A device and method for endovascular radiation therapy that prevents unwanted damage to blood vessels during treatment is described. The device is adapted to prevent the emission end of an energy delivery device from coming into contact with the wall of a blood vessel. The device is also adapted to sense the inputted radiation power and prevent irradiation above a preselected power or energy level. Both features serve to prevent radiation, having an overly high intensity or power density, from impacting the vessel wall and causing a puncture.