10260-RC-5 A case of a large cystic tumor in the right frontal lobe with histological diversity including rosette-forming glioneuronal tumor

Abstract Introduction Rosette-forming glioneuronal tumor (RGNT) is characterized by astrocytic components and neurocytic cells forming rosette-like structures. While there have been recent reports of RGNTs arising in supratentorial regions, large tumors with contrast enhancement and hemorrhage remain rare. We report a case of a large cystic tumor in the right frontal lobe initially diagnosed as pilocytic astrocytoma (PA), which later recurred and was suspected to be RGNT. Case Presentation A 12-year-old girl presented with symptoms of increased intracranial pressure. MRI revealed a 6 cm cystic tumor with contrast enhancement and intratumoral hemorrhage in the right frontal lobe. She underwent biopsy, drainage, and subsequent gross total resection. However, local recurrence occurred five years later, and a second surgery was performed at the age of 16. Pathological Findings The initial specimen was diagnosed as PA, showing BRAF V600E wild-type, IDH1/2 wild-type, and TERT wild-type. The recurrent tumor showed neurocytic cells with medium-sized round nuclei forming rosettes and perivascular pseudorosettes, positive for synaptophysin and showing EGFR amplification, suggestive of RGNT. However, histology varied by resection site and included features of oligo-like cells (OLC), PA, dysembryoplastic neuroepithelial tumor (DNT), ganglioglioma (GG), and RGNT. DNA methylation profiling of the recurrent tumor indicated a DNT methylation class. Discussion It is rare for RGNT, GG, or DNT to present as large hemorrhagic tumors with acute symptoms. The initial specimen showed a single histological pattern without DNT features, whereas the recurrent specimen demonstrated heterogeneous histology depending on the tumor region. Whether this represents a single tumor entity or a collision tumor is currently under investigation with region-specific methylation analysis. Conclusion We report a rare case of a large cystic tumor with recurrent components suggestive of RGNT, PA, DNT, and GG. Further accumulation of similar cases and detailed pathological analyses are needed.