Interstitial photodynamic therapy: diagnostic measurements and treatment in experimental malignant rat tumors

A recently developed multiple fiber system for treating malignant tumors with interstitial photodynamic therapy was used in studies on rats with colon adenocarcinoma inoculated into the muscles of the hind legs. The animals were intraperitonially administrated (delta) -aminolevulinic acid (ALA), which is metabolized to protoporphyrin IX (PpIX) in the tissue. The treatment system consists of a laser light source, a beam-splitting system dividing the light into three or six output fibers and a dosimetry program calculating the optimal fiber position within the tumor as well as the treatment time needed to obtain a given threshold value of the light dose. One aim of the study was to compare the treatment outcome with the modelled dosimetry predictions. Tumor reduction was examined three days post treatment. A volume decrease was found in 85% of the treated tumors. The mean volume reduction was 44%, with one tumor completely disappearing. Histopathological examination three days post treatment showed substantial necrotic parts which, however, to a smaller extent were present also for non-treated tumors. These results indicated that the tumors have been under treated and the light dose has to be increased. Measurements of the build-up and photo-induced bleaching of PpIX using laser-induced fluorescence were also performed during the experiments.