Interstitial Photodynamic Therapy (iPDT) for malignant gliomas using 5-ALA and cylindrical light-diffusing fiber applicators is currently under development. In a preceding investigation, the effects of 5-ALA iPDT on haemoglobin-related light absorption in a liquid phantom model with intact erythrocytes and protoporphyrin IX were experimentally assessed. Spectral monitoring identified a light-induced formation of deoxygenated haemoglobin and methaemoglobin after the onset of iPDT irradiation.
This study summarises the retrospectively analysed spectral data acquired during clinical iPDT interventions of malignant gliomas. The results were analysed for correlations with early postoperative T1-weighted MRI data obtained without a contrast agent. Intrinsic T1-hyperintensity signals were observed and found to be associated with the treatment volume in all patients. This may indicate the presence of methaemoglobin, possibly induced by iPDT. From spectrally resolved recordings of light detected through each of the inserted cylindrical diffusers during tissue illumination through one of the other diffusers, treatment light transmission through and PpIX fluorescence of the tissue could be evaluated. The optical absorption coefficient μa and its change due to iPDT irradiation were estimated for all tissue volumes between evaluable fiber pairs. In cases with intrinsic T1-hyperintensity involvement, the absorption increase during iPDT-irradiation was significantly higher than in cases with minor T1-hyperintensity (p=0.003).
The observations are consistent with in-vitro experiments and indicate iPDT-induced deoxygenation of haemoglobin and formation of methaemoglobin, potentially after injury to capillary vessels. Further clinical investigations are needed to provide more data on the time course of the observed changes, thus paving the way for optimized irradiation protocols.