What are pancreatic neuroendocrine tumors?
Pancreatic neuroendocrine tumors comprise a heterogeneous group of neoplasms that arise from cells of the neuroendocrine system. They all have malignant potential, but may be benign or malignant in their biologic behavior, and are typically associated with a better prognosis than pancreatic adenocarcinomas.
Pancreatic neuroendocrine tumors (PNETs) are neuroendocrine tumors that arise in the pancreas, most commonly in the islets of Langerhans. Most PNETs (50%–75%) are nonfunctioning—that is, they either do not produce any hormones or do not produce hormones that result in a clinical syndrome. Functioning PNETs are generally named for the hormones they secrete.
These include insulinomas, gastrinomas, glucagonomas, somatostatinomas, and vasoactive intestinal polypeptide tumors (VIPomas). PNETs can also occasionally secrete adrenocorticotropic hormone (ACTH), corticotropin-releasing factor (CRF), and growth hormone–releasing factor (GRF)
Are pancreatic neuroendocrine tumors usually benign or malignant?
Insulinomas are usually benign (80%–90%); other pancreatic endocrine tumors are frequently malignant and metastatic at presentation (50%–80%).
PNETs are a group of epithelial neoplasms originating from the islet cells of the pancreas (also known as islet cell tumors ).
They are categorized as either functional or nonfunctional, depending on the tumor’s ability to secrete hormones, such as insulin, gastrin, glucagon, vasoactive intestinal peptide (VIP), or somatostatin. The majority (60%–90%) of PNETs are nonfunctional.
Nonfunctional PNETs typically manifest similarly to pancreatic adenocarcinoma, with abdominal pain or biliopancreatic duct obstruction, secondary to mass effect. They are often diagnosed at advanced stages because of their slow growth pattern leading to a delay in onset of symptoms.
Nonfunctional PNETs are also increasingly being discovered incidentally with the increasing use of high-quality abdominal imaging. Functional PNETs are often diagnosed on the basis of presenting symptoms caused by overproduction of hormones.
They can be small and difficult to localize with imaging. PNETs are usually sporadic but can be associated with genetic syndromes about 10% of the time; these include multiple endocrine neoplasia types I and IV (MEN 1 or 4), von Hippel-Lindau (VHL) syndrome, neurofibromatosis-1 (NF-1), and tuberous sclerosis complex (TSC).
Are pancreatic neuroendocrine tumors associated with other endocrine disorders?
PNETs are usually sporadic tumors but can also be part of inherited endocrine tumor syndromes, such as multiple endocrine neoplasia type 1 (MEN 1), Von Hippel–Lindau (VHL) syndrome, and neurofibromatosis type 1 (NF-1). PNETs occur in ∼80% to 100% of patients with MEN 1, in ∼20% of patients with VHL, and in ∼10% of those with NF-1. PNETs also occur in ∼1% of patients with tuberous sclerosis.
Eighty-five percent are symptomatic due to the effects of secreted hormones and are typically detected when small in size, whereas 15% are symptomatic related to mass effect on adjacent abdominal structures and are typically detected when large in size.
On cross-sectional imaging, a pancreatic mass is seen, sometimes in association with a focal contour abnormality, typically with soft tissue attenuation, low-intermediate signal intensity on T1-weighted images, intermediate-slightly high signal intensity on T2-weighted images, as well as hyperenhancement on arterial phase contrast-enhanced images.
Cystic or necrotic change may sometimes be present, which appears as non enhancing components with attenuation and signal intensity characteristics similar to fluid.
Very high attenuation calcification may sometimes also be seen on CT. In general, small lesions tend to be homogeneous whereas large lesions tend to be heterogeneous in appearance.
Associated biliary ductal dilation, upstream pancreatic ductal dilation, upstream pancreatic parenchymal atrophy, local invasion of surrounding organs and tissues, regional lymphadenopathy, and distant metastatic disease may occur, but less commonly than with pancreatic adenocarcinoma.
Presence of regional lymphadenopathy or distant metastatic disease indicates that the pancreatic neuroendocrine tumor is malignant in behavior.