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What is photodynamic therapy?
Photodynamic therapy (PDT) is a treatment that uses a drug, called a photosensitizer or photosensitizing agent, and a particular type of light. When photosensitizers are exposed to a specific wavelength of light, they produce a form of oxygen that kills nearby cells
Each photosensitizer is activated by light of a specific wavelength. This wavelength determines how far the light can travel into the body. Thus, doctors use specific photosensitizers and wavelengths of light to treat different areas of the body with PDT.
How is PDT used to treat cancer?
In the first step of PDT for sun damage and sun cancer treatment, a photosensitizing agent is applied onto the skin for three hours. 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 application, when most of the agent has left normal cells but remains in cancer cells, the treated area is exposed to light. The photosensitizer in the tumour absorbs the light and produces an active form of oxygen that destroys nearby cancer cells.
In addition to directly killing cancer cells, PDT appears to shrink or destroy tumours in two other ways. The photosensitizer can damage blood vessels in the tumour, thereby preventing the cancer from receiving necessary nutrients. In addition, PDT may activate the immune system to attack the tumour cells.
The light used for PDT can come from a laser or other sources of light.
PDT may be repeated and may be used with other therapies, such as surgery, radiation, or chemotherapy.
What are the limitations of PDT?
The light needed to activate most photosensitizers cannot pass through more than about one-third of an inch of tissue (1 centimeter). For this reason, PDT is usually used to treat tumors on or just under the skin or on the lining of internal organs or cavities (3). PDT is also less effective in treating large tumors, because the light cannot pass far into these tumors (2, 3, 6). PDT is a local treatment and generally cannot be used to treat cancer that has spread (metastasized) (6).
Does PDT have any complications or side effects?
PDT temporarily makes the skin and eyes sensitive to light for up to 6 weeks after treatment (1, 3, 6). Thus, patients are advised to avoid unprotected direct sunlight and for at least 6 weeks.
Photosensitizers tend to build up in tumors and the activating light is focused on the tumor. As a result, damage to healthy tissue is minimal. However, PDT can cause burns, swelling, pain, and scarring in nearby healthy tissue (3). Other side effects of PDT are related to the area that is treated.
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