The Calypso® 4D Localization System™ is an FDA approved tumor localization system for adjunctive use in patients undergoing radiation therapy for prostate cancer. The system is designed to provide accurate, objective, and continuous 3-dimensional localization of the treatment target for patient alignment and target position monitoring relative to the isocenter of the linear accelerator. The Calypso system consists of implantable electromagnetic transponders, each with a unique resonant frequency. An array is placed over the patient and remains in place during radiation delivery. Source coils in the array emit electromagnetic signals that excite the transponders identifying the position of each individual transponder. The transponder position can be continuously monitored throughout treatment delivery reporting transponder motion in lateral, superior-inferior (SI), and anterior-posterior (AP) directions.