## Oligodendroglioma: Molecular and Histological Identity **Key Point:** Oligodendroglioma is uniquely defined by the combination of IDH mutation (IDH1 or IDH2) AND 1p/19q codeletion. This genetic signature is pathognomonic and is now part of the WHO 2021 classification. **High-Yield:** The 1p/19q codeletion is the single most important genetic marker for oligodendroglioma and is associated with better chemotherapy responsiveness (temozolomide and procarbazine-CCNU-vincristine regimen). ### Molecular Signatures of Diffuse Gliomas | Tumor Type | IDH Status | 1p/19q Codeletion | TP53 Mutation | CIMPG | Prognosis | | --- | --- | --- | --- | --- | --- | | Oligodendroglioma | Mutant | **Present** | Rare (~5%) | Yes | Better | | Diffuse Astrocytoma | Mutant | Absent | Common (~50–70%) | Yes | Intermediate | | Anaplastic Oligodendroglioma | Mutant | **Present** | Rare | Yes | Better than anaplastic astro | | Glioblastoma (IDH-wt) | Wild-type | Absent | Variable | No | Poor | **Clinical Pearl:** Oligodendrogliomas with 1p/19q codeletion show significantly improved overall survival (median 10–15 years for Grade II) compared to astrocytomas, and they respond well to combined chemotherapy (PCV regimen). **Mnemonic:** **OLIGO-19q** = Oligodendroglioma has 1p/19q codeletion. Remember: astrocytomas lack this codeletion; glioblastomas are usually IDH wild-type. [cite:Robbins 10e Ch 28] 
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