Evaluation of therapeutic effectiveness using PET

Positron emission tomography (PET) is a very useful technique for not only brain research but cancer diagnosis because it provides us quantitative image of cancer metabolism in living humans. The potentials of PET in cancer diagnosis are (1) detection of tumor, (2) grading of malignancy, (3) biological characterization of tumors and (4) evaluation of therapeutic effectiveness. From these view points, previous PET studies have been introduced. Concerning the first three points, visualization of intrahepatic tumors with 18F-fluorodeoxyglucose (18FDG), grading of glioma with 18FDG and characterization of hepatoma using 18F-fluorodeoxygalactose as an indicator of galactose metabolism in the liver were introduced. Then, a potential of PET for the evaluation of therepeutic effectiveness was focused on. Mouse tumor were irradiated a few days after transplantation. Tumor size increased until one day after irradiation and then started to decrease. Whereas 18FDG uptake in the tumor per gram of tissue decreased just after irradiation and the decrease of 18FDG uptake 5 days after irradiation was much greater than that of tumor size. Another experiments (Kubota et al.) showed that 11C-L-methionine uptake after irradiation responded earlier than that of 18FDG. These data indicated that PET method could detect the tumor response earlier than volume-metry and was more sensitive than morphological method. PET method is most promising for the evaluation of therapeutic effectiveness, because it reflects the tumor metabolism and therefore correspond the behavior of only viable cells. However, further experimental and clinical studies combined with radiobiology should be necessary. Furthermore the limitations of PET method, such as low spatial resolution and hard availability, should be also considered for the establishment of this diagnostic method. (author)