FNA diagnosis of CD99 positive neuroblastoma: A diagnostic dilemma.

Tissue diagnosis of small round cell tumors relies heavily on immunohistochemical staining. Two of the small round cell tumors, namely neuroblastoma and primitive neuroectodermal tumor, have considerable morphologic overlap. Many studies suggest that CD99 positivity virtually excludes the diagnosis of neuroblastoma. We report a case of poorly differentiated neuroblastoma in which aberrant CD99 positivity led to diagnostic dilemma.