A waveguide and radiation delivery system is disclosed to be used in topical applications of photodynamic therapy. One preferred embodiment consists of a rectangular waveguide that delivers electromagnetic radiation to a flexible conducting sheet that can be molded to the contours of the treatment sight. The sheet is surrounded by a reflective material or foil, except for the portion of the sheet in contact with the treatment area. That portion is covered with a semi-reflective sheet that is at least translucent with respect to the wavelength used in the treatment. This invention increases the speed and efficiency of the procedure by reducing energy loss due to reflection from the skin surface. It also increases safety by protecting sensitive areas of the body, and is particularly useful for head and neck procedures. Other embodiments include the use of a bladder-type applicator or a rectangular applicator designed to keep the optical fiber a specified distance from the treatment site.