What are the recommended radiological investigations for insulinoma?

Recommended Radiological Investigations for Insulinoma

For patients with suspected insulinoma, a multimodal imaging approach is recommended with endoscopic ultrasound (EUS) as the preferred initial localization method due to its high sensitivity of 82-93% for detecting pancreatic neuroendocrine tumors. 1, 2

First-Line Imaging Modalities

• Endoscopic Ultrasound (EUS): Highly sensitive (82-93%) for detecting small pancreatic tumors and is particularly valuable for obtaining tissue samples and determining surgical approach (enucleation vs. Whipple procedure) 1, 2

• Multiphasic CT: Sensitivity of 57-94% for pancreatic NETs, should be performed with arterial and venous phase imaging as insulinomas typically appear hypervascular in the arterial phase with a mean enhancement of 147 HU 1, 3

• MRI: Sensitivity of 74-94% for pancreatic NETs; insulinomas typically appear hypointense on T1-weighted sequences and isointense to slightly hyperintense on T2-weighted sequences 1, 3

Second-Line Imaging Modalities

• Intra-arterial calcium stimulation with hepatic venous sampling (Imamura-Doppman procedure): Particularly valuable for occult insulinomas with success rates up to 90% in localization when first-line imaging is inconclusive 1, 2

• Somatostatin Receptor Scintigraphy (SSRS): Limited sensitivity (50-60%) specifically for insulinomas compared to other pancreatic NETs, but may be useful when other imaging is inconclusive 1

• 68Ga-DOTATOC PET: Higher sensitivity (87-96%) than conventional SSRS for pancreatic NETs, though specific sensitivity for insulinomas may vary 1

Intraoperative Imaging

• Intraoperative Ultrasound (IOUS): Excellent adjunct to surgical palpation with ability to identify small lesions in the pancreatic head and multiple lesions (92-97% sensitivity in MEN1 syndrome); also allows assessment of the liver 1, 4

Imaging Algorithm

1. Start with non-invasive cross-sectional imaging:

   • Multiphasic CT (arterial and venous phases) AND/OR
   • MRI with contrast enhancement 1, 3

2. If initial imaging is negative or inconclusive:

   • Proceed to EUS (which also allows for tissue sampling) 2, 5

3. For difficult-to-localize cases:

   • Consider intra-arterial calcium stimulation with hepatic venous sampling 1, 2
   • Consider nuclear medicine imaging (68Ga-DOTATOC PET preferred over conventional SSRS) 1

4. Plan for intraoperative ultrasound during surgery regardless of preoperative localization success 1, 4

Important Considerations

• Insulinomas are typically small (66% <2cm at presentation), making them challenging to localize radiologically 6

• The combination of biphasic thin-section helical CT and EUS approaches 100% sensitivity in localizing insulinomas 4

• Laparoscopic ultrasound is considered mandatory for intraoperative localization of these tumors 4

• For patients who cannot undergo EUS, the combination of high-quality CT and MRI provides the best non-invasive detection strategy 3

• Accurate preoperative localization is crucial as complete surgical resection is the only curative treatment, with a success rate of 90% 2, 4