Photodynamic therapy, or PDT, (also called photoradiation therapy, phototherapy, or photochemotherapy) is a type of cancer treatment based on the discovery, made over 90 years ago, that one-celled organisms, if first treated with certain photosensitive drugs, will die when exposed to light at a particular frequency. PDT destroys cancerous cells by using fixed-frequency light that can activate photosensitizing drugs, which have accumulated in body tissues.
In PDT, the photosensitizing agent is injected into a vein, followed by a period of time during which it is absorbed by all cells. The agent rapidly leaves most normal cells, but it remains in cancer cells for a longer period. The treated cancer cells are then exposed to light from a laser chosen for its ability to activate the photosensitizing agent. (Laser light is focused into a beam so it can be aimed at a specific area; laser light produces a narrow range of light frequencies.) The photosensitizing agent in the treated cells absorbs the light, producing an active form of oxygen that destroys surrounding cancer cells. Light exposure must be timed carefully so that it occurs when most of the photosensitizing agent has left healthy cells but is still present in cancerous ones.
The major side effect of PDT is skin sensitivity. The skin in the treated area may remain sensitive to light for 6 weeks or more after treatment. Sunscreens usually are not completely effective in protecting treated skin from the sun, so patients are advised to avoid direct and indirect sunlight for at least 6 weeks. Patients who must go outdoors should wear sunscreen and protective clothing, including sunglasses. Other side effects may include nausea, vomiting, a metallic taste in the mouth, and eye sensitivity to light. These sometimes occur as a result of the injection with the photosensitizing agent.
An advantage of PDT is that cancer cells can be selectively destroyed while most normal cells are spared. However, because the laser light currently in use cannot pass through more than about 3 centimeters of tissue (a little more than one and an eighth inch), PDT is mainly used to treat tumors on or just under the skin or on the lining of internal organs.
The U.S. Food and Drug Administration has approved a photosensitizing agent called dihematoporphyrin ether/ester (DHE), or Photofrin-R, to relieve symptoms of esophageal cancer that is causing obstruction and for esophageal cancer that cannot be satisfactorily treated with lasers alone. The National Cancer Institute and other institutions are supporting clinical trials (treatment studies) to evaluate the use of photodynamic therapy for other cancers. Researchers are looking at different laser types and new photosensitizers that may increase the effectiveness of PDT against tumors that are located further below the skin or within an organ.
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