Abstract: Apparatus is disclosed for the uniform irradiation of the inner wall of a hollow organ. A catheter has a translucent balloon on its end. An optical sensing fiber is affixed to or within the balloon wall material. The apparatus is employed according to the following steps: inserting the catheter into the interior of the organ; inflating the balloon until it forms a predetermined configuration and distends the inner wall of the organ to be irradiated into a contiguous and congruent configuration; inserting a light transmission fiber having an isotropic light diffuser tip into the catheter so that the tip is positioned at the center of the balloon and, hence, automatically positioned at the center of the wall to be irradiated; transmitting light through the fiber out the tip, whereby light of uniform intensity irradiates the wall; and monitoring with the sensing fiber on the wall of the cavity the light to which the sensing fiber and, hence, the cavity wall are exposed.