Monte Carlo dosimetry for and eye plaque brachytherapy with various seed models

Abstract

PurposeDose distributions are calculated for various models of and seeds in the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). The sensitivity to seed model of dose distributions and dose distributions relative to TG‐43 are investigated.MethodsMonte Carlo simulations are carried out with the EGSnrc user‐code BrachyDose. Brachytherapy seeds and eye plaques are fully modeled. Simulations of one seed in the central slot of a 20 mm Modulay (gold alloy) plaque backing with and without the Silastic (silicone polymer) insert and of a 16 mm fully loaded Modulay/Silastic plaque are performed. Dose distributions are compared to those calculated under TG‐43 assumptions, i.e., ignoring the effects of the plaque backing and insert and interseed attenuation. Three‐dimensional dose distributions for different and seed models are compared via depth‐dose curves, isodose contours, and tabulation of doses at points of interest in the eye. Results are compared to those of our recent BrachyDose study for COMS plaques containing model 6711 or 200 seeds [R. M. Thomson et al., Med. Phys. 35, 5530–5543 (2008)].ResultsAlong the central axis of a plaque containing one seed, variations of less than 1% are seen in the effect of the Modulay backing alone for different seed models; for the Modulay/Silastic combination, variations are 2%. For a 16 mm plaque fully loaded with seeds, dose decreases relative to TG‐43 doses are 11%–12% (19%–20%) and 14%–15% (20%) at distances of 0.5 and 1 cm from the inner sclera along the plaque's central axis, respectively. For the same prescription dose, doses at points of interest vary by up to 8% with seed model. Doses to critical normal structures are lower for all seed models than for with the possible exception of the sclera adjacent to the plaque; scleral doses vary with seed model and are not always higher for than for .ConclusionsDose decreases relative to doses calculated under TG‐43 assumptions vary slightly with seed model (for each radionuclide). Dose distributions are sensitive to seed model; however, variations are generally no larger than the magnitudes of other systematic uncertainties in eye plaque therapy.