An exciting application for lasers in medicine is photodynamic therapy (PDT). PDT is a unique form of cancer therapy capitalizing on a photochemical reaction. Modern use of PDT is limited to the past 40 years, however ancient physicians used similar methods to treat diseases as early as 1200 ВС.PDT involves giving a light-activated drug, called a photosensitizer, to the patient and then irradiating the tumor with visible light. Because of the mechanism of action, PDT is a highly selective form of cancer therapy, especially when compared to ionizing radiation therapy or systemic chemotherapy. Neither the visible light nor photosensitizer causes any harmful tissue effects. Tissue damage only occurs with the presence of photosensitizer, light of the appropriate wavelength, and molecular oxygen simultaneously. The wavelength of light used for treatment will correspond to the peak absorption wavelength of the photosensitizer. The photosensitizer absorbs a photon and is then activated to an excited singlet or triplet state. The activated photosensitizer can then react with oxygen or any nearby molecule to cause oxidative cellular damage.
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