The patient went on to have a resection.

Histology

The sections show increased cellularity in the white matter. The glial cells show focal crowding. Some appear astrocytic with mildly enlarged, angulated and hyperchromatic nuclei. Other cells have rounder nuclei and perinuclear haloes. Mitoses are inconspicuous. No microvascular proliferation or necrosis is present. The overlying cerebral cortex shows preserved architecture. There is no evidence of malformation of cortical development.

• The glial cells are both GFAP and Nogo-A positive.
• Only occasional tumor cells are p53 positive.
• IDH1-R132H immunostain is positive (mutated).
• The topoisomerase index is 1%.
• MGMT is negative (likely methylated).
• ATRX shows no loss of staining (non-mutated).

1p/19q FISH. RPAH glioma FISH results:

• Chromosome 1
  • Mean copies 1p per cell: 1.25
  • Mean copies 1q per cell: 1.63
  • 1p/1q ratio: 0.77
• Chromosome 19
  • Mean copies 19q per cell: 1.2
  • Mean copies 19p per cell: 1.68
  • 19q/19p ratio: 0.71

Interpretation: 1p/19q co-deletion detected by FISH

FINAL DIAGNOSIS: Oligodendroglioma, IDH-mutant and 1p/19q codeleted (WHO Grade II).