Central Neurocytoma

Central neurocytoma was first described in 1982. It is a rare tumor of the central nervous system with neurocytic differentiation and a favorable prognosis. Although neurocytoma comprises only 0.25-0.5% of all intracranial tumors, they are the most frequent intraventricular tumor in adults. They occur predominantly in the third and fourth decades but can be seen in other age group. About three-quarter of the cases occur in the lateral ventricles with the foramina of Monro as the most common sites, the remaining one-quarter of the cases occur in the third ventricles. Rarely, they can occur in the cerebral hemispheres.

Their ventricular location makes symptoms and signs of hydrocephalus the most common manifestations. The acuteness of clinical manifestations is variable. Uncommonly intraventricular hemorrhage can be the initial cause for medical attention.  The MRI images of central neurocytoma are usually characteristic. Most of them occur as an exophytic, well circumscribed, globular mass that protrudes into the ventricles. Large tumors are not uncommon. Calcifications are common and easily identified by CT scans. Central neurocytomas that arise in the lateral ventricles typically adhere to the septum pellucidum. On T2-weighted image, they are isointense to gray matter. Contrast enhancement is common but variable and it can be intense. 

Macrosopically, the lesional tissue is soft, tan and well demarcated from the adjacent brain parenchyma tissue. Some of the tumors are partly cystic.  The consistency varies from soft to gritty, depending on the amount of calcification. Since they do not regularly permeate into the surrounding brain parenchyma, the amount of surrounding brain parenchyma attached to the tumor is often minimal in biopsy tissue. However, secondary attachment of the tumor to the surrounding ventricular surfaces made gross total excision not always possible.

Histological, the tumor is composed of solid sheets of small, round to polygonal, isomorphic tumor cells with distinct cell membranes. Within the neoplastic cells is a delicate vascular network. Microcalcifications, when present, are distributed throughout the tumor. The cytoplasm is fine and amphophilic. The summation of these features generates a “honey-comb” pattern similar to that of oligodendroglioma and clear cell ependymoma. The nuclei of neurocytoma are round, small, and contain fine and speckled chromatin. Nucleoli are indistinct. Mitotic figures are not readily seen or infrequent. Irregular and small islands of neuropils are present. Perivascular arrangement of neuropils in some areas may closely mimic the coronary perivascular fibrillary hypocellular mantles (perivascular pseudorosettes) that are considered evidence of ependymoma. The real ependymal pseudorosettes are more orderly and fibrillary. 

In intraoperative consultations, cytologic preparations and frozen sections often show features suggestive of a low-grade glial neoplasm. The variation of nuclear size in the cytologic preparation may exceed that in the frozen section and paraffin sections. Nuclei also tend to be more angulated in frozen sections.  The monotonous, small, and bland nuclei may raise the suspicion of an oligodendroglioma. The location of the tumor is very helpful to prevent calling these tumors glial tumors.

Ultrastructually, central neurocytomas have features of neuronal differentiation that include delicate cytoplasmic processes, microtubules, dense core granules and synaptic bouton-like structures. Intermediate filaments are exceptional. Dense-core granules of 60-160 nm are abundant. Clear vesicles of 40-60 nm of the pre-synaptic type and specialized syanptic junctions can also be see. Neurocytomas also bear immunoactivity for synaptophysin. Immunoreactivity for chromogranin and neurofilament proteins is usually lacking. Slightly more than half of the cases in one study express a photoreceptor antigen. Reactive astrocytes, often in the form of stellate shaped cells with long cytoplasmic process, are well demonstrated by immunohistochemistry for glial fibrillary acidic protein (GFAP).

 Interestingly, a small subset of neurocytoma cells coexpress synaptophysin and GFAP and suggest an origin from a pluripotential neuroglial precursor. Neurocytoma cells in culture form a cellular mosaic similar to subependymal layer that contains mitotically active cells, neurons and glial cells. This notion is supported by in vitro studies. These studies suggest that neurocytoma cells exhibit both neural and glial phenotypes and have the properties reminiscent of precursor cells derived from subventricular matrix. Perhaps it is best to view neurocytoma as a glial neuronal tumor with predominantly neurocytic differentiation.

This tumor is classically considered as benign tumor. However aggressive behavior has been described in some cases that atypical histological featuers and elevated proliferation index.  Features associated with aggressive behavior include microvascular proliferation, mitosis and necrosis. In one series, the histological and immunohistochemical criteria of biological aggressiveness appeared to be high mitotic activity, tumor necrosis, loss of neuronal differentiation. The proliferation index as evaluated by immunostaining for MIB-1 staining is under 2.3% in classic neurocytomas but over 5.2% in atypical neurocytomas  in one study. In another study, 22% of the neurocytomas with a MIB-1 labeling index below 2% relapse. In contrast, 63% of neurocytomas with MIB-1 labeling index over 2% relapse. These case are best termed atypical neurocytoma.
  [From Chimène Kesserwan, M.D. and Richard W. Leech, M.D. Department of Pathology, University of Oklahoma Health Science Center, Oklahoma City, Oklahoma ]

Revised 10/28/05.
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