Monitoring ALA-induced PpIX photodynamic therapy in the rat esophagus using fluorescence and reflectance spectroscopy

The presence of phased protoporphyrin IX (PpIX) bleach kinetics has been shown to correlate with esophageal response to 5-aminolevulinic acid-based photodynamic therapy (ALA-PDT) in animal models. Here we confirm the existence of phased PpIX photobleaching by increasing the temporal resolution of the fluorescence measurements using the therapeutic illumination and long wavelength fluorescence detection. Furthermore fluorescence differential pathlength spectroscopy (FDPS) was incorporated to provide information on the effects of PpIX and tissue oxygenation distribution on the PpIX bleach kinetics during illumination. ALA at a dose of 200 mg kg-1was orally administered to 15 rats, five rats served as control animals. PDT was performed at an in situ measured fluence rate of 75 mW cm-2using a total fluence of 54 J cm-2. Forty-eight hours after PDT the esophagus was excised and histologically examined for PDT-induced damage. Fluence rate and PpIX photobleaching at 705 nm were monitored during therapeutic illumination with the same isotropic probe. A new method, FDPS, was used for superficial measurement on saturation, blood volume, scattering characteristics and PpIX fluorescence. Results showed two-phased PpIX photobleaching that was not related to a (systematic) change in esophageal oxygenation but was associated with an increase in average blood volume. PpIX fluorescence photobleaching measured using FDPS, in which fluorescence signals are only acquired from the superficial layers of the esophagus, showed lower rates of photobleaching and no distinct phases. No clear correlation between two-phased photobleaching and histologic tissue response was found. This study demonstrates the feasibility of measuring fluence rate, PpIX fluorescence and FDPS during PDT in the esophagus. We conclude that the spatial distribution of PpIX significantly influences the kinetics of photobleaching and that there is a complex interrelationship between the distribution of PpIX and the supply of oxygen to the illuminated tissue volume.

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