After establishing a diagnosis of insulinoma, a variety of imaging modalities with different sensitivities can localize the tumor. The role of preoperative imaging is 2-fold. First, imaging is used to evaluate for evidence of metastatic disease; second, localization can better facilitate discussions with the patient with regard to extent and type of operation. Because virtually all sporadic insulinomas are small and intrapancreatic, preoperative localization fails 10% to 27% of the time.13
The extent of imaging necessary to ensure an operative cure has not been clearly defined and varies between institutions. In fact, some suggest that preoperative localization within the pancreas is not even necessary because the insulinoma can be localized successfully intraoperatively.14
Results of noninvasive localization studies, including transabdominal ultrasound, multiphase helical computed tomography (CT), magnetic resonance imaging (MRI), and somatostatin receptor scintigraphy (SRS) are disappointing. The success rate of transabdominal ultrasound for localization varies widely across institutions, from 9% to 66%.7,13,15–17
Multiphase helical CT localizes 50% to 80% (), MRI 40% to 70%, and SRS 17% of all insulinomas.7,13,15–19
All of the these imaging studies combined can localize around 80% of tumors.13,16,17
CT and MRI are useful to evaluate for metastatic disease, although MRI may be more sensitive than CT in identifying liver metastases.9
Lesion successfully localized by computed tomography scan. A round, well-circumscribed, hyperenhancing lesion can be seen at the tail of the pancreas (arrow). (Courtesy of National Institutes of Health, Bethesda, MD.)
When preoperative noninvasive studies fail to localize tumors, invasive studies may aid in regional localization. Pancreatic arteriography was historically considered the gold standard, with early reports quoting success rates of 90%.13
However, more recent studies show a much lower rate of localization in the range of 25% to 50%.7,13,15–17
Transhepatic portal venous sampling (THPVS) involves percutaneous and transhepatic catheterization of a branch of the portal vein followed by advancement into the small draining veins of the pancreas, including the superior mesenteric, portal, and splenic veins, to sample blood for insulin.13,20
A step-up in the insulin level reflects the region of the pancreas where the insulinoma resides. This technique has shown a 77% to 100% success in localization.17,20,21
However, THPVS requires special skills and experience and is associated with slight, but significant morbidity; it has therefore been abandoned.
THPVS has been replaced by intra-arterial calcium stimulation (IAC), which relies on calcium as a secretagogue for insulin secretion from the tumor.15,16
IAC involves catheterization of the gastroduodenal, superior mesenteric, and proximal and distal splenic arteries, which are then subsequently injected with calcium.15,16
Blood is sampled for insulin from a second catheter, which is placed first in the right hepatic vein and then the left hepatic vein for corroboration. A step-up in the insulin concentration localizes the insulinoma to a particular region of the pancreas ( and 4
). IAC has a reported sensitivity from 80% to 94% in localizing insulinomas to a particular region of the pancreas.15,16
Fig. 3 Left hepatic vein insulin concentrations after intra-arterial calcium injection. Injections of the superior mesenteric artery (SMA), gastroduodenal artery (GDA), and proper hepatic artery do not show any suspicious areas. However, the increase in insulin (more ...)
The use of endoscopic ultrasound for tumor localization has steadily increased over the past several years. Reported sensitivities range from 40% to as high as 93%.21,22
The sensitivity has shown to vary by tumor location, and is also operator dependent. The authors' experience with endoscopic ultrasound is limited, as this modality is not employed at our institution.
The use of intraoperative ultrasound (IOUS), introduced in 1981, is useful to localize intrapancreatic, nonpalpable lesions, and to determine the proximity of those lesions to the pancreatic or biliary duct. IOUS performed during an open or laparoscopic exploration can localize an insulinoma in 86% of cases.16,21
The practice at the authors' institution once the diagnosis of insulinoma has been made is to obtain a CT scan to evaluate for metastatic disease and to assist with localization of the lesion. If the CT scan successfully localizes the lesion, the patient is taken to the operating room for either an open or laparoscopic exploration with IOUS. If the CT scan does not show a lesion, the patient undergoes intra-arterial calcium stimulation to regionalize the tumor, then is taken to the operating room.