Investigation of Choice for Gastrinoma Localization
Somatostatin receptor scintigraphy (SRS) is the investigation of choice for gastrinoma localization, with a sensitivity of 72-97% for primary gastrinomas and 92% for liver metastases. 1, 2
Why SRS is Superior
SRS plays a central role in locating gastroenteropancreatic neuroendocrine tumors and should be the first imaging study performed for gastrinomas. 3, 1 The evidence strongly supports this recommendation:
- SRS detects gastrinomas in 58-70% of patients as a single modality, which equals the combined sensitivity of all conventional imaging studies together 4, 2
- For liver metastases specifically, SRS achieves 92% sensitivity compared to only 42-62% for CT/ultrasound/angiography and 71% for MRI 5, 2
- SRS is significantly better than all conventional imaging methods in identifying gastrinomas later confirmed at surgery (P = 0.004) 2
Comparative Sensitivities of Other Modalities
The other options perform substantially worse for gastrinoma localization:
- CT/MRI (Option A): Detect gastrinomas in only 22-75% of cases, with CT at 38% and MRI at 45% sensitivity for primary tumors 3, 2
- SPECT CT (Option B): This enhances SRS sensitivity through fusion imaging but is not a standalone modality—it augments SRS 3, 1
- OGD/Upper endoscopy (Option D): Essential for duodenal gastrinomas (80% of MEN1 gastrinomas are duodenal) but has limited sensitivity as a standalone test and should be performed alongside SRS, not instead of it 3, 1
Practical Algorithm for Gastrinoma Localization
- First-line: Perform SRS as the initial imaging study 1, 2
- Add upper GI endoscopy and CT/MRI for extrapancreatic/duodenal gastrinomas, as EUS sensitivity is lower for these locations 3, 1
- If SRS is negative: Proceed to triple-phase CT of thorax and abdomen after completing upper and lower GI endoscopy 3
- For difficult cases: Consider intra-arterial calcium stimulation with digital subtraction angiography, which may be particularly important for localizing occult gastrinomas 3, 1
- Add EUS for suspected pancreatic gastrinomas (90-100% sensitivity for pancreatic lesions) 1
Important Caveats
- SRS misses approximately one-third of gastrinomas found at surgery, particularly small duodenal tumors (<1.1 cm detected in only 30% of cases) 4
- Negative SRS should not preclude surgery in patients without hepatic metastases, as surgical exploration detects 33% more gastrinomas than SRS 4
- The sensitivity of SRS correlates with tumor size: 30% for lesions ≤1.1 cm, 64% for 1.1-2 cm, and 96% for >2 cm 4
Answer: C. SRS (Somatostatin Receptor Scintigraphy)