Simulating surgical procedures is still a complex challenge. Either modeling methods or simulators have to take into account specific geometries and properties of the patient organs. In this paper, a new soft tissue modeling method dedicated to prostate surgical interventions is presented. The chosen medical application requires the modeling of a complex anatomical environment including intricate interactions between organs and surgical instruments. We present a discrete model that has been specifically developed to fulfill these requirements. The influence of two particular instruments is studied: needles and ultrasound probe, according to two surgical interventions: biopsy and brachytherapy.