Answer: Solid pseudopapillary tumor

Histology: This is a solid cellular neoplasm, featuring minimal stroma and high cellularity. Neoplastic cells are epithelioid, with indistinct cytoplasm and round, bland nuclei. The cells appear somewhat dyscohesive, and they are scattered macrophages between the neoplastic cells. The neoplastic cells show strong labeling for CD10, and show nuclear b-catenin labeling. These findings support the diagnosis of solid pseudopapillary tumor

Discussion: Islet tumors are typically strongly immunoreactive for endocrine markers like chromogranin and synaptophysin and do not usually show labeling for CD10. They do not show nuclear B-catenin labeling. Acinar cell carcinomas label for markers such as trypsin and lipase. Only a small proportion of these lesions show nuclear B-catenin labeling. These neoplasms frequently demonstrate prominent nucleoli. Pancreatic ductal adenocarcinoma demonstrates marked cytologic atypia in addition to prominent desmoplastic stroma, neither of which is present in the current case.

Solid pseudopapillary neoplasms typically affect young females, and most pursue a benign course if completely resected. Some are locally aggressive, so these are best considered low grade malignancies.