In the first step of PDT for cancer treatment, a photosensitizing agent is injected into the bloodstream. The agent is absorbed by cells all over the body but stays in cancer cells longer than it does in normal cells. Approximately 24 to 72 hours after injection (1), when most of the agent has left normal cells but remains in cancer cells, the tumor is exposed to light. The photosensitizer in the tumor absorbs the light and produces an active form of oxygen that destroys nearby cancer cells (13).

In addition to directly killing cancer cells, PDT appears to shrink or destroy tumors in two other ways (14). The photosensitizer can damage blood vessels in the tumor, thereby preventing the cancer from receiving necessary nutrients. PDT also may activate the immune system to attack the tumor cells.

The light used for PDT can come from a laser or other sources (2, 5). Laser light can be directed through fiber optic cables (thin fibers that transmit light) to deliver light to areas inside the body (2). For example, a fiber optic cable can be inserted through an endoscope (a thin, lighted tube used to look at tissues inside the body) into the lungs or esophagus to treat cancer in these organs. Other light sources include light-emitting diodes (LEDs), which may be used for surface tumors, such as skin cancer (5).

PDT is usually performed as an outpatient procedure (6). PDT may also be repeated and may be used with other therapies, such as surgery, radiation therapy, or chemotherapy (2).

Extracorporeal photopheresis (ECP) is a type of PDT in which a machine is used to collect the patient’s blood cells, treat them outside the body with a photosensitizing agent, expose them to light, and then return them to the patient. The U.S. Food and Drug Administration (FDA) has approved ECP to help lessen the severity of skin symptoms of cutaneous T-cell lymphoma that has not responded to other therapies. Studies are under way to determine if ECP may have some application for other blood cancers, and also to help reduce rejection after transplants.