Optical monitoring of singlet oxygen generation during photodynamic treatment of tumors

Optical detection of singlet oxygen production accompanying continuous-wave (ON) laser irradiation of subcutaneous murine tumors at 630 nm following prior intraperitoneal injection of a Photofrin H sensitizer has been demonstrated, 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 auto fluorescence, chopping of the incident laser beam was required. 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. This demonstration provides the rationale for the development of a clinically useful electro-optical system to provide a realistic means for monitoring treatment effectiveness during the photo-dynamic therapy of tumors as well as the establishment of much-needed dosimetry standards. It is expected that, in the future, development of this capability will provide an analytical, quantitative means to remove some of the mysteries now impeding progress in this important area of medicine.