Monitoring photosensitizer concentration by use of a fiber-optic probe with a small source-detector separation.

We present a noninvasive method to track the concentration of photodynamic therapy drugs in real time. The method is based on measurements of backscattered and fluorescent light with a steady-state fluorescence spectrometer. The ratio of the fluorescent light to the scattered light is found to be linearly proportional to the absorption coefficient of the photosensitizer. The fiber-optic probe used for the measurements has a small source-detector separation; therefore the measurements could be performed through the working channel of an endoscope.