Validation of a non-invasive fluorescence imaging system to monitor dermatological PDT

Methyl-aminolevulinate (MAL) photodynamic therapy (PDT) involves selective accumulation of a photosensitiser, protoporphyrin IX (PpIX), primarily in tumour tissue, which in combination with visible light and tissue oxygen results in reactive oxygen species (ROS) production and thus cellular destruction.

A non-invasive fluorescence imaging system (Dyaderm, Biocam, Germany) has been employed to acquire colour (morphological) and fluorescent (physiological) images simultaneously during dermatological PDT. This system had been previously utilised for fluorescence diagnosis, however, here changes in PpIX concentration within the skin lesions and normal tissue were followed after MAL application. Measurements were also recorded from a synthetic PpIX standard.

Results indicated that imaging distance, imaging angle, position of the region of interest and light conditions all altered the PpIX levels acquired from the synthetic PpIX standard. The imaging system was therefore adapted and a standard operating procedure developed allowing reproducible images of dermatological lesions to be acquired. Different concentrations of synthetic PpIX were analysed with the system and a linear relationship was observed between the PpIX concentration and the mean greyscale value calculated for the images acquired up to 10 microM.

The Dyaderm imaging system can now be used reproducibly with confidence to semi-quantify PpIX (within the range of 0-10 microM) within dermatological lesions using the standard operating procedure derived from this work.