Surgical simulation requires to have an operating scenario as similar as possible to the real conditions that the surgeon is going to face. Not only visual and geometric patient properties are needed to be reproduced, but also physical and biomechanical properties are theoretically required. In this paper a physically based patient specific simulator for solid organs is described, recalling the underlying theory and providing simulation results and comparisons. The main biomechanical parameters (Young's modulus and density) have been integrated in a Mass-Spring-Damper model (MSDm) based on a tetrahedral structured network. The proposed algorithms allow the automatic setting of node mass and spring stiffness, while the damping coefficient have been modeled using the Rayleigh approach. Moreover, the method automatically detects the organ external layer, allowing the usage of both the surface and internal Young's moduli: for the capsule (or stroma) and for the internal part (or parenchyma). Finally the model can be manually tuned to represent lesions with specific biomechanical properties. The method has beed tested with various material samples. The results have shown a good visual realism ensuring the performance required by an interactive simulation.