To provide to and receive radiation from a photodynamic drug in neoplastic tissue, a laser system transmits radiation through an interface into a radiation delivery system, which is in juxtaposition with neoplastic tissue containing a photodynamic drug. The laser system may be a single argon laser pumping a dye laser, two parallel sets of argon lasers pumping a dye laser, a krypton laser or a xenon laser. In addition to transmitting radiation from the laser to the delivery system, the interface system may: (1) direct a portion of the light back to the laser's power supply to control the intensity of the radiation emitted from the laser; and/or (2) receive light from the light conductors of the delivery system. The interface channels light to radiation sensing devices which are either from a beam splitter indicating the magnitude of the radiation delivered from the laser system to the radiation delivery system or radiation leaking through the light conductor. Luminescent light from the photodynamic drug is selected and provides an indication of drug density and in some embodiments depth of the activity.