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  • GEP-NETs are neuroendocrine tumors that originate in the gastrointestinal tract.

  • About 60% GEP-NETs are nonsecretory or secretory without clinical manifestations; they may be detected incidentally or may present with weight loss, abdominal pain, or jaundice.

  • Carcinoid tumors arise from the intestines or lung, secrete serotonin, and may metastasize.


GEP-NETs are a subset of neuroendocrine tumors (NETs) that arise from the stomach, intestines, or endocrine pancreas. Pancreatic neuroendocrine tumors (pNETs) arise from different islet cells: the A cells (20%) secrete glucagon, the B cells (70%) secrete insulin,2 the D cells (5%) secrete somatostatin or gastrin, and the F cells secrete pancreatic polypeptide. Gastrin-producing cells are also found in the duodenum. Serotonin-producing cells are found throughout the gut, particularly near the terminal ileum.

The reported incidence of GEP-NETs has increased to about 37 per million yearly in the United States due to the incidental detection of small tumors on abdominal scans. About 40% are functional, producing hormones that also serve as tumor markers, important for diagnosis and follow-up. At presentation, 65% of GEP-NETs are unresectable or metastatic. Up to 25% of GEP-NETs are associated with one of four different inherited disorders: MEN 1, von Hippel-Lindau disease (VHL), neurofibromatosis 1 (NF-1), and tuberous sclerosis complex (TSC). In MEN 1, GEP-NETs are usually gastrinomas, carcinoids, or nonfunctioning tumors and are a common cause of death. In VHL, GEP-NETs are usually benign and multiple.

Insulinomas are the most common functional type of GEP-NET and are usually small, intrapancreatic, and benign (90%) and secrete excessive amounts of insulin. Insulinomas also produce proinsulin and C-peptide. Insulinomas are solitary in 95% of sporadic cases but are multiple in about 90% of cases arising in MEN 1. (See Chapter 27.)

Gastrinomas secrete excessive quantities of the hormone gastrin (as well as “big” gastrin). About 50% of gastrinomas are malignant and metastasize to the liver. Gastrinomas are typically found in the duodenum (49%), pancreas (24%), or lymph nodes (11%). Sporadic gastrinoma is rarely suspected at the onset of symptoms; typically, there is a 5-year delay in diagnosis. About 22% of gastrinomas arise in patients with MEN 1, who usually present at a younger age, often with multiple tumors; hyperparathyroidism can occur many years before or after the discovery of a gastrinoma.

Glucagonomas are rare and usually malignant, despite their benign histologic appearance. They usually arise as a large intrapancreatic tumor with 60% having liver metastases apparent by the time of diagnosis. Besides glucagon, they usually secrete additional hormones, including gastrin.

Somatostatinomas are very rare and usually single. They arise in the pancreas (50%) or small intestine. They secrete somatostatin.

VIPomas are quite rare and usually single intrapancreatic tumors with metastases usually evident (80%) at diagnosis. They produce vasoactive intestinal polypeptide (VIP).

CCKomas are rare tumors of the endocrine pancreas that secrete cholecystokinin.


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