Photodynamic therapy (PDT)

Treatment happens in two steps. First the photosensitizing drug is given — into a vein, by mouth, or onto the skin. Over the next 24–72 hours it clears from most healthy cells but stays concentrated in cancer cells. The treated area is then exposed to light: a lamp or laser aimed directly at a skin lesion or, for tumors inside the body, light delivered through thin fiber-optic cables positioned with an endoscope. The activated drug produces a reactive form of oxygen that destroys the cells; PDT may also damage the blood vessels that feed the tumor and prompt the immune system to attack it. The procedure is usually done on an outpatient basis and can be repeated or combined with other treatments. Its main limitations are the shallow reach of light — large or deep tumors respond poorly — and temporary light sensitivity: with porfimer sodium, the skin and eyes remain sensitive to light for about six weeks, so strong light must be avoided. Local side effects can include burns, swelling, pain, and scarring; other effects depend on the area treated.