dose distributions for all possible ring weighting and radionuclide permutations for a given plaque size and source strength resolution, assuming a 75 Gy apical prescription dose. These dose distributions were analyzed for conformity and homogeneity and compared to reference dose distributions from uniformly-loaded 125I plaques. The most conformal and homogeneous dose distributions were reproduced within a reference eye environment to assess organ-at-risk (OAR) doses in the Pinnacle3 treatment planning system (TPS). The gamma-index analysis method was used to quantitatively compare MC and TPS-generated dose distributions.ResultsConcentrating > 97% of the total source strength in a single or pair of central 103Pd seeds produced the most conformal dose distributions, with tumor basal doses a factor of 2-3 higher and OAR doses a factor of 2-3 lower than those of corresponding uniformly-loaded 125I plaques. Concentrating 82-86% of the total source strength in peripherally-loaded 131Cs seeds produced the most homogeneous dose distributions, with tumor basal doses 17-25% lower and OAR doses typically 20% higher than those of corresponding uniformly-loaded 125I plaques. Gamma-index analysis found > 99% agreement between MC and TPS dose distributions.ConclusionsA method was developed to select intra-plaque ring radionuclide compositions and source strengths to deliver more conformal and homogeneous tumor dose distributions than uniformly-loaded 125I plaques. This method may support coordinated investigations of an appropriate clinical target for eye plaque brachytherapy.

Model‐based dose calculations for COMS eye plaque brachytherapy using an anatomically realistic eye phantom

“…Doses delivered to critical ocular structures within the eye model, normalized in terms of dose to the tumor apex, are generally lowest for 103 Pd and highest for 131 Cs, with the exception of the tumor where average doses are comparable for 125 I and 103 Pd, and are lowest for 131 Cs. These observations have previously been reported for a water-equivalent eye [46][47][48] and have been shown in this work to hold when ocular media are considered, although specific comparison of doses delivered with the different radionuclides are sensitive to the simulation geometry.…”

Visual outcome after posterior uveal melanoma episcleral brachytherapy including radiobiological doses

“…Plaque arrangements can vary in source strengths and ring sizes [ 33 ]. An independent check of treatment time by redundant calculation was performed.…”

AAPM recommendations on medical physics practices for ocular plaque brachytherapy: Report of task group 221

“…COMS plaques may be used in a non‐standard way: for example, the gold alloy plaque backing alone with no Silastic insert and seeds directly affixed to the plaque backing, with a very thin reusable seed‐guide insert, or with non‐uniform loading to improve homogeneity of the dose distribution . In 2005, Astrahan et al reported a dosimetry study that used the Plaque Simulator © treatment planning system (Eye Physics, LLC, Los Alamitos, CA) to calculate dose distributions for the COMS plaque backing with the 0.3 mm thick gold alloy seed guide.…”