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Difficulty: HardOrdering Imaging Studies

A 45-year-old woman is diagnosed with Zollinger-Ellison syndrome after presenting with chronic diarrhea and recurrent, medically refractory peptic ulcers. Her fasting serum gastrin level is 2,400 pg/mL (normal: < 100 pg/mL) while off proton pump inhibitors, and her gastric fluid pH is 1.5. A secretin stimulation test demonstrates a paradoxical rise in serum gastrin of 350 pg/mL. An initial multiphase contrast-enhanced computed tomography (CT) scan of the abdomen and pelvis performed at a community hospital reveals no discrete pancreatic or hepatic masses. She is referred to a tertiary care center for surgical evaluation.

Arrange the following advanced imaging and localization modalities in the most appropriate chronological sequence to pinpoint the location of the primary neuroendocrine tumor.

  1. 1Gallium-68 DOTATATE positron emission tomography-computed tomography (PET/CT)
  2. 2Endoscopic ultrasonography (EUS) of the pancreas and duodenum
  3. 3Selective arterial secretin injection (SASI) with hepatic venous blood sampling

Answer

The correct diagnostic imaging sequence is to first perform a Gallium-68 DOTATATE PET/CT, followed by Endoscopic Ultrasonography (EUS) if further anatomic detail is needed or the PET/CT is negative, and finally utilizing Selective Arterial Secretin Injection (SASI) only if the tumor remains occult on all prior imaging.
The stepwise evaluation of a biochemically confirmed gastrinoma (Zollinger-Ellison syndrome) with an initially negative standard CT scan prioritizes the most sensitive, least invasive modalities first. Gallium-68 DOTATATE PET/CT is a functional scan that binds to somatostatin receptors (highly expressed on gastrinomas) and provides excellent whole-body localization for both the primary tumor and potential metastases, making it the ideal first step. If the PET/CT is negative, Endoscopic Ultrasonography (EUS) is the next logical step, as it provides extremely high-resolution imaging of the 'gastrinoma triangle' (the duodenum and pancreatic head) and can detect tumors smaller than 5 millimeters. Finally, if both PET/CT and EUS fail to localize the tumor, Selective Arterial Secretin Injection (SASI) with hepatic venous sampling is performed. This highly invasive angiographic procedure localizes the tumor by mapping the vascular territory that secretes gastrin in response to secretin stimulation.

Step-by-Step Solution

1
Order whole-body functional receptor imaging
Perform Gallium-68 DOTATATE PET/CT
To conduct a highly sensitive, non-invasive whole-body assessment for somatostatin receptor-expressing primary tumors and distant metastases, which is the standard next step when conventional CT is unrevealing.
2
Order high-resolution regional luminal imaging
Perform Endoscopic Ultrasonography (EUS)
To evaluate the gastrinoma triangle with high sensitivity for microadenomas (less than 1 cm) in the duodenal wall or pancreatic head, utilized if functional imaging is negative or equivocal.
3
Order invasive regional venous sampling
Perform Selective Arterial Secretin Injection (SASI)
To serve as a functional mapping test of last resort to locate the regional blood supply of a biochemically proven but anatomically occult gastrinoma prior to exploratory laparotomy.

Key Concept

Sequencing of diagnostic imaging for localization of occult neuroendocrine tumors (gastrinomas) in Zollinger-Ellison Syndrome
Estimated Time:1m 30s
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Topics

Ordering Imaging Studies

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