Spatial measurement of oxygen levels during photodynamic therapy using time-resolved optical spectroscopy

Tissue oxygenation is one of the key dosimetric factors involved in the application of photodynamic therapy (PDT). However, quantitative studies of oxygenation levels at and surrounding the treatment site have been lacking both before, during and after treatment. With the recent development of sensitive, non-invasive, optical spectroscopic techniques based on oxygen-dependent phosphorescence quenching of probe compounds, oxygenation levels can now be measured quantitatively at selected sites with spatial resolution on the millimeter scale. We present results using the phosphorescent compound, palladium meso-tetra(carboxyphenyl)porphine, for measurement of in vivo microvascular oxygen tensions in rat liver during PDT. Time-resolved phosphorescence detection was carried out using fibre-optic sensoring, and oxygen tensions were determined from the phosphorescence lifetimes using Stern-Volmer analysis. During PDT treatment using 5-aminolaevulinic (ALA) acid-induced protoporphyrin IX (PPIX) with a 50 mg/kg ALA dose, oxygen levels near the irradiation fibre placed on the surface of the liver showed a significant decrease by a factor of ten from 20 to 2 torr after an energy dose of 60 J using 100 mW at 635 nm. Areas farther from the treatment site which were exposed to lower light doses exhibited lower reductions in oxygen levels. This spectroscopic technique is a highly sensitive means of investigating tissue oxygenation during and after treatment, and should help not only to advance the understanding of hypoxia and microvascular damage in the PDT mechanism but also contribute to improving the dosimetry of PDT.