Non-Contrast CT is NOT Suitable for Diagnosing Insulinoma
Non-contrast CT should not be used for insulinoma diagnosis—contrast-enhanced multiphasic CT with arterial and venous phases is essential, as insulinomas are hypervascular tumors that appear enhanced in the arterial phase and would be missed without contrast. 1, 2
Why Contrast is Critical
- Insulinomas are hypervascular neuroendocrine tumors that demonstrate characteristic enhancement patterns during arterial phase imaging, making them visible only with contrast administration 1, 2
- Multiphasic CT (with both arterial and venous phases) has a sensitivity of 57-94% for detecting pancreatic neuroendocrine tumors, but this diagnostic accuracy is entirely dependent on contrast enhancement 1, 2
- Without contrast, these small tumors (often <2 cm) would be isodense to normal pancreatic tissue and essentially invisible 3
Appropriate Imaging Algorithm for Suspected Insulinoma
First-line imaging approach:
- Start with contrast-enhanced multiphasic CT (arterial and venous phases) AND/OR contrast-enhanced MRI to rule out metastatic disease and attempt initial tumor localization 1, 2
- MRI demonstrates comparable sensitivity of 74-94% and serves as an excellent alternative when CT is contraindicated 1, 2
If initial cross-sectional imaging is negative or equivocal:
- Proceed to endoscopic ultrasound (EUS), which has the highest sensitivity (82-93%) for detecting small pancreatic tumors and allows simultaneous tissue sampling 1, 2
- EUS is particularly valuable for detecting small lesions that cross-sectional imaging may miss 1
For truly occult cases:
- Consider selective arterial calcium stimulation with hepatic venous sampling (Imamura-Doppman procedure), which achieves up to 90% success rate for localizing occult insulinomas 1, 2
- 68Ga-DOTATOC PET/CT demonstrates the highest sensitivity (87-96%) and should be considered when conventional imaging is negative 1, 2
Intraoperative confirmation:
- Intraoperative ultrasound (IOUS) is mandatory regardless of preoperative imaging results, with sensitivity of 92-97% for identifying small lesions 1, 2
Critical Pitfall to Avoid
- Do NOT rely on somatostatin receptor scintigraphy (SSRS) for insulinoma detection, as its sensitivity is only 50-60% for insulinomas specifically—significantly lower than for other pancreatic NETs 1, 2
Context for This Patient
In your 80kg adult male with suspected sulfonylurea toxicity, the diagnostic priority is actually biochemical confirmation through a supervised 48-72 hour fast, not imaging 1. At the termination of the fast, diagnostic criteria include insulin >3 mcIU/mL, C-peptide ≥0.6 ng/mL, and proinsulin ≥5 pmol/L, with a negative sulfonylurea screen to differentiate from exogenous causes 1, 4. Only after biochemical confirmation of endogenous hyperinsulinism should you proceed to contrast-enhanced imaging for tumor localization 1, 2.