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Modeling the irregular three dimensional geometry of the prostate in 3D TRUS is an important task useful in a variety of clinical procedures of the prostate such as visualization, prostate volume estimation, treatment planning. Traditionally, the prostate volumetric shape has been often modeled with primitive shapes that oversimplify its complex and deformable structure. In this paper, we present a new model, we call it Finite Sphere Model or FSM, which is composed of variable-size spheres packed inside the prostate volume. This robust method accommodates different prostate shapes and does not need prior knowledge or shape approximation. First, the prostate region is extracted and isolated from the TRUS image. The algorithm applies morphological processing in order to obtain the medial axis of the prostate. Along the medial axis, the prostate space is searched for the optimal number of spheres that can be fitted inside its volume. Finally, small patches left by the algorithm are detected by masking the image with smaller discs until the filling criteria is satisfied. A number of experiments are used to verify the efficiency of the developed model.