Phototherapy is a treatment that uses special drugs called photosensitizing agents, along with light to kill cancer cells. The drugs only work after they have been activated or turned on by certain kinds of light. Depending on the part of the body being treated, the photosensitizing agent is either put into the bloodstream through a vein or put on the skin. Over a certain amount of time the drug is absorbed by the cancer cells. Then light is applied to the area to be treated. The light causes the drug to react with oxygen, which forms a chemical that kills the cells. Phototherapy might also help by destroying the blood vessels that feed the cancer cells and by alerting the immune system to attack the cancer.