What is the best imaging study to detect insulinoma?

Best Imaging Modality for Insulinoma Detection

Endoscopic ultrasound (EUS) is the best initial imaging study for detecting insulinomas with a sensitivity of 88%, followed by angiography with calcium stimulation which has a sensitivity of 95% for difficult-to-localize cases. 1, 2

Initial Imaging Approach

• EUS is highly sensitive (82-93%) for detecting small pancreatic insulinomas and provides the additional benefit of obtaining tissue samples to guide surgical approach (enucleation vs. Whipple procedure) 2
• Multiphasic CT (arterial and venous phases) has a sensitivity of 30-54% for insulinomas, as these tumors typically appear hypervascular in the arterial phase 1, 2
• MRI has a sensitivity of 10-63% for insulinomas; these tumors typically appear hypointense on T1-weighted sequences and isointense to slightly hyperintense on T2-weighted sequences 1, 2, 3

Advanced Imaging for Difficult-to-Localize Cases

• Intra-arterial calcium stimulation with hepatic venous sampling (Imamura-Doppman procedure) has a success rate up to 95% for localizing insulinomas when first-line imaging is inconclusive 1, 2
• Somatostatin Receptor Scintigraphy (SSRS) has limited sensitivity (25%) specifically for insulinomas compared to other pancreatic NETs (unlike gastrinomas which have 97% sensitivity with SSRS) 1, 4
• 68Ga-DOTATATE PET/CT has shown promising results with a sensitivity of 90% in recent studies, significantly outperforming conventional imaging 5

Intraoperative Imaging

• Intraoperative Ultrasound (IOUS) is an excellent adjunct to surgical palpation with ability to identify small lesions in the pancreatic head and multiple lesions 1, 2
• The combination of preoperative localization and intraoperative ultrasound significantly improves surgical outcomes 6

Imaging Algorithm

1. Start with EUS as the first-line imaging modality due to its high sensitivity (88%) 1, 2
2. Complement with either multiphasic CT or MRI for additional anatomical information and to rule out metastatic disease 1, 2
3. If initial imaging is negative or inconclusive, proceed to intra-arterial calcium stimulation with hepatic venous sampling 1, 2
4. Consider 68Ga-DOTATATE PET/CT as an alternative advanced imaging option for difficult cases 5
5. Plan for intraoperative ultrasound during surgery regardless of preoperative localization success 2

Common Pitfalls and Caveats

• Insulinomas are typically small (less than 2 cm) and can be missed on conventional imaging 3, 7
• Unlike other neuroendocrine tumors, insulinomas have lower expression of somatostatin receptors (only 50% express SSTR2), making SSRS less sensitive for primary insulinoma detection 1, 4
• Metastatic insulinomas may show positive SSTR more often than primary tumors, making SSRS more useful for detecting metastases than primary lesions 1
• Accurate preoperative localization is crucial as complete surgical resection is the only curative treatment, with a success rate of 90% 2, 8