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Rosette-Forming Glioneuronal Tumor: A Rare Posterior Fossa Tumor in an Old Adult
Rosette-forming glioneuronal tumor (RGNT) is an uncommon, slow growing low-grade (World Health Organization [WHO] grade I) glioneuronal type of primary brain tumor. It was included as a distinct glioneuronal neoplasm in the 2016 WHO classification of tumors of the central nervous system.  Initially, it was described as dysembryoplastic neuroepithelial tumor of cerebellum. RGNTs more often occur in young adults with mean age of 23.5 years with a female predominance.  Patient presents clinically with headache, which is the most common symptom and clinical manifestations of raised intracranial pressure or ataxia, depending on the location of the tumor. The primary treatment modality is surgery and gross total resection is almost curative. 
A 55-year-old male presented in our hospital with complaints of headache for the last 6 months. Magnetic resonance evaluation study revealed a solid cystic lesion with hyperintense cystic component and intermediate-hypointense solid component on T2 axial images with no evidence of perilesional edema, centered in posterior fossa, involving midline vermis without any significant mass effect on cerebellar structures. The lesion showed few foci of blooming on gradient sequences that appeared as bright foci on corresponding phase sequences, likely suggestive of calcification. On post-contrast sequences, the solid component in lesion showed enhancing nodular component.
Sagittal post-contrast sequences showed mild mass effect on the fourth ventricle with concomitant expansion and upstream dilatation of bilateral lateral and third ventricles suggestive of early onset hydrocephalus ([Fig. 1]).
Surgical intervention in the form of gross total resection was planned and the patient underwent suboccipital craniotomy with tumor decompression and external ventricular drain placement.
Intraoperative findings showed a solid cystic tumor, whitish in appearance arising from the fourth ventricle. Microscopically, the tumor was moderately cellular and had a biphasic neurocystic and glial architecture. The neurocystic component had ring-shaped neurocystic rosettes around eosinophilic neuropil cores. The neurocystic tumor cells had spherical nuclei with abundant cytoplasm in myxoid background. The glial component showed spindle to stellate shaped nuclei with dense chromatin in a fibrillary background and focal occurrence of Rosenthal fibers with hemosiderin deposits. No necrosis or calcification was seen. On immunohistochemistry (IHC), synaptophysin strongly highlighted center of rosettes. S-100 was positive in tumor cells. Glial fibrillary acidic protein was positive in glial component. Ki-67 index was 1% ([Fig. 2]). Diagnosis of RGNT (WHO grade I) was proposed based on these characteristic pathological features.
Article published online:
20 September 2021
© 2021. Indian Society of Medical and Paediatric Oncology. This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial-License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/).
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