Also known as photodynamic therapy, it was developed at Roswell Park Memorial Institute in Buffalo, New York in the early 1970's. A non-toxic drug, Hpd, is injected and is absorbed only by malignant cells. It sensitizes these malignant cells to light. About three days later, an intense laser light is shined on the tumor for 8 to 10 minutes, producing high-powered singlet oxygen inside the cell so reactive that it burns up everything in sight, destroying the cancerous growth. Since the light can only penetrate 5 to 10 millimeters, it does not work well on treating thick or deep-seated tumors. It appears to work best on early to middle-stage cancers of the lung, bronchi and bladder. The use of die laser is increasing dramatically in many major cities, but it is still generally considered an experimental therapy.