Monte Carlo‐based dosimetry of proposed bi‐radionuclide (125I and 106Ru/106Rh) eye plaque: A feasibility study

Background Combining the sharp dose fall off feature of beta‐emitting 106Ru/106Rh radionuclide with larger penetration depth feature of photon‐emitting125I radionuclide in a bi‐radionuclide plaque, prescribed dose to the tumor apex can be delivered while maintaining the tumor dose uniformity and sparing the organs at risk. The potential advantages of bi‐radionuclide plaque could be of interest in context of ocular brachytherapy. Purpose The aim of the study is to evaluate the dosimetric advantages of a proposed bi‐radionuclide plaque for two different designs, consisting of indigenous 125I seeds and 106Ru/106Rh plaque, using Monte Carlo technique. The study also explores the influence of other commercial 125I seed models and presence or absence of silastic/acrylic seed carrier on the calculated dose distributions. The study further included the calculation of depth dose distributions for the bi‐radionuclide eye plaque for which experimental data are available. Methods The proposed bi‐radionuclide plaque consists of a 1.2‐mm‐thick silver (Ag) spherical shell with radius of curvature of 12.5 mm, 20 µm‐thick‐106Ru/106Rh encapsulated between 0.2 mm Ag disk, and a 0.1‐mm‐thick Ag window, and water‐equivalent gel containing 12 symmetrically arranged 125I seeds. Two bi‐radionuclide plaque models investigated in the present study are designated as Design I and Design II. In Design I, 125I seeds are placed on the top of the plaque, while in Design II 106Ru/106Rh source is positioned on the top of the plaque. In Monte Carlo calculations, the plaque is positioned in a spherical water phantom of 30 cm diameter. Results The proposed bi‐radionuclide eye plaque demonstrated superior dose distributions as compared to 125I or 106Ru plaque for tumor thicknesses ranges from 5 to 10 mm. Amongst the designs, dose at a given voxel for Design I is higher as compared to the corresponding voxel dose for Design II. This difference is attributed to the higher degree of attenuation of 125I photons in Ag as compared to beta particles. Influence of different 125I seed models on the normalized lateral dose profiles of Design I (in the absence of carrier) is negligible and within 5% on the central axis depth dose distribution as compared to the corresponding values of the plaque that has indigenous 125I seeds. In the presence of a silastic/acrylic seed carrier, the normalized central axis dose distributions of Design I are smaller by 3%–12% as compared to the corresponding values in the