Currently, physicians are using a new technique based on radiofrequency (RF) induced hyperthermia, to treat tumors in the musculoskeletal system. This method uses a RF probe, which is placed inside the tumor, which in turn is heated within the active volume of the probe, eliminating in this way the tumor cells without affecting healthy cells. For this treatment, the surgeon must know the exact shape and location of the tumor. Usually a nuclear magnetic resonance imaging (NMRI) study is performed before surgery. Ultrasound imaging is used to locate the tumor intraoperatively. In this work is presented a computer system to build a 3D model of the tumor and adjacent bones based on segmented images from the preoperative NMRI study. Then, during surgery, the model will be registered with the anatomy of the patient, using intraoperative ultrasound images segmented by the surgeon. This work is part of a larger project, which will be a complete computer aided surgery (CAS) system to help train physicians in radiofrequency treatment and to make the procedure more reliable and efficient.