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A rare diagnosis of an extraventricular neurocytoma

Background: Extraventricular neurocytoma (EVN) is an extremely rare neoplasm of the central nervous system. As reported, it arises in a variety of locations, but mainly within the cerebral hemispheres. Despite its histological similarity with central neurocytoma (CN), EVN occurs outside the ventricular system and, in 2007, was recognized by the World Health Organization as a separate entity. Case Description: A 39-year-old man, with a ventriculoperitoneal shunt inserted for communicating hydrocephalus, was admitted at our Unit of Neurosurgery with a 1-month history of gait disturbance, postural instability, speech disorders, and occasional incontinence. Computed tomography scan and magnetic resonance imaging showed a mixed-density neoplasm in the left frontotemporal area, with anterior cerebral falx shift, and perilesional edema. The patient underwent surgical procedure; microsurgical excision of the lesion was performed through left pterional approach. Histopathological and immunohistochemical examination revealed monomorphic round cells of the neuronal lineage, with a percentage of Ki-67 positive nuclei <5% and no evidence of mitosis or necrotic areas. According to radiologic features, this pattern was compatible with the diagnosis of EVN. Patient had a favorable recovery and he is still in follow-up. Conclusion: Because of their rarity, clinical, radiologic, and histopathological characteristics of EVNs are not yet well defined, as well as the optimal therapeutic management. Whereas EVNs are rarely described in literature, we aimed to share and discuss our experience along with a review of the published literature.


INTRODUCTION
Extraventricular neurocytoma (EVN) is an extremely rare neuronal tumor with just approximately 100 reported cases in literature; [16] it has been considered for a long time as a variant of the central neurocytoma (CN), a rare tumor in itself with an estimated incidence of 0.25-0.50% of all primary brain tumors. [8,17,22,29] In facts, neurocytic neoplasms generally arise within the lateral ventricles and they are occasionally located in the context of brain parenchyma, without any continuity to the ventricular system. Compared with CN, EVN shows similar histopathological characteristics but a wider spectrum of locations and morphological features; both CN and EVN usually presents an indolent behavior, even though, since in 1989 Ferreol et al. described the first case of EVN, a higher rate of recurrence was observed in the latter. [7] Only in 2007 EVN was introduced as a separate entity in the World Health Organization (WHO) classification of tumors of the central nervous system. [19] Apart from systematic reviews, published literature on EVNs consists of sporadic case reports and a few small case-series. In consideration of the low number of reports, we aim to share our experience and describe a case of a 39-year-old patient with a left frontotemporal EVN.

CASE DESCRIPTION
A 39-years-old man was admitted to the emergency room of our Institution for a 1-month history of gait disturbance, postural instability, speech disturbance, and episodes of incontinence. His history was remarkable for communicating hydrocephalus, which had been surgically treated at the age of 14 months with the placement of a ventriculoperitoneal shunt, followed by a distal shunt revision at the age of 16 years. At the admission, brain computed tomography (CT) scan was performed and it showed the presence of 5 cm mixed-density pseudonodular lesion. His neurological examination revealed temporospatial disorientation, absence of focal motor deficits and sensory alteration, normal and symmetrical osteotendinous reflexes, walk of little steps, and wide-based gait, several body oscillations during the Romberg's test. A contrast-enhanced brain CT study [ Figure 1] confirmed the presence of an enhancing 57 mm left frontotemporal non-homogeneous neoformation, with several calcifications, hypodense colliquated areas, and perilesional edema. Preoperative magnetic resonance imaging (MRI) with gadolinium contrast medium, even though strongly fouled by motion artifacts due to poorly cooperative patient, showed a lesion with lobulated margins, non-homogeneous post contrast enhancement and heterogeneous signal intensity in relation to the presence of solid component mixed with necrotic-colliquative areas and calcifications. In particular, in the peripheral frontal portion, a small oval-shaped component was visible, surrounded by strongly hypointense peripheral border in FFE sequences, arguably due to hemosiderin deposition. e tumor presented a maximum diameter of 6.2 × 4.1 cm on the axial plane [ Figure 2a], 6.3 × 3.5 cm on the sagittal plane [ Figure 2b], and 3.6 × 3.3 cm on the coronal plane [ Figure 2c], and it extended cranially to the knee of the corpus callosum, displacing the large falx to the right and causing a compressive effect on the frontal horn of the right lateral ventricle.
Surgical procedure was performed with the aid of optic neuronavigation, intraoperative monitoring and the resection was performed though a left pterional approach. [1,24] Intraoperatively, the neoplasm appeared of soft and partly fibrous consistency, of reddish-grey color and with a fairly good cleavage plane from the adjacent cerebral parenchyma. With the aid of ultrasonic aspirator and microsurgical technique, macroscopical total resection of the exposed neoplastic lesion was carried out. Autologous fibrin glue was used to seal the dural plane.
Fragments of the lesion were sent for pathological examination, which revealed a population of neuronal lineage (Sinaptophysin+, neuron specific enolase+, glial fibrillary acid protein−) consisting of roundish monomorphic cells mixed with a proliferation of capillary vessels and the presence of widespread calcifications; the Ki-67 index was <5%, there was no evidence of mitosis or areas of necrosis. Histopathological and immunohistochemical pattern, according to radiological findings, was consistent with a diagnosis of EVN, classified by the WHO as a Grade II tumor [ Figure 3a-c].
e postoperative course was uneventful and control CT scan [ Figure 4] showed no complications. A further MRI  with two distinct peaks of incidence in the second and fifth decades. [35] EVNs may occur in various site outside the ventricular system; the most common location reported is the frontal lobe, followed by the parietal, temporal, and occipital lobes. [2] Less frequently involved regions are sellar region, [13,16] thalamus, [21] cerebellum, [5,15,27] and spine. [12,31] EVNs have also been described outside the context of central nervous system, in particular inside the pelvis, [9] adrenal gland, [6] and ovary. [11] Clinical presentation of EVN is heterogeneous, non-specific and it varies depending on the location of the tumor and the mass effect exerted. [34] According to the literature, the main clinical manifestations comprehend headache, vomiting, seizures, and limb motor deficit. In addition, visual disturbance has been described as the first symptom of EVN of the sellar region. [16] Our patient complained, among other symptoms, urinary incontinence, which is an uncommon symptom that has been only once reported. [3] In our case, the radiologic features of patient's lesion are consistent with the previous reports. [26] In fact, EVNs usually show focal cystic components and calcification; the solid portions of EVNs are often described as isodense or slightly hyperdense on CT, isointense or hypointense on T1WI, and isointense or hyperintense on T2WI. e lesion, in our patient, showed heterogeneous enhancement, a feature frequently noted in EVNs and other low-grade tumors. It also presented perilesional edema, which is described from 10% to more than 80% of EVNs cases. [13,37] However, EVNs may exhibit a wealth of non-specific imaging features, leading to a difficult differential diagnosis mainly toward oligodendrogliomas and oligoastrocytomas with neurocytic differentiation, ganglioglioma, pilocytic astrocytoma, ependymoma, and dysembryoblastic neuroepithelial tumors. [32,36] As a consequence, the integration of radiological findings and histopathological examination is essential for definitive diagnosis.
Furthermore, CNs and EVNs exhibit a shared microscopic appearance. In facts, neurocytic neoplasms are mainly was performed [ Figure 5a-c], which showed on the left frontal area, in correspondence of the surgical cavity, a large area of structural alteration, consisting predominantly of cerebrospinal fluid and a tissue portion with calcifications.
After the hospital discharge patient underwent at home neurorehabilitation with consistent improvement of his gait and speech. He also underwent adjuvant radiotherapy and at the present his follow-up is still ongoing.

DISCUSSION
Although the exact incidence is still not precisely defined, in a systematic review the incidence of cerebral EVN has been estimated to be 0.13% of approximately 7000 cases of intracranial tumors. [10] In a larger case series, among 868 patients with neurocytoma 19.3% showed an extraventricular location. [18] According to the literature, EVN exhibits no predilection for any particular sex, sometimes showing a slight predominance in males, and it seems to be distributed uniformly across all age groups. [2,4,10,13] Recently, it has been shown that EVN has a bimodal age distribution,  composed of uniform roundish cells with neuronal differentiation, strongly immunoreactive for synaptophysin. EVN, however, has been described with a wider spectrum of proliferation rates and cellularity, showing tendency for ganglionic or glial differentiation; in 2001, Brat et al. proposed "atypical EVN, " a variant associated with a higher recurrence rate. [4] e atypical histological features identified included increased mitotic activity, high Ki-67 proliferation index (>2%), focal necrosis, and vascular proliferation. [30] In our patient, histopathological examination revealed low Ki-67 index without evidence of mitotic activity or necrosis, while proliferation of capillary vessels was observed.
Age and atypical histological features have been suggested as negative prognostic factors associated with poorer outcomes. [14,23] Progression from typical to atypical EVN [3] and to neuroblastoma [28] is reported in the literature, although EVN usually shows non-aggressive behavior and has been classified by the WHO as a Grade II tumor since its introduction. [19,20] e therapeutic management for EVNs is based on surgical removal, while radiotherapy or chemotherapy should be considered in cases of subtotal resection or as salvage for local recurrence after surgery. Compared with gross total resection, subtotal resection followed by radiotherapy may offer a reasonably good outcome with a similar overall survival. [14] e efficacy of postoperative chemotherapy is not clear and it is rarely employed; however, in two different case-reports EVN showed sensitivity to vincristine. [25,34] Nevertheless, Dutta et al. recently investigated different patterns of care in a large cohort of patients with diagnosed neurocytomas, founding that tumor location and use of radiation were not predictive for improved survival. [4] Considering the usually benign nature of EVNs and the risks associated with adjuvant therapies, these should be administered to a selected subgroup of patients; the presence of unfavorable histopathological features should guide postoperative treatment decisions.

CONCLUSION
Since they are rare tumors, clinical, radiologic, and histopathological characteristics of EVNs are not yet well defined, as well as the optimal therapeutic management. Whereas EVNs are rarely described in the literature, we described our singular case of extra-ventricular neurocytoma, with our surgical management and an exhaustive review of the published literature.

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Conflicts of interest
ere are no conflicts of interest.