we describe a methodology for the calculation and animation of volumetric deformable objects. The goal of this work is to obtain realistic models of internal organs in order to simulate their motion and their form alteration during a radiotherapy process. Thus, these models should be able to represent the internal movements due to rhythmic motions, respiration, filling/emptying processes and organs' interactions. So we show how this can be done using particle systems and implicit surfaces and how to mix both models in an hybrid scene making organ's interaction simulation easier.