We show how nowadays it is possible to achieve the goal of accuracy and fast computation response in radiotherapic dosimetry using MonteCarlo methods, together with a grid computing model. We present a complete, fully functional prototype system for brachytherapy, entirely based on open source software systems originally developed for High Energy Physics experiments. It integrates a Geant4-based simulation component, an AIDA-based dosimetric analysis, a web-based user interface, and distributed processing either on a local computing farm or on geographically spread nodes. Thanks to the object-oriented approach adopted for the architecture, the work presented can be easily extended to become a general purpose dosimetric system, capable to address all radiotherapic techniques. An extension for application to dosimetric studies for IMRT is in progress. © 2005 Published by Elsevier B.V.