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The present research activities of Professor Foster's group involve medical diagnostic and therapeutic applications of visible and near-infrared light. In particular, the group is studying various aspects of photodynamic therapy (PDT), a relatively new cancer intervention that recently received limited approval in the U.S. and several other countries. PDT uses certain tumor seeking compounds which, when irradiated with visible light, initiate cytotoxic photochemical reactions that produce local tumor necrosis. These photochemical reactions depend on oxygen consumption. Professor Foster's group is studying the detailed kinetics of the photochemical depletion of tissue oxygen and its re-supply through diffusion. Microelectrode measurements of oxygen consumption in multi-cell tumor spheroids during laser irradiation have been an important source of information regarding the rates of these processes and the rates and mechanisms of photosensitizer degradation (bleaching). Recently we have extended these experiments to include direct measurements of photosensitizer fluorescence and photoproduct formation in vivo. In parallel experiments, we are using laser scanning confocal fluorescence imaging and spectroscopy to monitor these quantities in spheroids. In close collaboration with molecular biologists, we have begun to explore the use of fluorescent reporters of gene induction by PDT.
The group is also interested in optical properties of tissue and specifically, in spectroscopic methods to characterize tumor oxygenation and other aspects of the tumor micro-environment non-invasively. Quantitative spectroscopy is complicated by the fact that tissue is strongly scattering at visible and near infrared wavelengths. Using approaches based on radiative transport theory, the group has recovered accurate hemoglobin absorption spectra from strongly scattering, tissue-simulating phantoms and from the surface of laboratory tumor models in vivo. We have begun to use confocal fluorescence imaging of tumor sections to observe the recruitment of immune cells into the tumor and to measure the activity of enzymes that may contribute to tumor micro-invasion and metastasis.
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