National Cancer Centre - Cancer Update

Positron emission tomography (PET) is a functional anatomic imaging modality of nuclear medicine with direct applications in oncology, cardiology and neurology. F-18-fluoro-deoxyglucose (FDG) is an analogue of glucose and currently is the most commonly used PET radiotracer. Various molecular derangement in malignant cells including increased glycolytic rates result in increased cellular uptake of FDG. A FDG PET scan can detect and localize such abnormal concentrations of FDG. FDG PET scans are often useful in pretreatment staging (for local and distant disease), evaluation of response to treatment and the detection of recurrence of disease. At our facility in SGH, we currently perform PET/CT studies, either with or without IV contrast. So far, published literature has shown superior diagnostic correlation and accuracy with PET/CT compared to PET alone. This brief article is intended as a very concise description for clinicians on the utility of FDG PET scans in upper gastrointestinal malignancies.

Oesophageal cancer
FDG PET is highly sensitive in the detection of esophageal carcinoma an overall detection rate of 80%, approaching 100% for larger (T3 and T4 tumours). Higher intensity uptake suggests a poorer prognosis. FDG PET is useful for detecting distant metastases and distant nodal disease, particularly in the pericoeliac region, although endoscopic ultrasonography is probably more sensitive in local nodal evaluation. Post treatment scans can be useful in documenting the response to chemotherapy/radiotherapy with persisting activity suggesting non-responders.

Gastric Cancer
FDG PET scans are useful for the detection of gastric cancers. It is well-known that the degree of FDG uptake varies with the histological subtype of gastric carcinoma with relatively low levels of uptake seen in signet ring cell carcinoma and mucinous adenocarcinoma , a potential cause of false negative results. However, accuracy is relatively high for the primary lesion, distant nodal disease and distant metastases. Initial studies suggest that FDG PET may also help to differentiate responders and non-responders early in the course of chemotherapy.

Pancreatic Cancer
FDG PET is helpful for detecting pancreatic adenocarcinoma (sensitivity of 92%), but the uptake is relatively low for cystic masses. FDG PET is useful in detecting local and distant disease such as hepatic lesions (>1cm), response to therapy and recurrence. While chronic pancreatitis can cause false positives, 87% of cases of chronic pancreatitis will have negative PET findings.

Hepatocellular carcinoma
Hepatocellular carcinoma is known to give rise to a variable FDG uptake and in some cases, no uptake at all. This may be related to its metabolic pathways. The use of IV contrast PET/CT fusion imaging may help to characterise hepatic neoplasms more effectively than PET alone.

Cholangiocarcinoma
Choloangiocarcinoma has a striking FDG uptake and can often be easily detected with FDG PET. Small tumours in the gallbladder, common bile ducts and intrahepatic ducts have been successfully visualised on FGD PET. FDG PET is also useful in evaluating biliary strictures where the issue of malignancy is not resolved by anatomic imaging.

Early Therapy Response
PET is fundamentally a molecular/metabolic imaging modality. Due to the intrinsic metabolic nature of PET imaging, studies of various tumour types have noted the potential of PET to differentiate early response/no-response to treatment, even before anatomic changes occur. This may allow early stratification of patients into responders and non-responders based on molecular/biochemical behaviour and provide important information to direct therapy or early modification of treatment.