Answer: D - Central neurocytoma, CNS WHO grade 2

Histologic sections show a monomorphic cellular neoplasm with round nuclei, crisp nuclear contours and speckled chromatin. Tumor cells are embedded in a fibrillary background, with occasional formation of neurocytic rosettes (nuclei surround large, irregular areas of neuropil). The cytologic smear shows sheets of well-dispersed, monotonous, round nuclei, similar to the appearance of a pituitary adenoma. Immunohistochemical stains show the tumor cells to be positive for synaptophysin and NeuN, consistent with a neuronal immunophenotype. GFAP highlights background  (entrapped) reactive astrocytes, with likely some tumor cell expression as well. These findings, along with the clinical presentation, are in keeping with a diagnosis of "Central neurocytoma, CNS WHO Grade 2."

Central neurocytomas are intraventricular tumors composed of uniform round cells of neuronal immunophenotype. These tumors are often attached to the septum pellucidum near the foramen of Monro. The majority of patients present with these tumors between the ages of 20 and 40 years. Central neurocytomas are distinguished by their immunoreactivity for both synaptophysin and NeuN, but there is evidence of these tumors having potential for both neuronal and glial differentiation. Hence, while GFAP may highlight a significant element of entrapped reactive astrocytes, some immunoreactivity may correspond to tumor cell expression. Olig2 expression can be seen in occasional tumor cells, but is not diffusely expressed as is seen in oligodendroglioma. Ependymomas are typically GFAP-positive and Olig2-negative, however they lack the synaptophysin and NeuN expression that is seen in central neurocytoma.  Finally, expression of NeuN and a lack of a ganglion cell component distinguishes the current case from a ganglioglioma. 

Mitotic activity and Ki67 proliferative index ( < 2%) is typically low in central neurocytomas, and the tumor corresponds to CNS WHO grade 2. The clinical course is generally favorable, with the extent of resection and Ki67 labeling index having been reported as important prognostic factors. Interestingly, the molecular underpinning of central neurocytoma is still unknown, with no distinct recurrent mutations or chromosomal alterations yet reported. Methylation profiling, however, does map to a distinct methylation cluster for tumors with a histologic diagnosis of central neurocytoma.