A 35-year-old presents with seizures and a cortical lesion. MRI shows a well-circumscribed T2-hyperintense mass with no gadolinium enhancement. Biopsy shows moderate cellularity with atypical glial cells, rare mitoses, no necrosis, and no microvascular proliferation. Molecular testing reveals IDH1 R132H mutation and 1p/19q co-deletion. According to WHO 2021 CNS classification, this tumor is classified as:
- A Astrocytoma, IDH-mutant, CNS WHO grade 2
- B Glioblastoma, IDH-mutant, CNS WHO grade 4
- C Diffuse glioma, NOS, CNS WHO grade 2
- D Oligodendroglioma, IDH-mutant and 1p/19q co-deleted, CNS WHO grade 2 ✓
Explanation
In the WHO 2021 (5th edition) CNS tumor classification, oligodendroglioma is defined by the combination of IDH mutation AND 1p/19q co-deletion; this molecular definition supersedes histological appearance. The 1p/19q co-deletion results from a translocation mechanism (t(1;19)(q10;p10)) and is the molecular hallmark of oligodendroglial lineage. IDH mutation without 1p/19q co-deletion would classify as astrocytoma, IDH-mutant. The grade (2, 3) depends on histological features: grade 2 has no significant mitotic activity, no necrosis, and no microvascular proliferation. This represents a significant departure from 2016 WHO criteria by making molecular classification definitive.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.