From the Guidelines
No, insulinoma does not always have a visible mass on imaging. The detection rate of insulinomas varies depending on the imaging modality used, with endoscopic ultrasound (EUS) having a high sensitivity of approximately 82% 1, followed by CT scans and MRI. However, approximately 10-30% of insulinomas may not be detected on conventional imaging studies despite thorough evaluation, due to their small size, deep location within the pancreatic tissue, and similar density to surrounding tissue.
Imaging Modalities
- EUS has been shown to localize approximately 82% of pancreatic NETs, including insulinomas 1
- CT scans have a sensitivity of 54-88% for detecting primary insulinomas 1
- MRI has a sensitivity of 63-90% for detecting primary insulinomas 1 For insulinomas that remain occult on conventional imaging, more specialized techniques may be required, such as selective arterial calcium stimulation testing with hepatic venous sampling (SACST) or intraoperative ultrasound 1.
Diagnostic Approach
- Biochemical evidence, such as elevated insulin and C-peptide levels, is crucial in diagnosing insulinoma, even if imaging studies are negative 1
- A 48- to 72-hour observed or inpatient observed fast may be helpful in diagnosing insulinoma, with an insulin level greater than 3 mcIU/mL and C peptide concentrations of at least 0.6 ng/mL indicating the presence of the tumor 1 It is essential to recognize that a negative imaging study does not exclude the diagnosis of insulinoma when biochemical evidence strongly suggests its presence. Therefore, a comprehensive diagnostic approach, including biochemical testing and specialized imaging techniques, is necessary to accurately diagnose and manage insulinomas.
From the Research
Insulinoma Visibility on Imaging
- Not all insulinomas have a visible mass on imaging, as some may be small or not detectable by conventional imaging procedures 2, 3.
- The combination of biphasic thin section helical CT and endoscopic ultrasonography (EUS) has an almost 100% sensitivity in localizing insulinomas, but this does not necessarily mean that all insulinomas will have a visible mass on imaging 4.
- Some studies have shown that EUS can correctly diagnose and localize insulinomas in a high percentage of patients, but this may not be the case for all patients 2, 5.
Diagnostic Challenges
- Preoperative localization of an insulinoma can be challenging, and non-invasive procedures can fail to detect the tumor 2, 3.
- A selective arterial calcium stimulation test can be a sensitive diagnostic procedure for localizing insulinomas, especially when non-invasive radiological imaging does not allow the detection of an occult insulinoma 2, 6.
- The use of modern magnetic resonance imaging (MRI) in conjunction with calcium stimulation catheterization can improve the accuracy of insulinoma localization 3.