Islet Cell Tumors of the Pancreas

Clinical, Radiologic, and Pathologic Correlation in Diagnosis and Localization

Abstract Introduction and Historical Perspective Clinical Features

Pathologic and Histologic Features Radiologic Features Differential Diagnosis Treatment and Prognosis

Summary Source Information References

Differential Diagnosis

In patients with functioning islet cell tumors, the radiologist is called on only to localize the lesion. There is no differential diagnosis. However, when the patient has no endocrine syndrome and a pancreatic lesion is detected, differential diagnosis becomes important. Both small and large lesions may be detected in asymptomatic patients. Larger islet cell tumors are more common in asymptomatic patients or in patients presenting with endocrine syndromes that are relatively subtle and have gone unrecognized. There is an association between the size, consistency, behavior, and function of islet cell tumors (3). Smaller tumors are homogeneous without malignant behavior (gastrinoma is an exception) (Figure 4, Figure 5, Figure 6, Figure 8, Figure 13). Larger tumors more commonly demonstrate cystic degeneration, necrosis, calcification, local invasion, vascular invasion, and distant metastases (Figure 3, Figure 7, Figure 9, Figure 10, Figure 11). Overall, the larger clinically silent tumors have a worse prognosis than the smaller lesions and are associated with a 3-year survival rate of about 60% (60,61).

The radiologic differential diagnosis for islet cell tumors includes adenocarcinoma; microcystic adenoma; metastatic tumor; solid and papillary epithelial neoplasms; and, rarely, paraganglioma and sarcoma. The hypervascularity of islet cell tumor is a key feature that helps differentiate it from other pancreatic neoplasms, particularly adenocarcinoma, which is hypovascular. Microcystic adenomas may also be hypervascular, but these tumors usually demonstrate characteristic small cysts and typically occur in older women. Islet cell tumors have no significant sex predilection and occur in young to middle-aged patients. In addition, microcystic adenomas are benign lesions that do not demonstrate local or vascular invasion or metastatic disease. Hypervascular metastases to the pancreas may mimic features of islet cell tumors. In the Mayo Clinic experience (62), the most frequent metastasis to the pancreas was from renal cell carcinoma, which is hypervascular. In these cases, the clinical history of a prior primary malignancy was the distinguishing feature. We have seen a case of a pancreatic paraganglioma that demonstrated hypervascularity similar to that of an islet cell tumor; this, however, is extremely unusual.

Solid and papillary epithelial neoplasms are typically seen in young female patients and characteristically demonstrate areas of hemorrhagic degeneration (63). Degenerated areas may mimic certain features of larger islet cell tumors. However, the peripheral portions of solid and papillary epithelial neoplasms do not demonstrate the hypervascularity typical of islet cell tumor. There are extremely rare sarcomas of the pancreas that may demonstrate central necrosis and malignant behavior similar to that seen in large islet cell tumors. These sarcomas do not typically demonstrate a hypervascular component, and they are much rarer than islet cell tumors.

MR imaging is helpful in characterizing islet cell tumors, which have marked increased signal intensity on T2-weighted images and fat-suppressed inversion-recovery images. Gadolinium enhancement in the nonnecrotic or nondegenerated portions of the tumor is also a characteristic feature (Figure 6) that helps differentiate islet cell tumors from the more common adenocarcinoma (which is hypovascular, does not enhance, and is of lower signal intensity on T2-weighted images).

Abstract Introduction and Historical Perspective Clinical Features

Pathologic and Histologic Features Radiologic Features Differential Diagnosis Treatment and Prognosis

Summary Source Information References