Introduction
In order to treat uveal Melanoma , first, radioactive seeds are laid on a silicone- made substance which is called Silastic after that they are inserted in the plaque, and finally, this plaque containing silicone-made substance is stitched to the sclera surface. The dose gradient within the tumor and healthy tissues can be varied due to changing the Silastic thickness between sclera surface and radioactive seeds. In turn, this leads to difference in the amount of absorbed dose of tumor and healthy tissues. Present study is to investigate the optimum Silastic thickness in uveal Melanoma brachytherapy. 
Materials and Methods
To measure changes of depth dose of the plaque in a sphere with a radius of 12 mm, MCNP4C code was applied. Exact specifications of a 20-mm Collaborative Ocular Melanoma Study (COMS) plaque, Silastic and three I-125 seed sources, 6711 model were integrated in simulation. Dose calculations were performed using F6 tally in spheres with a radius of 0.2 mm.
Results
By measuring the changes of dose rate of plaque in distances of 0.2 to 18 mm from the sclera surface and having the prescribed dose for the absolute treatment of eye melanoma, final absorbed doses by tumor and healthy tissues for each different Silastic thicknesses of 0, 0.5, 1, 1.5, and 2 were calculated.
Conclusion
Considering the results and sclera tolerance, it was concluded that the thickness of Silastic must not exceed 0.5 mm, because increasing the Silastic thickness from this area, increases absorbed dose by healthy tissues and also the treatment time.