Patient anatomy and position during the course of radiation therapy can vary from those used for treatment planning; a function of patient movement, uncertainty in positioning system, and organ motion. Traditionally, treatment margins are designed to compensate for interfraction prostate setup variability. This approach has the potential to lower the overall effectiveness of treatment because the prostate gland is a continuously moving target whose motion cannot be accurately accounted for solely on the basis of interfraction movement. More recently, the dosimetric relevance of intra-fraction prostate motion has been recognized, and may be compensated for by continuous real-time adaptive radiation therapy afforded by the Calypso 4D Localization System™. In the current study, the investigators propose to characterize intrafraction prostate motion. The investigators hypothesize that intrafraction prostate motion is correlated with respiratory motion.