A system and method for accurately locating and tracking the position of a target, such as a tumor or the like, withina body. In one embodiment, the system includes one or more excitable beacons positionable in or near the target, an external excitation source that remotely excites the beacons to produce an identifiable signal, and a plurality of sensors spaced apart in a known geometry relative to each other. A computer is coupled to the sensors and configured to use the beacon measurements to identify a target isocenter within the target. The computer compares the position of the target isocenter with the location of the machine isocenter. The computer also controls movement of the patient and a patient support device so the target isocenter is coincident with the machine isocenter before and during radiation therapy.
