A 35-year-old woman undergoes brain MRI for new-onset seizures. A 3 cm frontal lobe mass is found with T2/FLAIR hyperintensity, minimal enhancement, and no significant mass effect. Biopsy shows a diffusely infiltrating glioma with IDH1 R132H mutation, intact ATRX, and 1p/19q codeletion. Per WHO 2021 CNS classification, this tumour is best classified as:
- A Astrocytoma, IDH-mutant, CNS WHO grade 2 or 3
- B Glioblastoma, IDH-mutant (secondary GBM)
- C Diffuse midline glioma, H3 K27-altered
- D Oligodendroglioma, IDH-mutant and 1p/19q-codeleted, CNS WHO grade 2 or 3 ✓
Explanation
Per the 2021 WHO Classification of Tumors of the Central Nervous System (5th edition), IDH-mutant gliomas are classified based on the presence or absence of 1p/19q codeletion (and TERT promoter mutation). IDH mutation + 1p/19q codeletion = Oligodendroglioma, IDH-mutant and 1p/19q-codeleted (WHO grade 2 if low mitotic activity, grade 3 if mitotic activity or necrosis/microvascular proliferation). IDH mutation + intact 1p/19q = Astrocytoma, IDH-mutant. ATRX loss occurs in IDH-mutant astrocytomas (not oligodendrogliomas). The 1p/19q codeletion in this case, combined with IDH1 mutation and intact ATRX (typical of oligodendrogliomas where ATRX is preserved), classifies this as oligodendroglioma. IDH-mutant GBM (secondary GBM) shows necrosis and microvascular proliferation on histology. Diffuse midline glioma is H3 K27-mutated in the pons/thalamus, not IDH-mutated.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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