How to diagnose an insulinoma using CT scan in an 80kg adult male with suspected sulfonylurea toxicity and hypoglycemia?

How to Look for Insulinoma on CT

Dual-phase multidetector CT should be performed as the initial imaging modality with a sensitivity of 57-94%, though it may miss small lesions and should be followed by endoscopic ultrasound (EUS) if negative or equivocal. 1

CT Imaging Protocol

• Perform multiphasic contrast-enhanced CT (dual-phase or triple-phase) as the first-line cross-sectional imaging study to evaluate for insulinoma and rule out metastatic disease 2, 1
• CT has moderate sensitivity (57-94%) for detecting pancreatic neuroendocrine tumors, but this is lower than for other imaging modalities like EUS 1
• The multiphasic technique is critical because insulinomas are hypervascular and enhance during the arterial phase of contrast administration 2

What to Look For on CT

• Small, well-defined hypervascular lesions that enhance brightly during the arterial phase—most insulinomas are small tumors, often less than 2 cm 3, 4
• Look throughout the entire pancreas (head, body, and tail), as insulinomas can occur anywhere in the pancreatic parenchyma 3, 4
• Assess for metastatic disease, particularly hepatic metastases, though 90% of insulinomas are benign 2

Critical Limitations of CT

• CT frequently misses small insulinomas due to their small size, which is why EUS should follow if CT is negative or equivocal 1
• CT alone is insufficient for definitive localization in many cases—approximately 18% of insulinomas may not be detected by CT 2

Algorithmic Approach After CT

• If CT is positive: Proceed with surgical planning, but still perform EUS for precise localization and to rule out multiple lesions (especially in MEN1 syndrome) 1
• If CT is negative or equivocal: Immediately proceed to EUS, which has 82-93% sensitivity and is superior for detecting small tumors 2, 1
• If both CT and EUS are negative: Consider 68Ga-DOTATATE PET/CT (87-96% sensitivity) or selective arterial calcium stimulation test (90% success rate for occult insulinomas) 1, 5
• At surgery: Mandatory intraoperative ultrasound (IOUS) should be performed regardless of preoperative imaging results, as it improves sensitivity to 92-97% 1

Important Caveats

• Do not rely on somatostatin receptor scintigraphy (Octreoscan) for insulinomas—it has only 50-60% sensitivity for insulinomas compared to 75% for other pancreatic neuroendocrine tumors, making it an unreliable modality 2, 1
• MRI is an acceptable alternative to CT with comparable sensitivity (74-94%) and can be used if CT is contraindicated or as a complementary study 1
• In the context of suspected sulfonylurea toxicity, ensure sulfonylurea screening is negative before attributing hypoglycemia to insulinoma, as exogenous sulfonylureas can mimic insulinoma biochemically 3, 4, 6