Dosimetric Calculations of a Radioactive Eye Plaque used in Management of Ocular Melanoma Using Monte Carlo Simulations

Introduction & Brachytherapy using I-125 radioactive seeds in removable episcleral plaques (EP) is often used in treatment of ocular malignant melanoma. Some radioactive seeds are fixed in a gold bowl-shaped plaque. The plaque is sutured to the sclera surface corresponding to the base of the intraocular tumor, allowing for a localized radiation dose delivery to the tumor. Minimum target doses as high as 85Gy are directed at malignant tumor. The aim of this study was to develop a Monte Carlo simulation of an ocular plaque in order to calculate the resulting isodose distributions. The MCNP-4C Monte Carlo code is used to simulate the plan of an episcleral plaque treatment. A 20-mm Collaborative Ocular Melanoma Study (COMS) plaque with 3, I-125 seed of model 6711 was used. Resulting dose distributions, including central axis dose and off-axis dose profiles, were calculated in a water phantom with 12mm radius. The calculated dose distributions were compared to the corresponding dose measured by Knuten et al., 2001. Central axis dose calculations represent a rapid dose fall off, which is an important factor in selection of appropriate eye plaque for management of tumors with known dimension. Calculated off-axis dose profiles show decreased dose uniformity at distances close to the plaque. Increasing of distance from the plaque resulted in increasing of the dose uniformity. Monte Carlo simulation of eye plaques can be used as a useful tool in process of design, development and treatment planning of ocular radioactive plaques.