Recent advances in two-photon photodynamic therapy.

Photodynamic therapy (PDT) is a treatment for tumors and accepted in several countries in the world. Introduction of two-photon 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.