Photodynamic therapy in gynecology

PDT is a technique in which visible light is used in combination with photosensitizing agents to achieve a tumoricidal effect. Hematoporphyrins are the most commonly used photosensitizers in clinical practice. DHE is the active fraction of hematoporphyrin. Intravenously injected DHE is found in highest concentration in the liver followed by the spleen, kidney, tumor, skin, muscle, brain, and lungs. The strongest absorption bands for DHE are in the blue region of the spectrum, and this helps to account for the skin toxicity associated with PDT. Red light, at the wavelength of 630 nm, is usually used clinically because of its greater tissue penetration. Techniques such as photobleaching and use of photosensitizers that have weak absorption bands at the lower wavelengths may reduce cutaneous toxicity in the future. Other approaches, such as the use of monoclonal antibody-linked photosensitizers or cationic photosensitizers that are specifically localized in tumor cells, may also increase the effectiveness of PDT while decreasing toxicity. Light for PDT is usually provided by argon-pumped dye lasers or metal vapor lasers. Diode lasers will be used in the future. The use of fiber-optics and diffusing lenses allows the endoscopic and interstitial use of PDT. The mechanism of action of PDT involves the formation of singlet oxygen, which oxidizes biologic molecules and causes irreversible subcellular damage. The major in vivo effect of PDT is caused by its destruction of tumor vasculature, causing anoxia and necrosis. The use of PDT in gynecology has been limited. Several investigators have reported mixed results in treating lower genital tract intraepithelial and recurrent malignant tumors using a variety of approaches involving PDT. The use of PDT in other similar, though nongynecologic, tumors offers a direction for future research.