1. A target locating and tracking system usable with a radiation therapy delivery source that delivers radiation to a target in a body, the radiation being delivered to a predetermined volume configured around a machine isocenter spaced apart from the radiation delivery source, comprising: a marker fixable at a position relative to the target in the body, the marker being excitable by an external excitation source to produce an identifiable marker signal from the marker while in the body; sensors spaced apart from each other in a known geometry relative to each other and positioned to identify the marker signal from the marker, the sensors being configured to measure the marker signal and to provide marker measurement signals; a reference device positionable at a selected position relative to the radiation therapy delivery source and the machine isocenter, the reference device configured to produce a reference signal measurable by a plurality of the sensors; and a data processing unit coupled to the sensors to receive the marker measurement signals, the data processing unit being configured to use the marker measurement signals to determine the location of the target volume with a target isocenter and the location of the reference device relative to the plurality of sensors, and the data processing unit being configured to identify the location of the target isocenter relative to the machine isocenter.