Optical monitoring of singlet oxygen generation during photodynamic treatment of tumors

The author reports optical detection of singlet oxygen production accompanying continuous-wave (CW) laser irradiation of subcutaneous murine tumors at 630 nm following prior intraperitoneal injection of a Photofrin II sensitizer. The detection method is optical, remote, and, thus, noninvasive. Chopping of the incident laser beam was required in order to separate the spectrally discrete, time-delayed, singlet oxygen emission from the dominant, spectrally diffuse, coherent background provided by the combination of sensitizer infrared fluorescence and tissue-related autofluorescence. Using the infrared fluorescence to provide a reference, the singlet oxygen emission is shown to be given directly by the frequency-dependent quadrature component of the detector output. Maximum detector quadrature output for the in vivo case was obtained for a chopping frequency between 10 and 20 kHz. The spectral variation of the emission from the tumor was obtained and identified as that characteristic of singlet oxygen.