FINAL DIAGNOSIS AND DISCUSSION:
Diagnosis: Extraventricular atypical neurocytic neoplasm ("cystic ganglioneurocytoma").
Neuronal neoplasms of the central nervous system show a wide morphological spectrum of tumors and may contain immature neuroblastic cells, neurocytes and ganglion cells. Tumors composed solely of neurocytes are uncommon and usually localized intraventricularly in young adults. These tumors were separated as a distinct entity, i.e. central neurocytoma . They are composed of uniform oval-to-round cells with neuronal differentiation. However, rare neurocytic tumors were also described in extraventricular location [2-6]. Their nosological status is unclear and terminology variable, depending on a localization, morphological pattern and a cellular composition. Apart from the monotonous population of neurocytes, these neoplasms may have an admixture of ganglion or ganglioid cells and reactive astrocytic elements [3,6]. An important constituent of some of these neoplasms is vascular proliferation, frequently forming a network intermixed with the neurocytes [3,6].
The present tumor belongs to the spectrum of extraventricular neurocytic neoplasms. However, in contrast to those described previously, its cellular composition was pleomorphic, as, in some areas, atypical multinucleated cells could be found. Moreover, the areas of tissue necrosis were also present. In the vicinity of necrotic areas, a proliferation of glomeruloid vessels was discernible (Fig. 2). Tumor necrosis was uncommonly reported in central neurocytomas [7-13], and it was not identified in extraventricular neurocytic neoplasms [2-6]. However, the tumor necrosis seems to be an irrelevant indicator of unfavorable course in central neurocytomas, as such cases showed the range of survival comparable to classical intraventricular neurocytomas [7,13].
In central neurocytomas the vascular proliferation and the elevated proliferation index correlated with less favorable clinical outcome, and such tumors were referred to as atypical neurocytomas . In the extraventricular neurocytic neoplasms two types of vascular proliferation may be distinguished. More commonly, the vascular proliferation makes a component of the tumor in the form of dense capillary network and these vessels have densely hyalinized walls and flattened and atrophic endothelial cells. This feature was identified in some cases reported by Giangapero et al.  and mixed neuronal-glial tumors including ganglioglioma  and papillary glioneuronal tumor . Their mural hyalinization seems to be an important clue to the neural (neurocytic) origin of this tumor and indirectly may indicate its indolent clinical behavior. This type of vascular proliferation contrasts with vascular proliferations identified in classical neurocytomas which may have delicate arborizing pattern resembling that observed in endocrine tumors [10,15].
The second type of vascular proliferation was identified in atypical neurocytomas and in one case of extraventricular neurocytic neoplasm . It was formed by glomeruloid structures of vessels with cuboidal endothelial cells. This type of proliferation may be influenced by tissue necrosis and local ischemia, and it resembles similar structures found in glioblastomas. In the present case, these structures were found focally around necrotic foci.
In the present case, the proliferation index, as determined by Ki-67 (MIB-1) immunoreactivity, was low (0.7%). It was in the range of MIB-1 index reported in other series (0.5-2.5%) [3,6]. However, the tissue necrosis, glomeruloid vascular proliferations and the cytologic features seem to justify the designation of atypical extraventricular neurocytic tumor. The clinical significance of these features is unclear.
The differential diagnosis embraces other neuronal tumors and the most important is distinguishing this tumor from the neoplasms containing neuroblastic elements as the latter carry less favorable prognosis . In the present tumor, we could not identified pure neuroblastic elements and due to the presence of tissue necrosis, prominent atypia in seemingly differentiated neurocytes or ganglioid cells, which significance is unclear, we preferred the term extraventricular atypical neurocytic neoplasm to designate these features.
Contributed by Wojciech Biernat, M.D, Krzysztof Zakrzewski, M.D. and Pawel P. Liberski, M.D.