Intraoperative cytology of central neurocytoma mimicking oligodendroglioma.

Central neurocytomas (CNs) are uncommon tumors of the central nervous system. These tumors have a predilection for the lateral ventricles of young adults and are known to display characteristic neuroimaging and histomorphologic features. Typically, CNs are associated with a favorable outcome, although cases with more aggressive clinical course with recurrences are not unknown. Most descriptions of this tumor are available in the form of isolated histopathology-based case reports and small series. Cytology-based publications of CN are rare. Here, we report a case of CN in a 22-year-old girl. Intra-operative squash cytology and subsequent histopathology of the tumor simulated an oligodendroglioma and a clear cell ependydoma. Final confirmation was obtained on immunohistochemistry. This paper discusses the salient cytological, histomorphological and immunohistochemical features of CN that are useful in distinguishing from its mimickers.

Introduction

Central neurocytomas (CNs) are slow-growing neuroectodermal tumors, which account for approximately 0.1-0.5% of all brain neoplasms. These typically affect young adults in the third decade of life, and have a predilection for the lateral ventricles at the foramen of Monro. Rare cases of extraventricular neurocytomas (EVN) have also been reported.[1]

Most of the available literature on CNs is based on histopathological diagnosis, with only few case reports elaborating on the cytological features seen on intra-operative squash preparation. In the present report, we describe the cytomorphology of CN, along with its differential diagnoses and, finally, the approach to a definitive confirmation.

Case Report

A 22-year-old right-handed woman presented with global headache of 4 months duration, which was insidious in onset and gradually progressive, along with gradually deteriorating vision of both eyes. She denied any history of vomiting, seizures or loss of consciousness. There were no associated co-morbidities. Clinical examination of the central nervous system (CNS) revealed normal higher mental functions and speech, with reduced visual acuity of both eyes. Pupils were normal sized and were equally reactive to light. Examination of other cranial nerves, sensory-motor system and cerebellum were within normal limits. Fundoscopy revealed bilateral papilledema.

Magnetic resonance imaging (MRI) scan showed a lobulated heterogeneously T2-hyperintense right-sided intra-ventricular mass measuring 3 cm, which was extending up to the foramen of Monro. Differential diagnoses of CN, germinoma and oligodendroglioma were considered [Figure 1a]. Intra-operatively, the mass was found to be arising from the right third ventricle with extension into the foramen of Monro. It was soft, moderately vascular and was suggestive of a high-grade glioma. Total resection was performed and the mass was submitted initially for intra-operative evaluation, followed by histopathological examination.

Figure 1

(a) MRI skull (T1-weighted image): Right intra-ventricular mass with heterogenous hypointensity. (b) Photomicrograph of squash preparation of tumor bits showing round monomorphic uniform-sized cells with finely stippled granular chromatin (MGG, ×200). (c) Histopathology of tumor tissue showing isomorphous cells, with round to oval nucleus, fine speckled chromatin and inconspicuous nucleoli; few of the cells showed a perinuclear halo (H and E, ×100), with (d) synaptophysin-immunoreactivity (IHC, ×100)

Intra-operative cytology

The squash preparation showed a cellular tumor composed of round monomorphic uniform-sized cells set in a background of fibrillary matrix of neuropils. Tumor cells displayed finely stippled granular chromatin, prominent micronucleoli and ill-defined cytoplasm. Nuclear pleomorphism, mitotic figures or necrotic debris were absent [Figure 1b]. A diagnosis of low-grade tumor with differential of neurocytoma and oligodendroglioma were offered.

Histopathological examination

All the tumor bits were processed and 5-μm-thick sections were cut from routine formalin-fixed, paraffin-embedded tissue, and stained with hematoxylin and eosin (H and E) stain. Immunohistochemistry (IHC) was done using streptavidin-biotin immunoperoxidase technique (Envision Kit, M/s Dakopatts, Denmark) using monoclonal antibodies to glial fibrillary acidic protein (GFAP), synaptophysin and MIB-1 (all antibodies were prediluted and obtained from M/s Dakopatts, Denmark).

Sections from the resected mass showed a tumor composed of monomorphic cells, with clear cytoplasm, round to oval nucleus, finely speckled chromatin and inconspicuous nucleoli. Numerous arborising capillary-sized blood vessels were noted. No evidence of calcification, anisonucleosis, increased mitosis, necrosis or microvascular proliferation was seen [Figure 1c]. Differential diagnoses of oligodendroglioma, neurocytoma and clear cell ependymoma were considered.

Immunohistochemistry, however, displayed synaptophysin positivity in the tumor cells and neuropils, with GFAP expression restricted to the entrapped astrocytes only. MIB-1 labeling index (MIB-1LI) was <1% [Figure 1d].

Follow-up

Post-operative period was uneventful. She was followed up for a period of 3 months, during which she remained symptom-free.

Discussion

In 1982, Hassoun et al.[2] reported a tumor with round cells having a central round nuclei and perinuclear halo, which on immunohistochemistry and electron microscopy revealed to be neuroectodermal in nature. It is generally accepted that CNs develop from the nuclei of the septum pellucidum.[2] Subsequently, Nishio et al.[3] have extrapolated this hypothesis to include the third ventricle and the subependymal plate of the lateral ventricles, as well.

CN is an intra-ventricular neoplasm, typically located in the foramen of Monro, with a predilection for young to middle-aged adults, similar to the index case. A majority of the patients present with features of raised intracranial tension, while visual and mental disturbances with hormonal dysfunction may also be observed.[4] This was similar to the present case that manifested with headache and visual symptoms.

A classical CN is either isodense or mildly hyperdense, with strong, uniform contrast enhancement on computed tomography (CT) scan, while MRI shows a high signal on both T1- and T2-weighted images, with moderate to strong gadolinium enhancement.[4]

Intra-operative cytological evaluation of brain tumors is generally practiced either to render a preliminary interpretation, or more often as a complement to frozen-section examination. Since the latter has a drawback of freezing artifacts of nuclear and cytoplasmic details, a combination of the two procedures is considered ideal. Considering the rarity of the neoplasm, and paucity of published literature, intra-operative analysis of CNs is a diagnostic challenge to all cytopathologists.[5]

Imprint cytological preparations of CNs typically display sheets or clusters of uniform round cells without appreciable pleomorphism, which are set against a background of neuropils and admixed short straight capillaries. Few variations in cytology have been reported. Sugita et al.[6] reported numerous giant cells with phagocytosed hemosiderin granules between the tumor cells. Ng et al.[7] detected calcospherites, neuropil islands and rosette-like structures as well. Further, Kobayashi et al.[8] noted typical perinuclear halo in the tumor cells, as observed in the present case, which simulated an oligodendroglioma. An isolated report by Kiehl et al.[9] documented pigmented neurocytoma resembling a melanocytic lesion.

Intra-operative cytological preparations of CN often simulate oligodendroglioma and clear cell ependymoma. Oligodendroglial cells, with monomorphic vesicular nuclei and ill-defined scant, wispy cytoplasm may resemble CN. However, the latter contains a characteristic background of fibrillary neuropil-rich matrix. Further, oligodendrogliomas contain a population of fibrillary astrocytes or minigemistocytes, and display a loose aggregation and arrangement of cells around empty spaces, forming a ‘ring-like’ pattern, contrary to CN. Ependymomas, in contrast to CN, have perivascular rosettes and poorly formed acini, cords and fibrillary processes tumor cells.[10]

The typical histopathology of neurocytoma shows uniform, small to medium-sized cells with rounded nuclei, finely stippled chromatin and inconspicuous nucleoli, along with scant cytoplasm. Cells are usually closely apposed but may also contain a background of finely filamentous stroma having a neuropil-like quality. H and E stained sections often reveal fixation artifacts in the form of cytoplasmic vacuolations. Vascularity is represented by long, thin-walled, capillary-sized vessels, which are arranged in a linear arborising pattern, imparting an endocrine appearance.[4]

Supplementing these cytological and histological features with confirmatory immunostaining for synaptophysin is imperative for CN. Typically, synaptophysin immunoreactivity is noted in the neuropil, especially in the fibrillary zones and perivascular cell-free areas. False cytoplasmic immunopositivity may be attributed to pre-existent neuropil or neuronal structures, or faulty antigen-retrieval techniques and/or the use of polyclonal anti-synaptophysin antibody. Expression of GFAP has been observed more frequently in EVN as compared to CN. CN ultrastructure shows cell processes containing parallel arrays of microtubules, with both clear vesicles and dense core granules in their terminations, thus confirming its neuronal origin.[4]

Surgical resection is presently considered the ideal therapeutic option, with the best prognosis in terms of local control and survival. The role of adjuvant radiotherapy apparently seems to benefit patients with incomplete resection and in atypical neurocytoma. Most CNs are World Health Organization (WHO) grade II tumors associated with a favorable prognosis. However, occasional cases may display histologic features of anaplasia or malignancy without clinical evidence of poor outcome, which have been termed as ‘proliferating neurocytoma’.[4]

Conclusion

To conclude, CNs possess distinct cytomorphological features that aid in distinguishing them from other intra-ventricular tumors. However, definitive confirmation warrants an immunohistochemical workup, and correlation with the clinical-radiological and histomorphological profile of the case.