Pancreas on FDG PET/CT

Pancreatic Adenocarcinoma

Primary pancreatic adenocarcinomas vary widely in their FDG avidity, ranging from markedly FDG avid to not appreciable on FDG positron emission tomography (PET). Do not allow mild or lack of FDG avidity in a primary pancreatic adenocarcinoma to suggest a benign lesion, because almost all pancreatic adenocarcinomas are highly aggressive, even if not apparent on FDG PET. The variability in FDG avidity makes detection of the primary pancreatic neoplasm unpredictable on FDG PET/computed tomography (CT), and contrast-enhanced CT or magnetic resonance (MR) imaging at a pancreatic contrast phase (approximately 45 seconds) remains the mainstay for imaging primary pancreatic malignancies and determining their resectability. Pancreatic adenocarcinomas are often hypoattenuating to normal pancreatic parenchyma on contrast-enhanced CT; however, a small percentage are isoattenuating, and FDG PET may provide value in visualizing the primary pancreatic adenocarcinoma in this small percentage of patients. Overall, the detection of the primary malignancy is usually not the primary purpose of FDG PET/CT, even if the primary malignancy is FDG avid in many patients ( Fig. 14.1 ).

As with malignancies from many other organ systems, FDG PET/CT is far more useful for the detection of nodal and distant metastases from pancreatic adenocarcinoma than for detection of the primary pancreatic malignancy. FDG avidity within borderline-sized local lymph nodes may help to raise confidence in diagnosis of nodal metastases, and FDG-avid foci within the liver of a patient with pancreatic adenocarcinoma is highly suspicious for hepatic metastases ( Fig. 14.2 ). Peritoneal carcinomatosis may be visualized on CT, FDG PET, both (see Fig. 14.1 ), or neither. It is not unusual for peritoneal carcinomatosis to be detected during surgery which was occult on both CT and FDG PET.

Surgery is the only current potentially curable treatment modality for pancreatic adenocarcinoma; unfortunately, most patients present with unresectable disease. Systemic chemotherapy with or without radiation is used in most patients. Despite the method of initial therapy, most patients will have residual or recurrent malignancy within 2 years. Postsurgical changes may make detection of recurrent malignancy difficult on anatomic imaging, and FDG PET/CT has shown promise in detecting recurrent malignancy following therapy ( Fig. 14.3 ).

Pancreatic Neuroendocrine Tumors

Neuroendocrine tumors represent a minority of pancreatic neoplasm, maybe 2%. Neuroendocrine tumors may be nonfunctioning, without secretion of hormones, or functioning, which may present with specific endocrine syndromes. The most common functioning pancreatic neuroendocrine tumor is insulinoma, followed by gastrinomas. Neuroendocrine tumors often demonstrate arterial phase enhancement on CT or MR, in contrast to pancreatic adenocarcinomas, which are usually hypoattenuating to normal pancreatic parenchyma. PNETs vary widely in their extent of FDG avidity. FDG avidity often correlates with histologic grade of a PNET, with well-differentiated PNETs demonstrating little or no FDG avidity, whereas poorly differentiated PNETs may be markedly FDG avid. Remember that this is unlike pancreatic adenocarcinomas, which also vary widely in their extent of FDG avidity but are almost always high-grade malignancies. The usefulness of FDG PET/CT for detection of primary and metastatic PNETs will vary based on the extent of FDG avidity. Of note, well-differentiated PNETs with low FDG avidity may be substantially better visualized by somatostatin-based nuclear imaging, such as 18F-DOTA-octreotate (18F-DOTATATE) PET/CT or indium-111 octreotide single-photon emission computerized tomography (SPECT)/CT.