Paranuclear Dot-like Immunostaining for CD99: A Unique Staining Pattern for Diagnosing Solid-Pseudopapillary Neoplasm of the Pancreas

Solid-pseudopapillary neoplasm of the pancreas is a rare tumor of uncertain histogenesis that is characterized by a cystic and solid growth pattern with pseudopapillary structures. The differentiation of solid-pseudopapillary neoplasm from other pancreatic tumors is of great importance. However, it is sometimes difficult because of similarities in morphologic features and immunophenotype. CD99 is a diagnostically useful marker for Ewing sarcoma/primitive neuroectodermal tumor. The aim of this study was to investigate the diagnostic value of CD99 in solid-pseudopapillary neoplasm. We investigated immunohistochemical staining for CD99 in tissue microarray blocks from 55 cases of pancreatic solid-pseudopapillary neoplasm, 51 cases of pancreatic neuroendocrine tumor, and 54 cases of pancreatic adenocarcinoma. Biopsy specimens from 7 solid-pseudopapillary neoplasms, 6 acinar cell carcinomas, and 1 pancreatoblastoma were also investigated. All the solid-pseudopapillary neoplasm cells exhibited paranuclear dot-like immunoreactivity for CD99 regardless of the clinicopathologic or morphologic features. Forty of the 51 pancreatic neuroendocrine tumors were positive for CD99. Staining here was membranous, or membranous and cytoplasmic. Four of the 54 pancreatic adenocarcinomas and 1 pancreatoblastoma showed faint membranous staining. None of the acinar cell carcinomas was reactive for CD99. Our study has identified for the first time that pancreatic solid-pseudopapillary neoplasm exhibits a unique dot-like staining pattern for CD99. This could prove to be the most useful aspect of its immunoprofile for the definitive diagnosis of solid-pseudopapillary neoplasm and differentiation from other pancreatic tumors.