Monitoring PDT by means of superficial reflectance spectroscopy

Monitoring of relevant parameters during photodynamic therapy (PDT) and correlating these with treatment response is necessary to guarantee optimal and reproducible treatment outcome. In this paper we study the correlation between changes in the local tissue optical properties (absorption and scattering coefficients) during ALA-PDT and changes in PpIX fluorescence. The optical properties are measured extremely superficially by employing a single fiber for the delivery and collection of white light to and from the tissue. The measured reflectance spectrum is modeled in terms of four relevant parameters: blood saturation, relative blood volume fraction, scattering intensity and wavelength dependence of the scattering. All these parameters, except the relative blood volume fraction, are shown to correlate with the rate of photobleaching of PpIX, which in turn has previously been shown to correlate with the response of tissues to PDT. These results yield valuable insight in the behavior of these parameters during PDT and their suitability to predict PDT-response for other photosensitizers for which monitoring through photobleaching is not possible.