Introduction to photodynamic therapy

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Lara Bonner Millar, MD
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: August 16, 2011

Photodynamic therapy (PDT) is a treatment that uses light to damage malignant or abnormal tissues. PDT is FDA-approved for providing relief of obstruction caused by esophageal cancer or non-small-cell lung cancer (NSCLC); treatment of microinvasive endobronchial NSCLC in patients not recommended for surgery or radiotherapy; treatment of Barrett's Esophagus and high grade dysplasia; actinic keratosis; and age-related macular degeneration.

PDT requires a light source, such as a laser, combined with a drug that makes the tissues light-sensitive, which is known as a "photosensitizer." When the light and photosensitizer are combined, oxygen-free radicals that are able to destroy cancer cells are released. Photosensitizers are taken up in greater amounts by cancer cells compared to normal cells.

A photosensitizing drug is given to the patient by an intravenous (IV) line a few days prior to the light exposure, but the drug is not activated until it is exposed to a particular wavelength of light. When the light is directed at the area of the cancer, the photosensitizer is activated and the cancer cells are destroyed. This wavelength determines how far the light can travel into the body. Typically, the depth of penetration is on the order of millimeters. Therefore, PDT is generally not used to treat large tumors, because the light cannot reach the necessary depth to treat those tumors. Different photosensitizers are activated by different wavelengths of light; therefore, depending on the area of the body to be treated, there are different photosensitizing drugs and different wavelengths of light that can be used.

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