Rosette-forming glioneuronal tumour of the fourth ventricle arising in the cerebellar hemisphere: diagnostic challenges including cytological features in intra-operative squash preparations

Rosette-forming glioneuronal tumour (RGNT) is an entity newly codified in the 2007 WHO classification of tumours of the central nervous system. RGNTs are rare and typically arise in the mid-line in the region of the fourth ventricle. Origin at other sites such as cerebellar hemisphere, spinal cord and supratentorial sites has been described. The tumour consists of distinct neurocytic and astrocytic components, the former featuring distinctive neuro-cytic rosettes and perivascular pseudorosettes, and the latter resembling pilocytic astrocytoma. An atypical site of presentation and a predominance of either the neurocytic or astrocytic components at intra-operative consultation present diagnostic challenges. The distinctive appearance of the tumour in intra-operative squash preparations is a clue to the diagnosis in this setting. We describe the case of a RGNT which arose in the left cerebellar hemisphere. The intra-operative squash preparations showed the neurocytic rosettes and pseudorosettes within a loose fibrillary and mucinous background. The biphasic nature of the lesion was manifest in the frozen section, but more apparent in subsequent tissue sections. The differential diagnoses of RGNT with oligo-dendroglioma, pilocytic astrocytoma, central and extraventricular neurocytoma and dysembryoplastic neuroepithelial tumour are discussed.