A 35-year-old patient with a frontal lobe tumour undergoes resection. Histology shows infiltrating glioma with moderate cellularity, nuclear atypia, no necrosis, and no microvascular proliferation. IDH1 R132H IHC is positive. FISH shows co-deletion of 1p/19q. MGMT promoter is methylated. According to the WHO 2021 CNS classification, what is the diagnosis and grade?
- A Astrocytoma, IDH-mutant, ATRX-retained, CNS WHO Grade 2
- B Oligodendroglioma, IDH-mutant and 1p/19q-codeleted, CNS WHO Grade 2 ✓
- C Glioblastoma, IDH-wild type, CNS WHO Grade 4
- D Diffuse midline glioma, H3K27-altered, CNS WHO Grade 4
Explanation
The WHO 2021 CNS tumor classification integrates molecular parameters into diagnosis. The combination of IDH mutation (R132H) plus 1p/19q codeletion is the defining molecular signature of oligodendroglioma regardless of histological appearance. Without necrosis or microvascular proliferation, this is Grade 2 oligodendroglioma. If mitoses are brisk or necrosis/MVP is present, it would be Grade 3. Astrocytoma, IDH-mutant would show ATRX loss and no 1p/19q codeletion. GBM, IDH-wild type requires TERT promoter mutation, EGFR amplification, or +7/-10 signature. MGMT methylation predicts response to temozolomide chemotherapy and is prognostically favourable.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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