Monitoring blood volume and saturation using superficial fibre optic reflectance spectroscopy during PDT of actinic keratosis

Optically monitoring the vascular physiology during photodynamic therapy (PDT) may help understand patient-specific treatment outcome. However, diffuse optical techniques have failed to observe changes herein, probably by optically sampling too deep. Therefore, we investigated using differential path-length spectroscopy (DPS) to obtain superficial measurements of vascular physiology in actinic keratosis (AK) skin. The AK-specific DPS interrogation depth was chosen up to 400 microns in depth, based on the thickness of AK histology samples. During light fractionated aminolevulinic acid-PDT, reflectance spectra were analyzed to yield quantitative estimates of blood volume and saturation. Blood volume showed significant lesion-specific changes during PDT without a general trend for all lesions and saturation remained high during PDT. This study shows that DPS allows optically monitoring the superficial blood volume and saturation during skin PDT. The patient-specific variability supports the need for dosimetric measurements. In DPS, the lesion-specific optimal interrogation depth can be varied based on lesion thickness.