A 45-year-old man presents with seizures. MRI shows a 4 cm frontal lobe lesion with no ring enhancement, minimal edema, and calcification. The WHO 2021 CNS classification assigns this to the oligodendroglioma category. Which COMBINED molecular requirement defines oligodendroglioma in the 2021 WHO CNS classification?

• A IDH mutation alone + ATRX loss on IHC
• B IDH mutation (IDH1/2) PLUS 1p/19q co-deletion — both required for oligodendroglioma diagnosis ✓
• C TERT promoter mutation + EGFR amplification — defining glioblastoma, IDH-wildtype
• D H3 K27M mutation on immunohistochemistry — defining diffuse midline glioma

Explanation

The 2021 WHO CNS5 classification mandates both IDH mutation AND 1p/19q codeletion for the diagnosis of oligodendroglioma, CNS WHO grade 2 or 3. This co-deletion results from a centromeric translocation (der(1;19)) and is a defining feature — its presence reclassifies IDH-mutant gliomas from astrocytoma to oligodendroglioma and confers better prognosis and alkylating agent sensitivity. IDH mutation alone with ATRX loss and no 1p/19q co-deletion = astrocytoma, IDH-mutant. H3 K27M defines diffuse midline glioma. TERT promoter mutation + EGFR amplification + Chr7 gain/Chr10 loss supports glioblastoma, IDH-wildtype.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.