In a heart catheter which consists of a hose having a sonde with an electrode structure for locating pathological areas arranged at its distal end, a light transmitting fiber is movably disposed in the hose and the hose has sensors associated therewith so as to be movable together with the fiber out of the distal end of the hose for engagement with the tissue of the pathological area for positioning the distal end of the hose at a predetermined distance and location relative to the pathological area in order to provide for appropriate irradiation of the pathological area by radiation energy supplied through the light transmitting fiber.