Rectal balloon with radiation sensor and/or markers

A prostate immobilizing rectal balloon, said rectal balloon comprising:

• a. a flexible shaft having a fluid passageway extending therethrough and having a distal end and a proximal end,
• b. a balloon having an upper surface, a bottom surface, a distal end near said distal end of said shaft and an proximal end that is affixed to the proximal end of said shaft, such that said fluid passageway communicates with an interior of said balloon,
• c. wherein said balloon comprises a top layer connected to a bottom layer to form said balloon,
• d. said balloon further comprising a middle layer extending between said top layer and said bottom layer, wherein said middle layer is connected to said top layer at a groove overlying said shaft,
• e. wherein said balloon has a non-inflated condition,
• f. wherein said balloon has an inflated condition, wherein said groove has depth and forms a central seating area that is configured to engage with a prostate, and
• h. said balloon further comprising i) a radiation sensor in said groove for determining radiation dose from an external beam radiation device, or ii) one or more fiducial markers, or iii) both of i) and ii).