Photodynamic therapy (PDT) is also called photoradiation therapy, phototherapy, or photochemotherapy. It involves using drugs, called photosensitizing agents, along with light to kill cancer cells. The drugs only work after they have been activated by certain kinds of light.

Depending on the part of the body being treated, the photosensitizing agent is either injected into the bloodstream or applied to the skin. After the drug is absorbed by the cancer cells a light source is applied only to the area to be treated. The light causes the drug to react with oxygen, which forms a chemical that kills the cancer cells. PDT may also work by destroying the blood vessels that feed the cancer cells and by alerting the immune system to attack the cancer.

The period of time between when the drug is given and 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 and depends on the drug used.

Studies have shown that PDT can work as well as surgery or radiation therapy in treating certain kinds of cancers and pre-cancerous conditions. It may have some advantages, such as:

• It has no long-term side effects when used properly.
• It is less invasive than surgery.
• It can be targeted very precisely.
• Unlike radiation, it can be repeated several times at the same site if necessary.
• There is little or no scarring after the site heals.

However, PDT has its limits, too. It can only treat areas where light can reach, so it is mainly used to treat problems on or just under the skin, or in the lining of internal organs. While the drugs may travel throughout the body, the treatment only works at the area exposed to light, so PDT can’t be used to treat extensive cancers. Also, the drugs that are now in use leave people very sensitive to light, and during this time special precautions must be taken.

Revised: 03/20/2007