Image 3.
Case 4. A 36-year-old man had a 2 × 2-cm nonenhancing mass in the left frontal lobe (radiology not shown). Histologic features of the initial biopsy (A) suggested a low-grade glioneuronal tumor with disordered ganglion cells, such as a dysembryoplastic neuroepithelial tumor or ganglioglioma. Four years later the tumor recurred, this time showing unequivocal grade II oligodendroglioma round cell morphology (B). Immunohistochemical staining for R132H IDH1 mutation was strongly positive in the tumor cells (D). Retrospective analysis of the original tumor also showed immunopositive cells (C), many of which were wrapping around and physically distorting immunonegative neurons (C, inset).