Monte Carlo is the algorithm that most closely models the actual physics of the energy deposition process. Thus it would be very interesting to use this method in the cancer treatment planning by radiation. Indeed, existing treatment planning systems (TPS) are limited in the accuracy of dose calculations for certain specific cases. In this approach, we are interested in the validation GATE Monte Carlo toolkit for dosimetric applications in medical physics (Radiotherapy and Brachytherapy). However, the use of Monte Carlo method for complex geometries or images with a large resolution requires a large computing time. Indeed, a precise result can be obtained only by generating a large number of events. We thus study the performances of the computing grid to reduce computing time by deploying GATE simulations on the grid architecture developed within the framework of EGEE European project.