Somatostatinoma

Somatostatinomas:

• very uncommon islet cell tumours, accounting for less than 5% of all functional pancreatic neuroendocrine tumours
• the majority (more than 60%) are large tumours (mean diameter 5 cm) located in the head and body of the pancreas
• other sites for somatostatinomas are in the periampullary region of the duodenum and in the small intestine
  • these tumours are generally smaller

Characteristically pancreatic somatostatinomas present with a triad of gallstones, steatorrhoea and diabetes mellitus. The plasma immunoreactive levels are usually increased to more than 10 times normal levels.

Extrapancreatic somatostatinomas can present in association with neurofibromatosis type I and phaeochromocytoma

Presenting features of extrapancreatic somatostatinomas include:

• obstructive pancreatitis
• obstructive jaundice
• small intestinal haemorrhage
• small intestinal obstruction
• abdominal pain

Although a definitive threshold for fasting plasma somatostatin levels has not been universally established, latest guidance recommends a diagnostic cutoff exceeding 2 to 3 times the upper reference limit (ie, >60 pg/mL when the upper limit is >30 pg/mL). (3) Additional non-specific markers, eg, chromogranin A and pancreatic polypeptide, are often elevated in pancreaticoduodenal neuroendocrine tumours.

Localization of somatostatinoma:

• pancreatic somatostatinomas and metastases - detected with transabdominal ultrasonography and CT scanning
• duodenal somatostatinomas - generally found during endoscopy
• functional imaging (PET) scans - somatostatin receptor-based positron emission tomography (PET) using radiolabelled ligands such as 68Ga-DOTATATE, 68Ga-DOTATOC,68Ga-DOTANOC, or 64Cu-DOTATATE demonstrates excellent sensitivity and specificity for tumour localization and staging.

Management:

• seek specialist advice
• pancreatic somatostatinomas (because of their large size) - only rarely can be cured by extensive surgery - often a Whipple's procedure often has to be performed
  • cytoreductive debulking surgery may result in palliation
  • prognosis ranges from weeks to years
• duodenal somatostatinomas may be treated with surgery alone, although again a Whipple's procedure often has to be performed
• hepatic embolization - used for the palliation of metastatic disease
• for patients with unresectable or metastatic (stage IV) disease, treatment goals shift toward symptom control and disease stabilization. International guidelines recommend a tiered therapeutic strategy based on tumour grade, somatostatin receptor expression, and tumour progression. (2)

Reference:

1. Bevere M, Gkountakos A, Martelli FM, Scarpa A, Luchini C, Simbolo M. An Insight on Functioning Pancreatic Neuroendocrine Neoplasms. Biomedicines. 2023 Jan 21;11(2)
2. Del Rivero J et al. Systemic Therapy for Tumor Control in Metastatic Well-Differentiated Gastroenteropancreatic Neuroendocrine Tumors: ASCO Guideline. J Clin Oncol. 2023 Nov 10;41(32):5049-5067
3. Hofland J et al. European Neuroendocrine Tumor Society 2023 guidance paper for functioning pancreatic neuroendocrine tumour syndromes. J Neuroendocrinol. 2023 Aug;35(8):e13318