How should functional PNETS be imaged?
Given the small size of many functional PNETs, preoperative localization can be difficult. Multiphasic CT or MRI is often the first imaging modality pursued. Somatostatin receptor scintigraphy (octreotide scan) is highly sensitive (60%–90%) in locating most PNETs and is especially useful in identifying metastatic disease but is less sensitive for insulinomas because of their low level of expression of type 2 somatostatin receptors. An emerging imaging technique is the use of gallium-labeled radioligands (e.g., 68Ga-DOTATATE and 68Ga-DOTATOC), for which uptake is measured with positron emission tomography/CT. This technology has been shown to be superior to standard imaging (CT, MRI, octreotide scanning) in the detection of primary NETs and metastases in multiple studies (sensitivity and specificity of 97% and 92%, respectively). Endoscopic ultrasonography has also emerged as a valuable imaging modality (sensitivity and specificity of 82% and 92%, respectively), particularly for small insulinomas often missed on CT, and has the added advantage of the ability to perform a biopsy and even tattoo lesions to aid in identification during minimally invasive surgical approaches. Provocative arterial stimulation studies (secretin for gastrinomas and calcium for insulinomas) with hepatic venous sampling have higher sensitivity than portal vein sampling and have, therefore, replaced it; however, their invasiveness and ability to only regionalize a tumor make them less desirable options for localization.