The long-term objective of this project is to develop a framework for quantitative dosimetry in photodynamic therapy. Two parallel avenues of investigation will be pursued: i)Explicit dosimetry, where the goal is to evaluate the photosensitizer concentration and light fluences in individual patients. This approach attempts to quantify the first step (i.e. excitation of the photosensitizer) in PDT which may be predictive of tumor response. Three methods will be investigated for noninvasive measurement of photosensitizer concentration and tissue optical properties. For uniform tissues, fluorescence spectroscopy will be used to quantify the amount of photosensitizer present and spatially-resolved reflectance spectroscopy will be used to determine the absorption and scattering coefficient of the tissue and to correct the fluorescence measurements for tissue attenuation. For layered tissues, noninvasive spatially- resolved frequency domain reflectance spectroscopy will be used to determine the optical properties of each of the tissue layers. This information will be combined with fluorescence spectroscopy to measure the concentration of photosensitizer in each layer. Finally, for tissues not accessible to surface measurements, interstitial measurements of light fluence and fluorescence will be used. ii) Implicit dosimetry, where the goal is to incorporate effects which occur after the initial step of photosensitizer excitation and which may affect biological response, for example, reduced oxygen tension due to photochemical depletion or reduced blood flow during PDT. If the photosensitizer itself reacts with the toxic photoproduct (e.g. singlet oxygen) and is altered, it may be possible to use measurement of this alteration as an implicit measure of the local photodynamic effect. It will be determined whether photosensitizer fluorescence, which is often observed to decrease during PDT, can be used as a reliable predictor of biological response.