A rectal balloon apparatus has a shaft with a fluid passageway extending therethrough. A balloon is affixed over an end of the shaft such that the fluid passageway communicates with an interior of the balloon. The balloon has a laterally flat surface thereon when inflated. The laterally flat surface of the balloon has a longitudinal groove formed thereon. The balloon has a radiation sensor positioned within the groove and a plurality of fiducial markers positioned on a surface of the balloon. A stopper is slidably mounted on the shaft.
