Photodynamic therapy (PDT) is a treatment that uses a drug, called a photosensitizer or photosensitizing agent, and a particular type of light. When photosensitizers are exposed to a specific wavelength of light, they produce a form of oxygen that kills nearby cells.PDT was approved by the U.S. FDA for clinical application in 1996 and approved for clinical application by Chinese SFDA in 2003.