Pancreatic malignant fibrous histiocytoma, inflammatory myofibroblastic tumor, and inflammatory pseudotumor related to autoimmune pancreatitis: characterization and differential diagnosis.

Malignant fibrous histiocytoma (MFH) and inflammatory myofibroblastic tumor (IMT) are uncommon primary non-epithelial cell tumors of the pancreas. In addition, there are inflammatory pseudotumors (IPT) that may arise in the course of autoimmune pancreatitis (AIP). In the English language literature, only 24 cases of IMT and nine cases of MFH in the pancreas have been reported to date. We investigated three individual spindle cell tumors of the pancreas that were identified as MFH, IMT, and IPT, respectively, using immunohistochemical and molecular analysis. Both the MFH and the IMT, but not the IPT, showed nuclear p53 expression and mutations of the p53 gene. The MFH and the IMT also had higher mitotic and Ki-67 (MIB-1) indexes than the IPT. The IPT was found to be a tumor-like case of AIP. Many IgG4-positive plasma cells, which are considered to be a feature of AIP, were found in all three tumors. It is concluded that in this series of spindle cell tumors of the pancreas, apart from immunohistochemical features, the demonstration of p53 mutations may be helpful in distinguishing true neoplastic tumors from pseudotumors such as IPTs arising in the context of AIP.