An apparatus for applying x-rays to an interior surface of a body cavity includes a catheter assembly, and one or more flexible probe assemblies. An x-ray generator assembly, including an optically activated x-ray source, is coupled to a distal end of each flexible probe assembly. The catheter assembly includes a body member defining one or more interior channels; an x-ray absorption control layer surrounding the body member; at least one inner tube enclosing the body member and the absorption control layer; at least one outer tube; and one or more inflatable elements coupled to the inner tube. The inflatable elements, when inflated, fixedly position the catheter assembly within the body cavity. Each flexible probe assembly is slidably positionable within at least one of the interior channels, and includes a transmission path adapted to transmit an activating energy, such as light from laser, onto a cathode within the x-ray source.