Abstract: A photodynamic therapy method administers, to a patient to be treated, a photosensitive medication which fluoresces when illuminated. The fluorescent light is an indication of the instantaneous phototherapy dose. A catheter according to the invention includes a probe or sensor arrangement for generating a signal representing the instantaneous amplitude of the fluorescence or the incident illumination. The signal is integrated to produce a signal representing the total dose of phototherapy. The photodynamic therapy is terminated when the integrated signal indicates a particular value. The catheter in one embodiment includes an array of semiconductor light sources near its distal end, energized from the proximal end. In another embodiment, the light source is external to the patient, and the light is conducted to the distal end of the catheter by one or more optical fibers.