Photodynamic therapy (PDT) is a treatment for tumors and accepted in several countries in the world. Introduction of twophoton absortion (2PA) into PDT allows spatially selective treatment of cancers. Possibilities and limitations of the use of two-photon excitation in PDT are discussed, and many literatures in this area are reviewed. The conclusions are that 2PA-PDT has an advantage for higher selectivity than one-photon absorption PDT, and femtosecond pulsed laser is more suitable for 2PA-PDT than pico- and nanosecond pulses. However, most of photosensitizers used in the past studies had low 2PA cross section values less than 50 GM, and resulted in a low PDT efficiency under two-photon irradiation conditions. To realize 2PA-PDT, much larger 2PA cross sections must be required.
Keywords: Photodynamic therapy, two-photon absorption, porphyrin, Photofrin, cancer, Hela cell, laser, singlet oxygen
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