Solid pseudopapillary tumor of the pancreas: emphasis on differential diagnosis from aggressive tumors of the pancreas.

Solid pseudopapillary tumor is an unusual primary tumor of the pancreas with a low potential for malignancy and unknown cell origin, seen mostly in young women. Although it is discussed among pancreatic epithelial tumors, many cases do not express cytokeratin but show neuroendocrine differentiation. Three cases (2 female, 1 male, aged 24, 45 and 50 years, respectively) of solid pseudopapillary tumor localized in the pancreas are presented. All cases displayed a well-circumscribed tumor, with an average diameter of 6 cm and a red-brown colored, hemorrhagic, cystic cut surface. Microscopically they were encapsulated with large areas composed of thin papillary formations and solid areas focally. Tumor cells were dyscohesive with small, round- to-oval, central nuclei, and vacuolated, clear or eosinophilic cytoplasm without mitotic activity. NSE, vimentin, synaptophysin, ER, PR, Ki-67, S-100, Pan CK, a1-antitrypsin, a2-antichymotrypsin, and antibodies were used in the immunohistochemical study. Vimentin, synaptophysin, NSE, PR, and a1-antitrypsin showed expression in all cases, while Pan-CK was expressed in two cases. Ki-67 expression was below 1% in all cases. Morphologic features of solid pseudopapillary tumor may be confused with pancreatic endocrine neoplasm and ductal adenocarcinoma. All cases showed features of histiocytic and neuroendocrine differentiation. Epithelial differentiation was identified in two cases. We conclude that immunohistochemistry is incapable of giving additional information for the diagnosis of solid pseudopapillary tumor due to different lines of differentiation of tumor cells. We believe that macroscopic and microscopic features (using hematoxylin and eosin stain) are more important for the diagnosis and differential diagnosis of this tumor.