Analysis of 18F-fluorodeoxy-glucose PET imaging data captured before and after Pc 4-mediated photodynamic therapy of U87 tumors in the athymic nude rat

Introduction: Several workers have proposed the use of PET (Positron Emission Tomography) imaging for the outcome assessment of photodynamic therapy (PDT), especially for deep-seated tumors. We report on our study of 18Ffluorodeoxy- glucose (18F-FDG) PET imaging following brain tumor Pc4-PDT. Our working hypothesis was that the tumor's metabolic activity would decline dramatically following Pc 4-PDT owing to tumor necrosis. Methods: Seven days after intraparenchymal implantation of U87 cells, the brains of 12 athymic nude rats were imaged by micro-CT and/or micro-MR. These animals were also 18F-FDG micro-PET (mPET) scanned before and after Pc 4-PDT. 18F-FDG was used to trace metabolic activity that was monitored via mPET. Occurrence of PDT was confirmed on histology. The analysis of 18F-FDG dose and animal weight normalized mPET activity was studied over the 90 minute µPET scan. Results: Currently, mPET data have been studied for: (1) three of the animals that did not indicate tumor necrosis on histology and were assigned to a "Non-PDT" group, and (2) six animals that exhibited tumor necrosis on histology and were assigned to a "PDT" group. The mPET-detected 18F-FDG uptake activity in the tumor region before and after photoirradiation increased in the Non-PDT group an average of 2.28 times, and in the PDT group it increased an average of 1.15 times. Discussion: We are investigating the cause of the increase in 18F-FDG mPET activity that we observed in the PDT group. The methodology used in this study should be useful in determining whether this or other PET, SPECT, or MR functional imaging protocols will detect both the specificity and sensitivity of brain tumor necrosis following Pc 4-PDT.