A 45-year-old man presents with headaches and personality changes over 3 months. MRI brain shows a right frontal mass with minimal enhancement, T2 hyperintensity, no midline shift. Stereotactic biopsy reveals IDH1-mutant, 1p/19q co-deleted oligodendroglioma. According to WHO 2021 classification, this is:
- A Oligodendroglioma, IDH-mutant, 1p/19q-codeleted — WHO Grade 3
- B Oligodendroglioma, IDH-mutant, 1p/19q-codeleted — WHO Grade 2 ✓
- C Astrocytoma, IDH-mutant — WHO Grade 2
- D Classification requires EGFR amplification status
Correct answer: B. Oligodendroglioma, IDH-mutant, 1p/19q-codeleted — WHO Grade 2
Explanation
WHO 2021 CNS tumor classification requires both IDH mutation AND 1p/19q co-deletion to define oligodendroglioma as a distinct entity. Without mitoses, necrosis, or microvascular proliferation, this is Grade 2. Grade 3 oligodendroglioma (anaplastic) requires high mitotic index (≥6/10 HPF), necrosis, or microvascular proliferation. Astrocytoma requires IDH mutation but intact 1p/19q (ATRX and TP53 mutations instead). The molecular-first paradigm of WHO 2021 means histology alone is insufficient — IDH and 1p/19q co-deletion must be confirmed.
Reference: Bailey & Love's Short Practice of Surgery, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.