Is gastrinoma (a type of tumor that causes Zollinger-Ellison syndrome) diagnosable by computed tomography (CT) scan?

Is Gastrinoma Diagnosable by CT Scan?

CT scan has limited sensitivity for detecting gastrinomas and should not be relied upon as the sole diagnostic imaging modality, though it plays an important role as part of the initial imaging workup when combined with other localization techniques. 1

Diagnostic Performance of CT for Gastrinoma

CT imaging has significant limitations in detecting gastrinomas:

• Sensitivity is poor for primary tumors: Only 25% of pancreatic gastrinomas are detected by CT scan alone 2, and approximately 50% of all gastrinomas remain undetectable on preoperative CT imaging 3
• Multiphase technique is essential: When CT is performed, it must be done as a multiphase contrast-enhanced study of the abdomen and pelvis for optimal detection 1
• Small tumor size is the main limitation: Most gastrinomas are small (often <2 cm), making them difficult to visualize on cross-sectional imaging 3, 2

Current Recommended Imaging Approach

The National Comprehensive Cancer Network guidelines establish a hierarchical imaging strategy:

• Gallium-68 PET imaging is now the gold standard: Ga-68 DOTATATE or similar radiotracers with positron emission tomography are currently the standard for tumor localization in ZES 1
• Somatostatin receptor scintigraphy (Octreoscan) is useful for initial evaluation when PET is unavailable 1, 4
• Endoscopic ultrasound (EUS) has superior sensitivity (up to 83%) for pancreatic gastrinomas and should be used for tumor localization 1, 2
• Combination approach is most effective: Using somatostatin receptor imaging plus EUS detects >90% of gastrinomas 4

Anatomical Considerations

Understanding gastrinoma location explains CT's limitations:

• Most gastrinomas are duodenal (42% in the duodenal wall), with the remainder in pancreas (17%) or lymph nodes (38%) 2
• Subepithelial location: Gastrinomas arise from deeper layers as submucosal lesions covered by normal-appearing epithelium, making them difficult to detect on both endoscopy and CT 1
• "Gastrinoma triangle": Most tumors occur within the anatomic triangle bounded by the junction of cystic and common bile ducts, junction of second and third portions of duodenum, and junction of neck and body of pancreas 1

Clinical Pitfalls to Avoid

• Do not exclude ZES based on negative CT: A normal CT scan does not rule out gastrinoma given the 50% false-negative rate 3
• Do not delay biochemical diagnosis: Diagnosis of ZES is primarily biochemical (fasting serum gastrin >100 pg/mL with gastric pH <2, positive secretin stimulation test), not imaging-based 3, 4
• Ensure proper CT technique: If CT is performed, it must include IV contrast with neutral oral contrast (water or dilute barium) and be multiphase 1

Practical Diagnostic Algorithm

When ZES is suspected clinically:

1. Confirm biochemically first: Measure fasting serum gastrin after stopping PPIs for 1-2 weeks, check gastric pH, perform secretin stimulation test 1, 3
2. Initial imaging: Obtain Ga-68 PET/CT if available, or somatostatin receptor scintigraphy 1
3. Add multiphase CT/MRI of abdomen and pelvis for anatomic detail and staging 1
4. Perform EUS for detailed evaluation, especially for small pancreatic and duodenal lesions 1, 2
5. Consider endoscopy to assess for peptic ulcer disease and evaluate duodenal mucosa 1