3. insulinoma

Discussion

The CT scan (Fig 1) demonstrated a focal, hyperdense lesion of approximately 1 cm in diameter in the tail of the pancreas that is more readily seen using a narrow window width (Fig 2). The increased density indicates a hypervascular mass. Intraoperative ultrasound with the transducer placed directly on the pancreas demonstrates the corresponding well-circumscribed hypoechoic (noncystic) mass (Fig 3), which was enucleated. Histologic evaluation confirmed an islet cell tumor consistent with an insulinoma.

Fig 3. — Intraoperative ultrasound with the transducer placed directly on the pancreas demonstrates the corresponding well-circumscribed hypoechoic (noncystic) mass, which was enucleated.

Insulinomas are rare tumors of the islets of Langerhans within the pancreas. The majority of these tumors occur sporadically as small benign lesions that can be located anywhere within the pancreas and occasionally outside of it. Although most are benign, they can induce profound effects on the patient. The diagnosis of insulinoma is often difficult because there may be a variety of presenting symptoms (dizziness, headaches, apathy, and behavioral disturbances attributable to neuroglycopenia). The diagnosis of hyperinsulinemia is confirmed by the biochemical estimation of fasting serum glucose and insulin as well as other supporting clinical and laboratory data. Surgical resection is usually the treatment of choice for insulinoma.

The utility of preoperative localization of insulinomas is controversial since the sensitivity for localization using intraoperative ultrasound with palpation can approach 100%.^1,2 Because management may require demanding surgery with the potential for significant morbidity, most surgeons still attempt preoperative localization since it can allow for a focused pancreatic exploration and may obviate the use of blind distal pancreatectomy in cases where intraoperative localization fails.

A variety of imaging modalities have been used for preoperative localization. These studies have traditionally included CT, ultrasonography (US), magnetic resonance imaging (MRI), and angiography. Transabdominal US typically detects less than 50% of lesions, may be limited by patient body habitus, and is operator dependent. With advances made in endoluminal US, endoscopic US has become another important modality for localization. The ability to direct the ultrasound probe closer to the pancreas has improved detection of pancreatic lesions. Sensitivity ranging from 57% to 93% has been reported.^3-5 However, the location of the lesion plays a role in the accuracy of detection. In a study by Schumacher et al,³ the sensitivity for the detection of lesions in the head of the pancreas was 83%; this dropped to 37% for lesions located in the tail of the organ.

Technologic advances made in CT equipment and greater availability of spiral CT scanners have improved the detection of insulinomas. To optimize detection, CT evaluation should be performed using spiral acquisition with thin-section collimation and rapid bolus contrast administration.⁶ The technique of dual-phase contrast-enhanced spiral CT allows scanning in the arterial and arteriovenous phase after contrast administration. This has the potential for improving detection of small insulinomas. In a study by King et al,⁷ high-resolution dual-phase contrast-enhanced spiral CT localized 6 of 7 tumors that were previously undetected by a variety of radiologic studies. The detected lesions ranged in size from 6 mm to 18 mm with better visualization of the lesions in the arterial phase. Similarly, MRI techniques have evolved from standard T1- and T2-weighted spin echo imaging to include newer sequences such as inversion recovery with motion suppression⁸ and dynamic gadolinium-enhanced imaging with fat suppression.^9,10 The number of patients evaluated is small, but lesions that may be missed on standard MR sequences have been detected.

When noninvasive studies fail to localize the lesion, angiography has traditionally been attempted to delineate the lesion due to the hypervascular nature of insulinomas. This approach also has limitations in detecting small lesions. This has led to the use of portal venous sampling as the procedure of choice. However, angiography is an invasive and technically demanding preoperative procedure requiring subselective catheterization of variable draining veins from the pancreas. A new provocative an

giographic technique, introduced in 1991 to improve and simplify the invasive procedure, couples arteriography with stimulation of various regions of the pancreas with calcium gluconate (a secretagogue to stimulate production of insulin by the insulinoma) and subsequent sampling of hepatic veins to measure increases in insulin levels. As with portal venous sampling, the intraarterial calcium stimulation test does not visualize the lesion. However, a finer degree of regional localization is obtained since specific regions of the pancreas can be stimulated in an isolated manner. The results obtained from this technique have shown greater sensitivity (89% to 94%) for lesion localization,^11-13 and it has supplanted portal venous sampling as the invasive procedure of choice when noninvasive studies are negative.

Conclusions

A variety of imaging methods is available for preoperative localization of insulinomas. Most research studies have small patient populations and wide variability in detection rates. Therefore, no single modality can be clearly recommended as the initial imaging study of choice. However, technical advances in spiral CT, MRI, and endoscopic US show promise in improving detection of smaller lesions. Local technical expertise and availability of newer imaging modalities may play a larger role in determining the best initial diagnostic test. For occult insulinomas not visualized by noninvasive studies, the intraarterial calcium stimulation test appears to be the study of choice when preoperative localization in required.

References

1. Galiber AK, Reading CC, Charboneau JW, et al. Localization of pancreatic insulinoma: Comparison of pre- and intraoperative US with CT and angiography. Radiology. 1988;166:405-408.

2. Kuzin NM, Egorov AV, Kondrashin SA, et al. Preoperative and intraoperative topographic diagnosis of insulinomas. World J Surg. 1998;22:593-597.

3. Schumacher B, Lubke HJ, Frieling T, et al. Prospective study on the detection of insulinomas by endoscopic ultrasonography. Endoscopy. 1996;28:273-276.

4. Pitre J, Soubrane O, Palazzo L, et al. Endoscopic ultrasonography for the preoperative localization of insulinomas. Pancreas. 1996;13:55-60.

5. Zimmer T, Stolzel U, Bader M, et al. Endoscopic ultrasonography and somatostatin receptor scintigraphy in the preoperative localisation of insulinomas and gastrinomas. Gut. 1996;39:562-568.

6. Chung MJ, Choi BI, Han JK, et al. Functioning islet cell tumor of the pancreas. Localization with dynamic spiral CT. Acta Radiol. 1997;38:135-138.

7. King AD, Ko GT, Yeung VT, et al. Dual phase spiral CT in the detection of small insulinomas of the pancreas. Br J Radiol. 1998;71:20-23.

8. Kier R, Kinder B. Insulinomas: MR imaging with STIR sequences and motion suppression. AJR Am J Roentgenol. 1992; 158:457-458.

9. Kraus BB, Ros PR. Insulinoma: Diagnosis with fat-suppressed MR imaging. AJR Am J Roentgenol. 1994;162:69-70.

10. Semelka RC, Cumming MJ, Shoenut JP, et al. Islet cell tumors: comparison of dynamic contrast-enhanced CT and MR imaging with dynamic gadolinium enhancement and fat suppression. Radiology. 1993;186:799-802.

11. Cohen MS, Picus D, Lairmore TC, et al. Prospective study of provocative angiograms to localize functional islet cell tumors of the pancreas. Surgery. 1997; 122:1091-1100.

12. Pereira PL, Roche AJ, Maier GW, et al. Insulinoma and islet cell hyperplasia: value of the calcium intraarterial stimulation test when findings of other preoperative studies are negative. Radiology. 1998;206:703-709.

13. Brown CK, Bartlett DL, Doppman JL, et al. Intraarterial calcium stimulation and intraoperative ultrasonography in the localization and resection of insulinomas. Surgery. 1997;122:1189-1193.

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