Approaches to the diagnosis of gut neuroendocrine tumors: the last word (today).
Because SRS identifies 90% of hepatic metastatic disease and the addition of other studies (ultrasonography, C.T. MRI, and selective mesenteric angiography) identities only 4% more, the identification of a primary lesion with SRS obviates for the most part the use of further investigations. If SRS is negative, additional studies should only be undertaken if surgery is contemplated. Because SRS may only localize 60%-70% of primary gut NETs, an additional 10%-15% may be identified by undertaking additional studies. The most sensitive test, STIR-MRI, should be undertaken next, but because it is not widely available, pancreatic protocol CT scan is almost as effective in identification of a primary lesion. If a primary gastrinoma cannot be identified by SRS or STIR-MRI, endoscopic ultrasonography should be undertaken because duodenal gastrinomas are often minute and multicentric. A similar strategy applies for insulinomas because up to 40% cannot be located by SRS and the majority are located in the pancreatic head. Thus, STIR-MRI followed by endoscopic ultrasonography is the most appropriate course. Although calcium provocation-angiography is highly effective in the identification of insulinomas, it is significantly more invasive and should be used only as a last resort. Of particular interest is the observation that in the study of gastrinomas, SRS altered clinical management in almost 50% of patients. This reflected the ability of SRS not only to identify the primary tumor location but clarify equivocal localization results generated by conventional imaging studies. It thus seems that the simplicity, superior sensitivity, high specificity, and cost-effectiveness of SRS mandate that it be the imaging modality in patients with gastrinomas. Because the cost of an SRS is $1800 and may obviate the need for multiple other topographic studies that are at least as expensive, the fiscal dictates further warrant the use of this study as the initial topographic investigation. These observations are probably applicable to all gut NETs, although the likelihood of primary identification in the instance of insulinoma patients may be somewhat less. The timely and cost-effective establishment of the type of NET, its primary site, and the detection metastatic spread will enable determination of the appropriate management strategy.