Intrafraction movement by the patient and the tumor during external-beam radiotherapy remains a signicant and challenging problem. Several strategies for automated interventional response to the motion are presently being pursued: (1) temporal gating of the beam; (2) realignment of the beam aperture or linear accelerator for x-ray therapy; (3) realignment of the patient; (4)  electromagnetic steering of a charged particle beam.  Each method requires online knowledge of changes in target position. is knowledge can presently be gained directly using uoroscopic (Shirato et al. 2000), radiographic (Murphy and Cox 1996), or electromagnetic (Kupelian et al. 2007) methods. In addition, there is the future prospect of intrafraction magnetic resonance imaging for motion detection (Dempsey et al. 2005; Raaymakers et al. 2004).