also called PDT. A treatment sometimes used for cancers of the skin, esophagus (swallowing tube), or lung. It is also being studied for use against some other tumors. A chemical is applied to the skin or injected into a vein and allowed to collect in the tumor for hours or days. A special type of laser light is then focused on the cancer. This light changes the chemical so that it can kill cancer cells with very little harm to normal cells.