Introduction: The rate of PpIX fluorescence photobleaching is routinely used as a dose metric for ALA-PDT. Diffuse
reflection spectroscopy is often used to account for variations in tissue optical properties at the photosensitizer excitation
and emission bands. It can be used to quantify changes in vascular parameters, such as blood volume fraction and
saturation, and can aid understanding of tissue response to PDT. The volume and(/or) depth over which these signals are
acquired are critical. The aim of this study is to use quantitative reflectance spectroscopy (DPS) to correct fluorescence
for changes in tissue optical properties and monitor PDT.
Materials & Methods: ALA was topically applied to hairless mice skin and the incubated spot was treated with PDT
according to fractionated illumination schemes. DPS measurements of vascular parameters and optical properties were
performed directly before and after illumination. Both the differential signal, delivery-and-collection-fiber signal and the
collection fiber signal, which all probe different measurement volumes, are analyzed.
Results & Conclusions: Analysis of DPS measurements shows that at the depth where most fluorescence originates,
there is almost no blood present. During PDT vascular parameters at this depth stay constant. In more oxygenated layers
of the tissue, the optical properties do change during PDT, suggesting that only a small part of PpIX fluorescence
originates from the interesting depths where vascular response occurs. Correcting fluorescence emission spectra for
optical changes at specific depths and not for the total of changes in a larger volume, as is usually done now, can
improve PpIX photobleaching based treatment monitoring.