Photodynamic Therapy (PDT) is a type of laser therapy where a special chemical (light-activated) that is injected into the bloodstream and absorbed by cells all over the body. This special chemical quickly leaves the normal cells but stays in cancer cells for a longer period of time.

When a laser light (a narrow beam of intense light) is aimed at the cancer cells the special chemical will cause them to light up. The laser beam then is used to destroy the cancer cells.