Because of the nature of the tumor, preoperative localization of gastrinoma may prove difficult or, in some cases, unreliable. The tumor may be single or multiple and can range in size for less than 1 cm to more than 3 cm. When associated with MEN-1, studies suggest that gastrinomas are usually multiple and commonly found within the duodenum and pancreas.1 ' Although it has been suggested that the tumors found in patients with MEN-1 may have a lower potential for metastases, they appear to metastasize with a similar frequency to their sporadic counterparts when evaluated in long-term follow-up.
   Approximately 80% of gastrinomas are found within the gastrinoma triangle, an area that includes the first, second, and third portions of the duodenum and the head of the pancreas.1 • In addition to the duodenum and pancreas, primary gastrinomas have been found in the jejunum, stomach, liver, spleen, mesentery, ovary, and heart.1 • 1 • A particularly confusing tumor is one that is found only within a lymph
node. Although rare, patients have been reported to be biochemically cured of ZES after excision of solitary gastrinomas that appear to have arisen within a lymph node. This has given rise to the idea of a lymph node primary gastrinomas • Gastrinomas of the duodenum and pancreas appear to have a similar incidence of metastases; however, pancreatic gastrinomas have a higher incidence of liver metastases than the
duodenal tumors, and duodenal tumors have a higher incidence of lymph node metastases than pancreatic tumors. This leads to a decreased long-term survival rate with pancreatic primary gastrinomas. LOCALIZATION PROCEDURES
The preoperative localization of tumor in patients with ZES continues to be imprecise. No one imaging or localization study alone can clearly identify total tumor extent. In general, preoperative imaging includes ultrasound (US), computed tomography (CT), magnetic resonance imaging, and selective angiography. More invasive studies, including portal venous sampling for gastrin levels or intra-arterial secretin with
hepatic venous sampling for gastrin levels, provide functional localization to a region of the pancreas. Conventional noninvasive localization studies may fail to detect tumor in as many as 40% of patients with ZES.
As a first-line study, US tends to have a sensitivity of no greater than 30% but its specificity approaches 92%.1 ' The accuracy of CT scanning is dependent on the size of the gastrinoma. Lesions of less than 1 cm are seldom visualized, even with current advanced helical sequences, whereas 30% of those between 1 and 3 cm are seen. CT can detect all primary and metastatic lesions of more than 3 cm. Overall, CT
imaging can identify approximately 80% of pancreatic and 35% of extrahepatic gastrinomas. MRI may be useful in identifying small lesions and, in particular, liver metastases. It is also useful in distinguishing metastatic lesions from benign hemangiomas. However,
it is expensive and, on the basis of most studies, images only about 25% of primary gastrinomas.
Somatostatin receptor scintigraphy (SRS) ( Fig. 8-1 ) with 111 In-DTPA-dPhe octreotide was first evaluated in 1993. It is regarded as the imaging test of choice for localizing both primary and metastatic gastrinomas. The radiolabeled somatostatin analogue has a high affinity for the type 2 somatostatin receptor, which is expressed in most gastrinomas. Ninety per cent of tumors can be imaged with this modality,
  with a specificity approaching 100%. I ' In the setting of ZES, when clinical suspicion of gastrinoma is high, it has a positive predictive value of 100% and can have a sensitivity exceeding that of all other imaging studies combined.I ' However, it still may be unable to identify small primary duodenal gastrinomas.
Endoscopic ultrasound (EUS) is a fairly new method of localizing gastrinomas. It is relatively invasive and attempts to detect small tumors by endoscopically placing a high-frequency ultrasonic transducer in the vicinity of the gastrinoma triangle and the liver. It has been used to successfully image pancreatic islet cell tumors such as insulinomas. The procedure is operator dependent and has not been able to reliably
identify small duodenal tumors. In addition, EUS may have difficulty differentiating normal lymph nodes from those containing tumor because the sonographic appearance is similar. One study found the sensitivity of EUS to be 50 to 75% for duodenal, 75% for pancreatic, and 63% for lymph node gastrinomas.
Because noninvasive studies may not image the gastrinoma, invasive imaging and regional localization studies have also been used extensively. Previously, selective angiography was the imaging study of choice and was able to identify 60% of tumors. Primary or metastatic gastrinomas were seen as tumor "blush" within the liver, pancreas, or wall of the duodenum. This study necessitated Figure 8-1 Somatostatin receptor scintigraphy showing a lesion that is consistent with a gastrinoma (arrow) within the area of the gastrinoma triangle. On exploration, the lesion was found in the medial wall of the duodenum.
Figure 8-2 Intraoperative ultrasound showing the echogenic characteristics of a gastrinoma within the head of the pancreas (arrows). The mass itself is about 2 cm in diameter. The pancreatic duct is seen in the lower left of the screen (arrowhead).
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