From the Guidelines
The preferred imaging for insulinoma diagnosis is a combination of contrast-enhanced CT scan and endoscopic ultrasound (EUS). This approach is supported by the most recent and highest quality study, which reported that CT scan with pancreatic protocol can detect tumors as small as 1-2 cm with sensitivity of 70-80% 1. If CT is negative, EUS should follow, offering superior sensitivity (90-95%) for small tumors and the ability to perform fine needle aspiration for tissue diagnosis.
Key Considerations
- MRI with gadolinium can be used as an alternative to CT, particularly in patients with contrast allergies or renal impairment.
- For difficult cases, selective arterial calcium stimulation testing with hepatic venous sampling (SACST) may be necessary, which can localize functional tumors even when not visible on conventional imaging.
- Somatostatin receptor scintigraphy (Octreoscan) has limited utility for insulinomas as only 50-60% express somatostatin receptors.
- PET/CT with specific tracers like 68Ga-DOTATATE may be used for metastatic disease evaluation.
Rationale
The multimodality approach is recommended because insulinomas are typically small (less than 2 cm), can be located anywhere in the pancreas, and accurate localization is crucial for successful surgical resection, which remains the definitive treatment. As noted in the guidelines, most experts recommend the Imamura-Doppman procedure only for patients with persistent or recurrent insulinoma or when other localization tests are equivocal or negative 1. Additionally, serum insulin, proinsulin, and C peptide should be tested, and a 48- to 72-hour observed or inpatient observed fast may also be helpful in uncertain cases 1.
Additional Recommendations
- Multiphasic CT or MRI scans should be performed to rule out metastatic disease, as 90% of insulinomas have an indolent course and can be cured surgically 1.
- Insulinomas are less consistently octreotide-avid than other pancreatic NETs, and somatostatin scintography may consequently be less useful as an imaging technique for insulinomas than for other tumor subtypes 1.
From the Research
Imaging Modalities for Diagnosing Insulinoma
The following imaging modalities are used for diagnosing insulinoma:
- Computed Tomography (CT) 2, 3, 4, 5, 6
- Endoscopic Ultrasonography (EUS) 2, 4, 5, 6
- Magnetic Resonance Imaging (MRI) 4, 5
- Arterial Stimulation and Venous Sampling (ASVS) 4
- B-ultrasonography 4
Sensitivity of Imaging Modalities
The sensitivity of different imaging modalities for diagnosing insulinoma is as follows:
Combined Imaging Protocols
Combined imaging protocols, such as dual-phase thin-section multidetector CT and endoscopic sonography, can increase the diagnostic sensitivity for insulinoma 6
- The overall diagnostic sensitivity for combined biphasic thin-section helical CT and endoscopic sonography was 100% 6