Pancreatic neuroendocrine tumors: somatostatinoma
What is a somatostatinoma?
Somatostatinoma is a very rare neuroendocrine tumor, with an incidence of one in 40 million persons. These tumors arise from the delta cells in the pancreas, although these cells can be present in duodenal tissue as well, where 40% of the tumors occur. Somatostatin is a normally occurring peptide that inhibits the function of almost all gut hormones. Bowel motility is slowed, and nutrient absorption may be impaired.
The most common presenting symptom of a patient with a somatostatinoma is abdominal discomfort. Conversely, however, owing largely to the rarity of this tumor, somatostatinoma is a very uncommon cause of abdominal pain. Patients are most often 40 to 60 years of age. Overall, pancreatic tumors comprise about 60% of somatostatinomas, with duodenal tumors making up almost all of the remaining tumors. Exceedingly rarely, a jejunal somatostatinoma has been reported. Pancreatic tumors tend to be larger than duodenal tumors, with a median size being greater than 4 cm, while most duodenal tumors are less than 2 cm at diagnosis. Unlike insulinomas and gastrinomas, which tend to become symptomatic due to hormone overproduction at a small size, it is thought that symptoms of somatostatinoma due to hormone excess only occur with very high hormone levels, and, therefore, with large tumors. The reported incidence of the "Somatostatin syndrome" is 10% or less. This syndrome includes gallstones, diarrhea and steatorrhea (or fat in stools), mild diabetes, decreased stomach acid, and weight loss. Severely low blood glucose levels have rarely been reported, as well.
About 50% of patients with somatostatinoma have other endocrine disorders, particularly those presenting with duodenal tumors. Neurofibromatosis type-1 (NF-1), and less frequently conditions such as MEN-1, von Hippel-Lindau disease, tuberous sclerosis, and gastrointestinal stromal tumors (GIST) have presented with somatostatinoma as well. Tumor markers, with the exception of somatostatin, are nonspecific, and include pancreatic polypeptide, ghrelin, andrenomedulin, neuron-specific enolase, and chromogranin A.
Most somatostatinomas are diagnosed by radiographic imaging studies, such as CT scan, in the work-up of unexplained abdominal pain, or by endoscopy for unexplained blood loss, diarrhea, or in an effort to localize a tumor producing another hormone, such as an insulinoma. It is common for somatostatinomas, particularly those of pancreatic origin, to secrete other hormones in addition to somatostatin). PET scan and somatostatin receptor scintigraphy also may localize these tumors.
Metastasis occurs most typically to the liver. Regional lymph nodes and bone are involved sites less commonly. Metastases are present at initial diagnosis in 80% with a pancreatic primary and in nearly 50% of patients whose primary tumor is duodenal. Late diagnosis is thought to play a significant role.