The patient underwent resection of the right frontal lesion.

Microscopy: The sections show cerebral cortex and white matter. Cortical laminar architecture is preserved with no features of cortical dysplasia seen. There is prominent perineuronal secondary structuring. White matter contains a population of atypical glial cells with oligodendroglial morphological features - enlarged round and oval hyperchromatic nuclei, some with perinuclear clearing and a paucity of processes. Several groups of overlapping nuclei are noted. No mitotic figures are identified. There is no microvascular proliferation and no necrosis is seen. A population of reactive astrocytes is also present.

Immunohistochemistry:

• GFAP negative in atypical oligodendroglial cells
• positive in reactive astrocytes
• NogoA positive in atypical oligodendroglial cells
• IDH-1 R132H positive (mutated)
• ATRX positive (not mutated)
• p53 negative
• MGMT positive (likely unmethylated)
• p16 CDKN2A positive

Topoisomerase labeling index: Approximately 1%.

The features are of IDH-1 mutated diffuse glioma (WHO II) strongly favoring oligodendroglioma.

Diagnosis: Right frontal tumor: IDH-1 mutated diffuse glioma (WHO II) with features strongly favoring oligodendroglioma. FISH for chromosome 1p/19q deletion is recommended.

Supplementary report: FISH for chromosome 1p/19q deletion. 1p36 NO DELETION DETECTED 19q13.3 NO DELETION DETECTED Number of cells scores 200 Number of sites scored 10 1p36/1q25 ratio 1.00 19q13.3/19p13 ratio 1.00

REFERENCE CRITERIA: 1p36/1q25 ratio <0.8 Deletion detected 19q13.3/19p13 ratio <0.8 Deletion detected

Therefore, the right frontal lesion represents a diffuse astrocytoma.