Photodynamic therapy or PDT is a treatment that uses special compounds, called photosensitizing agents, along with light (referred to as Blue Light) to treat Actinic keratosis (AKs), rough-textured, dry scaly patches on the skin that are caused by excessive UV exposure such as sunlight. Over a certain amount of time the compound is absorbed by the cancer cells. Then light is applied to the area to be treated. The light causes the compound to react with oxygen, which forms a chemical that kills the cells.