The role of positron emission tomography in colorectal carcinoma

Positron emission tomography (PET) with 18F-fluorodeoxyglucose (FDG) is a functional imaging modality that provides mapping of glucose metabolism in the whole body. The glucose analogue fluorodeoxyglucose is labeled with the cyclotron-produced, positron-emitting radioisotope fluorine-18. The resulting radiopharmaceutical FDG is a substrate for glucose transport proteins (Glut) in cell membranes and accumulates intracellularly. Increased metabolic activity in malignant tissue is accompanied by increased glucose uptake relative to that of surrounding normal tissue. This focal increase in glucose uptake can be identified with FDG PET, which allows identification of malignant tumor foci. Multiple reports have shown that positron emission tomography with 18F-fluorodeoxyglucose scanning (FDG-PET) is highly accurate in detecting early localized tumor recurrence with a sensitivity and specificity in the mid nineties. FDG-PET scanning evaluates abdomen, chest, and pelvis in one examination setting, permiting identification of local recurrence as well as distant metastasis. FDG-PET is also highly sensitive in detecting hepatic and extra-hepatic metastasis. Finally, FDG-PET scanning can distinguish post-treatment (postoperative and postradiation therapy) scarring from recurrent tumors since malignant tumors are metabolically active and FDG-avid on PET imaging and scar tissue is not. This high accuracy in identifying early stage recurrent tumors with FDG-PET is crucial for potential surgical cure and improving patient outcomes.