2.2.2 FDG. 18F-fluorodeoxyglucose (FDG) is the most common PET-tracer used in more than 95% of oncologic PET studies. FDG is a metabolic tracer depicting cellular glucose uptake, which is increased in cancer, inflammation and other conditions with a rapid cellular turnover. It is trapped inside the cells depending on the presence and activity of the enzyme glucose-6-phosphatase, which appear to be decreased in proportion to the aggressiveness of the individual malignancy. Increased cellular FDG accumulation is, therefore, not specific for malignancies, but at the same time an indicator of the degree of malignancy, which may cover the entire spectrum ranging from insignificant and non-aggressive to highly aggressive and rapidly metastasizing PCa FDG-PET imaging for PCa has been intensively investigated, but so far without convincing success. The reasons are several. Well-differentiated PCa's utilizes less glucose than other tumors in general and thus may give rise to results considered false negative. Moreover, since surplus radioactive FDG is excreted through the kidneys, intense accumulation in the ureteres and urine bladder may hamper interpretation of the prostate gland and nearby regional lymph nodes .