The period between when the drug is given and when the light is applied is called the drug-to-light interval. It can be anywhere from a couple of hours to a couple of days, depending on the drug used.
The light used in PDT comes from certain kinds of lasers or from light-emitting diodes (LEDs). The kind of light used depends on the type of cancer and where it is in the body. PDT is usually done as an outpatient procedure but is sometimes combined with surgery, chemotherapy or other anti-cancer drugs, or radiation therapy.