Photodynamic therapy

Photodynamic therapy treats disease by activating a light-sensitive drug in the presence of oxygen, producing reactive molecules that damage targeted tissue.

Photosensitizer: A molecule that absorbs light and transfers its energy or electrons to surrounding molecules. It is the drug component that absorbs the treatment light and initiates tissue damage.
Basal-cell carcinoma: A common skin cancer arising from basal cells in the epidermis. Superficial lesions can be treated with photosensitizers and externally applied light.
Photochemistry: The study of chemical reactions initiated by absorption of light. Photodynamic therapy is a biomedical application of light-driven chemical reactions.
Radiation therapy: A cancer treatment that uses ionizing radiation to damage tumor cells and their DNA. It can treat deep tumors without requiring a photosensitizer or direct access for light.
Photosensitivity: An increased tendency for skin or eyes to react abnormally to light. Some photosensitizers remain in the body and require precautions against bright light.
Reactive oxygen species: Chemically reactive oxygen-containing molecules, including singlet oxygen and superoxide. These molecules carry out much of the cell damage after a photosensitizer is activated.
Actinic keratosis: A rough, scaly skin lesion caused by ultraviolet damage that can develop into squamous-cell carcinoma. Topical photodynamic therapy is an established treatment for these sun-damaged lesions.
Light penetration in biological tissue: The depth to which light travels through tissue before absorption and scattering reduce its intensity. Limited penetration restricts treatment of tumors that cannot be reached by surface illumination.
Laser surgery: Surgery that uses focused laser energy to cut, remove, or destroy tissue. It may also use light, but destroys tissue directly rather than through a drug-oxygen reaction.
Skin phototoxicity: Skin injury caused when a light-sensitive substance reacts to light exposure. It is a characteristic adverse effect when photosensitizer reaches skin exposed to ambient light.

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