Photodynamic therapy (pdt) uses a drug, called a photosensitizer, and a light to kill cancer cells. Depending on the type of cancer, the drug is applied to the surface of the skin or injected. It can be used for certain cancers of the skin, esophagus, and lung. Studies are being done looking at the use of pdt for cancers of the head and neck, brain, and gastrointestinal tract.