Insulinoma

Diagnosis of Insulinoma

Lab studies

Failure of endogenous insulin secretion to be suppressed by hypoglycemia is the hallmark of an insulinoma. Thus, the finding of inappropriately elevated levels of insulin in the face of hypoglycemia is the key to diagnosis.

The biochemical diagnosis of insulinoma is established in 95% of patients during prolonged fasting (up to 72 h) when the following results are found:

  • Serum insulin levels of 10 µU/mL or more (normal < 6 µU/mL)

  • Glucose levels of less than 40 mg/dL

  • C-peptide levels exceeding 2.5 ng/mL (normal < 2 ng/mL)

  • Proinsulin levels greater than 25% (or up to 90%) of immunoreactive insulin levels

  • Screening for sulfonylurea negative

Imaging studies

Insulinomas can be located with the following imaging modalities:

  • Endoscopic ultrasonography: Detects 77% of insulinomas in the pancreas [

  • Real-time transabdominal high-resolution ultrasonography: 50% sensitivity

  • Intraoperative transabdominal high-resolution ultrasonography with the transducer being passed over the exposed pancreatic surface: Detects more than 90% of insulinomas

  • Computed tomography (CT) scanning: 82-94% sensitivity

  • Magnetic resonance imaging (MRI)

  • Arteriography: Previously the standard for insulinoma localization but highly operator dependent, with reported sensitivities ranging from 29–64%

  • Selective arterial calcium stimulation testing: Since calcium stimulates insulin secretion by insulinomas, selective injection of calcium into small arterial branches of the celiac system with measurement of hepatic vein insulin during each injection can localize tumors; most studies report ≥90% accuracy. [7]

  • PET/CT with gallium-68 DOTA-(Tyr3)-octreotate (Ga-DOTATATE): 90% sensitivity; possible adjunct study when other imaging studies are negative and minimally invasive surgery is planned