Clinicopathological analysis of unusual rosette-forming glioneuronal tumor in brain parenchyma

Abstract

Background Rosette-forming glioneuronal tumor (RGNT) is a rare and novel brain tumor. It affects mainly young adults and arises in the midline, primarily involving the cerebellum, and the walls or floor of the fourth ventricle. The tumor is composed of distinctive histological components, uniform neurocytes forming rosettes and (or) perivascular pseudorosettes, as well as astrocytic component resembling pilocytic astrocytoma. To our best knowledge, no more than 50 cases of RGNT have been described in the literatures to date and found commonly in association with the ventricular system. Only a few cases have been known to occur at sites outside of its usual location. Herein, we present a rare case of RGNT of brain parenchyma. Due to its rarity and non-specific appearance in radiological examination, it is a diagnostic challenge for radiologists and histopathologists to differentiate RGNT in unusual sites from other intracranial lesions because of its similarities in radiological and histological findings. The aim of this study is to summarize the clinicopathological features of RGNT and discuss the differential diagnosis of histologically similar tumors in brain. Methods The clinical manifestation of a patient with RGNT occurring in left frontal lobe was presented retrospectively. Resected mass was routinely paraffin-embedded and stained with Hematoxylin and Eosin. Dako EnVision immunohistochemical staining system was used to detect the tumor antigen expressions, including glial fibrillary acidic protein (GFAP), S-100 protein (S-100), cytokeratin (CK), neuronal nuclear antigen (NeuN), synaptophysin (Syn), neuron-specific enolase (NSE), chromogranin A (CgA), oligodendrocytes transcription factor-2 (Olig-2), epithelial membrane antigen (EMA) and Ki-67 (MIB-1). Results A 12-year-old girl presented with 2-year history of twitches and mild headache. MRI revealed a solid well-circumscribed lesion in left frontal lobe with mild heterogeneous enhancement. The lesion was observed to locate in brain parenchyma and there was no evidence of tumor infiltrating in ventricular system. Craniotomy was performed and the tumor was removed totally. Histological examination revealed that the tumor was distinctive in its juxtaposition of patterned neurocytic and pilocytic astroglial components. The neurocytic component showed the tumor cells had small, uniform round nuclei with scant cytoplasm and formed narrow perivascular pseudorosettes or Homer-Wright-like rosettes arrays of neurocytic nuclei around delicate eosinophilic neuropil cores. The glial component tended to exhibit pilocytic astrocytoma-like morphology with long, hair-like processes and Rosenthal fibers. Immunohistochemical staining showed that the tumor cells in glial component were diffusely positive for GFAP and S-100, but negative for NeuN, Syn and NSE. However, perivascular pseudorosettes or Homer-Wright-like rosettes were positive for Syn and Olig-2, and negative for GFAP. Ki-67 index was low and less than 1%. Based on clinical presentation and histological findings, a final histological diagnosis of RGNT in brain parenchyma, WHO grade Ⅰ, was made according to the criteria of WHO classification. The patient has not received chemotherapy and attended follow-up for 12 months, without any neurological deficit or signs of recurrence. Conclusions RGNT is a rare tumor and classified as mixed neuronal-glial tumor. RGNT probably derives from a common progenitor cell originated from subependymal plate or brain parenchyma, able to differentiate toward both glial and neuronal phenotype. RGNT in brain parenchyma is also observed to have the similar biological behaviors and histopathological characteristics with its intra-ventricular counterpart. With similarities in histological findings, it may be difficult to differentiate RGNT from extraventricular neurocytoma, dysembry oplastic neuroepithelial tumor (DNT), and ependymoma with neuronal differentiation or neuropil-like islands. The presence of distinct low-grade glial component in the tumor and appropriate immunohistochemical profile are necessary for correct diagnosis. doi: 10.3969/j.issn.1672-6731.2014.03.014