|Year : 2012 | Volume
| Issue : 4 | Page : 644-646
A rare posterior cranial fossa tumor
Bevinahalli N Nandeesh1, Manmeet Singh Chabra2, Manjaly K Babu1, Ashish K Chand2
1 Department of Pathology, St. John's Medical College, Bangalore, Karnataka, India
2 Department of Neurosurgery, St. John's Medical College, Bangalore, Karnataka, India
|Date of Web Publication||29-Jan-2013|
Bevinahalli N Nandeesh
Department of Pathology, St. John's Medical College, Bangalore, Karnataka
Source of Support: None, Conflict of Interest: None
Among tumors of the central nervous system, tumors of the mixed glioneuronal type form an important recognized subset. Some of the examples for mixed glioneuronal tumors include gangliocytoma, dysembryoplastic neuroepithelial tumor (DNT), ganglioglioma, anaplastic ganglioglioma, and central neurocytoma. The rosette-forming glioneuronal tumor (RGNT) of the fourth ventricle is a new entity that has only slowly emerged in the literature due to its prior classification with other low-grade mixed glial and neuronal tumors. These tumors are relatively infrequent lesions, and therefore, they can be challenging to diagnose for the practicing pathologist. This is a rare biphasic tumor with clearly defined neurocytic and glial components. The tumor is found exclusively in the posterior fossa, where it arises in the midline, usually occupying a substantial fraction of the fourth ventricle, and it is observed by magnetic resonance imaging (MRI) as a circumscribed, solid mass with heterogeneous contrast enhancement. We describe here a case of RGNT occurring in a 22-year-old male.
Keywords: Glial fibrillary acidic protein, glioneuronal tumor, posterior fossa, synaptophysin
|How to cite this article:|
Nandeesh BN, Chabra MS, Babu MK, Chand AK. A rare posterior cranial fossa tumor. J Can Res Ther 2012;8:644-6
| > Introduction|| |
In recent years, numerous new entities or variants of recognized central nervous system tumors have been described in the literature, and mixed glial and neuronal tumors represents an important category into which the World Health Organization (WHO) has recently added three new entities.  One of this recently described novel rare mixed glioneuronal tumor of the posterior fossa is the rosette-forming glioneuronal tumor (RGNT) which is described in this case report.
| > Case Summary|| |
A 22-year-old man presented to the neurosurgical department with a history of headache since four months. This was associated with dizziness, ataxia, and vomiting. The past and family history was not significant. His general physical examination and systemic examination were not contributory. Magnetic resonance imaging (MRI) of the brain showed a predominantly solid cerebellar-based tumor of 2.8 cm diameter with ill-defined borders and appeared bulging into the fourth ventricle. The solid component appeared isointense on T1 sequences, hyperintense in the T2 mode, and enhanced moderately [Figure 1]a and b. The combined clinical and radiological differentials were pilocytic astrocytoma or ependymoma. The patient underwent gross total resection of the tumor via suboccipital craniotomy. Frozen section suggested a diagnosis of pilocytic astrocytoma.
|Figure 1: (a,b) MRI images of brain (transverse and sagittal view) showing predominantly solid and cystic, cerebellar-based tumor (T) with ill-defined borders, bulging into the fourth ventricle, (c) microphotograph(H&E × 100) showing a biphasic pattern with glial component (G) and neurocytic (N) component|
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On studying the paraffin sections, a relatively moderate cellular neoplasm was observed infiltrating the cerebellar parenchyma and it included two components [Figure 1]c. The first was a cytologically bland and generally paucicellular astroglial proliferation with spindle, stellate, and piloid cells resembling pilocytic astrocytoma [Figure 2]a. The glial fibrillary acidic protein (GFAP) immunostain stained this glial component [Figure 2]c, inset. Eosinophilic granular bodies, Rosenthal fibers, and hyalinized blood vessels were also observed. The second component, in which cells with small, round, regular nuclei and speckled chromatin form perivascular pseudorosettes and miniature neurocytic rosettes were observed, represented the neuronal component [Figure 2]b, which was further highlighted by synaptophysin immunostain [Figure 2]c. In addition to this, areas of hemorrhage and delicate vasculature were observed. No mitotic activity or necrosis was noted.
|Figure 2: (a) Microphotograph (H&E × 400) showing glial component (G) with Rosenthal fibers, (b) microphotograph (H&E × 200) showing neurocytic (N) component with Homer Wright and perivascular rosettes; (c) microphotograph showing neurocytic (N) component with positive immunohistochemical staining for synaptophysin (X 200) and negative staining by glial component (G). Inset: microphotograph showing positive staining for GFAP (inset X100)|
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Follow-up of the patient with MRI and a computed tomography (CT) scan up to 11 months after the surgical treatment has not shown any evidence of tumor recurrence.
| > Discussion|| |
The category of mixed glial and neuronal tumors represents a vast and largely uncharted territory which is ripe for exploration. Three new entities have been recently added to the group of glioneuronal tumors in the most recent update of the WHO classification of tumors of the central nervous system: Papillary glioneuronal tumor (PGNT), RGNT with neuropil-like islands, and RGNT of the fourth ventricle.  These tumors are relatively infrequent lesions, and therefore, they can be challenging to diagnose for the practicing pathologist. They probably arise from a common progenitor in the subependymal plate.  The RGNT of the fourth ventricle is a new entity that has only slowly emerged in the literature due to its prior classification with other low-grade mixed glial and neuronal tumors. Following their initial report in 1998, Komori et al. published a detailed description of this tumor in 2002. , To date, less than 20 cases of RGNT have been reported in the world literature. 
Patients have mostly been young adults, and ages have ranged from 12 to 59 years. Presenting symptoms have varied in duration from days to 13 years and are typical of an intraventricular posterior fossa tumor, with headache and ataxia reported most often.  Also, patients may not suffer from any neurological symptoms and these may be detected incidentally. On imaging, they often appear as midline masses occupying the fourth ventricle or mainly involving cerebellum (especially vermis) with little mass effect or surrounding edema. The tumors are relatively circumscribed and often partially cystic by MRI, with the majority showing heterogeneous contrast enhancement. The unusual feature in the present case is the presence of intratumoral hemorrhage which made the radiologists consider ependymoma. There is often involvement of surrounding periventricular and periaqueductal tissue such as the dorsal pons, midbrain, thalamus, or pineal region and, in addition, multinodularity or multicentricity may be observed. These tumors are biphasic, with clearly defined and spatially separate neurocytic and glial components. The neurocytic component contains a homogeneous population of small, clear cells that are arranged in either a rosetted pattern with large central spaces or papillae around central vessels. A microcystic component with a blue mucinous extracellular matrix may be present as well. Delicate processes form a neuropil matrix adjacent to the tumor cells and extending to central vessels. The glial component of this tumor is solid and composed of highly fibrillated cells that resemble those of pilocytic astrocytoma.
The most hypocellular examples of this glial component may suggest reactive piloid gliosis, but other cases show a level of cellularity and architectural patterns that are incompatible with a reactive process. Eosinophilic granular bodies, Rosenthal fibers, hyalinized blood vessels, and microcystic change may further foster a false impression of pilocytic astrocytoma for the unwary pathologist. The clue to correct diagnosis lies in the second component, in which cells with small, round, regular nuclei and speckled chromatin form perivascular pseudorosettes and miniature neurocytic rosettes. 
Ultrastructurally, neuronal cells feature microtubule-containing processes and aberrant synaptic terminals, but dense core granules are rare.  For the histopathological differential diagnosis of RGNT, other glioneuronal tumors have to be considered. Columnar arrangements of neurocytic tumor cells are reminiscent of the 'glioneuronal element' seen in dysembryoplastic neuroepithelial tumor (DNT). However, glioneuronal elements of DNT have arrangements of tumor cells along bundles of axons, while tumor cells in RGNT arrange perivascularly. In addition, lack of 'floating neurons' and infratentorial localization discern RGNT from DNT. PGNT is another tumor type showing histopathological features similar to RGNT. RGNT lacks histopathological signs of malignancy. Tumor cell proliferation indices as assessed by MIB-1 labeling of Ki-67 antigen are low.
In conclusion, the spectrum of presenting symptoms of RGNT is wide, nonspecific, and typically depends on the size and extent of tumor. This tumor entity should be considered in the differential diagnosis of posterior fossa masses to avoid undue surgical aggressiveness. The numerous new entities or variants of recognized central nervous system tumors described in the literature in recent years have an incompletely delineated morphologic spectrum. The present case contributes in understanding the characteristics of a rare posterior fossa tumor. Also, this report stresses that accurate diagnosis and classification of these lesions is important to ensure that patients receive adequate therapy and prognostic information.
| > Acknowledgment|| |
The authors would like to acknowledge Dr. Pritilata Rout, MD (Pathology), Professor of Pathology, Dr. Namita Sinha, MD (Radiology), Associate Professor* Department of Radiology*, St. John's Medical College, Bangalore, India for their valuable guidance and diagnostic support.
| > References|| |
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[Figure 1], [Figure 2]