Clinical Applications

Initially, PET was used for studies of the brain and the heart, but its applications in oncology are now expanding more and more. Although many PET studies have focused on organ physiology and pathophysiology, it is increasingly being used in patients for the diagnosis and staging of malignant tumors. The clinical success of PET is primarily related to the use of 18F-fluorodeoxyglucose (FDG), a glucose analogue that assesses regional glucose metabolism within the body. Various malignant tu mors are characterized by increased glucose metabolism, as was first described by Warburg and his group in the early 1920s (Warburg et al. 1930; Warburg 1956). Recently, the molecular basis of this finding has been found. Increased metabolism, including increased rates of glucose consumption, was noted following activation of oncogenes or loss of tumor suppressor genes (Flier et al. 1987). Cells expressing the ras or src oncogenes exhibited increased rates of aerobic glycolysis and increased levels of glucose transporter proteins only hours after malignant transformation by oncogenic viruses. In addition to the increased glucose transport into cells, a five-fold overexpression of the type II hexokinase gene was found in a hepatoma cell line compared with normal hepatocytes (Rempel et al. 1996). Recent molecular studies suggest that cellular energy metabolism is predominantly affected by the expression of transcription factors that regulate the genes that encode metabolic enzymes following the development of malignancy.

Since FDG-PET is able to delineate malignant tissue, metabolic imaging can be used to differentiate between benign and malignant tumors, assess the extension of disease, detect tumor recurrence and monitor response to therapy.

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