Among the current challenges in human soft tissue modeling for medical purpose, the ability to model complex anatomical structures, their interactions and to accurately simulate them with physical realism are in the forefront of research. This paper describes a discrete soft tissue model which is geared toward solving these challenges. In this model, objects can be described as volumetric or surfacic sets of nodes depending on the level of precision required. Nodes have their own physical properties and a definition of their neighborhood. All these objects are submitted both to internal cohesive forces and to external attractive or interaction forces with other objects. Volume preservation is considered as a constraint. The model is applied to the simulation of the prostate and its surrounding organs.