A 35-year-old with a grade 2 glioma shows IDH1 R132H mutation and 1p/19q co-deletion. Per WHO 2021 CNS5 classification, this tumor is classified as:

• A IDH-mutant astrocytoma, CNS WHO grade 2
• B Glioblastoma, IDH-mutant grade 4
• C Diffuse midline glioma, H3 K27-altered
• D Oligodendroglioma, IDH-mutant and 1p/19q-codeleted, CNS WHO grade 2 ✓

Explanation

WHO 2021 CNS5 classification mandates that the combination of IDH mutation + 1p/19q co-deletion = oligodendroglioma by definition, regardless of histological appearance. The 1p/19q co-deletion results from an unbalanced translocation t(1;19)(q10;p10) and is always associated with IDH mutation and TERT promoter mutation (the 'triple-positive' signature). Astrocytomas are IDH-mutant but 1p/19q intact (and show ATRX loss and TP53 mutation instead). Glioblastoma requires H3 K27M for midline glioma or IDH-wildtype with specific molecular features for conventional GBM.

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

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Written and medically reviewed by the StethoPrep medical team.