We show how nowadays it is possible to achieve the goal of accuracy and fast computation response in radiotherapic dosimetry using Monte Carlo methods, together with a distributed computing model. Monte Carlo methods have never been used in clinical practice because, even if they are more accurate than available commercial software, the calculation time needed to accumulate sufficient statistics is too long for a realistic use in radiotherapic treatment. We present a complete, fully functional prototype dosimetric system for radiotherapy, integrating various components based on HEP software systems: a Geant4-based simulation, an AIDA-based dosimetric analysis, a web-based user interface, and distributed processing either on a local computing farm or on geographically spread nodes. The performance of the dosimetric system has been studied in three execution modes: sequential on a single dedicated machine, parallel on a dedicated computing farm, on a grid test-bed. An intermediate software layer, the DIANE system, makes the three execution modes completely transparent to the user, allowing to use the same code in any of the three configurations. Thanks to the integration in a grid environment, any hospital, even small ones or in less wealthy countries, that could not afford the high costs of commercial treatment planning software, may get the chance of using advanced software oncological therapy, based on Monte Carlo methods, by accessing distributed computing resources, shared with other hospitals and institutes belonging to the same virtual organization.