Localization of the transponders is achieved using an electromagnetic array consisting of radiofrequency signaling and receiving coils. Each of the three transponders that are implanted has a unique resonant frequency. e transponders absorb some of the radiofrequency energy and reemit that energy in the form of a decaying signal that is detected by the electromagnetic array. e transponder position is then detected relative to the array, which is calibrated to the room reference frame by three rigidly mounted infrared cameras. A misalignment of the target can be detected by a proprietary algorithm that identies shis from its prescribed location anytime throughout the treatment. e accuracy of the detection of a target in phantom is less than 1 mm. e Calypso system displays real-time graphs instantaneously highlighting shis in position that exceed a user-specied threshold. A radiation monitoring device inside the room makes it possible to generate reports synchronized to the radiation delivery. Figure 79.1 shows the beacon transponders and the Calypso system along with a sample report.