Overview
- Oligodendroglioma, IDH-mutant and 1p/19q-codeleted, is a diffuse infiltrating glioma molecularly defined by the concurrent presence of an IDH1 or IDH2 mutation and whole-arm codeletion of chromosomes 1p and 19q.
- Under the WHO 2022 (5th edition) classification of CNS tumours, this molecular signature is obligatory for diagnosis, without both alterations, the tumour cannot be classified as an oligodendroglioma regardless of its histological appearance.
- The tumour is graded as either grade 2 or grade 3 based on histological features.
- Among all diffuse gliomas, oligodendroglioma carries the most favourable prognosis; median overall survival exceeds 15-20 years for grade 2 and 10-15 years for grade 3 lesions.
Classification
- Oligodendrogliomas arise predominantly in adults, with peak incidence in the fourth and fifth decades.
- They constitute approximately 5-15% of all gliomas and show a predilection for the cerebral hemispheres, particularly the frontal and temporal white matter.
WHO 2022 Grading
| WHO Grade | Former Terminology | Key Criteria |
|---|---|---|
| Grade 2 | Oligodendroglioma | IDH-mutant + 1p/19q codeletion; low mitotic activity; no anaplastic features |
| Grade 3 | Anaplastic oligodendroglioma | IDH-mutant + 1p/19q codeletion; increased mitoses, high cellularity, nuclear anaplasia ± necrosis |
The diagnosis of mixed oligoastrocytoma is now virtually obsolete; with molecular testing, nearly all such lesions are reclassifiable as either oligodendroglioma (1p/19q codeletion) or astrocytoma (ATRX/TP53 mutation, intact 1p/19q).
Molecular Pathogenesis
| Molecular Event | Significance |
|---|---|
| IDH1 R132H (or less common IDH2 R172 mutations) | Initiating driver mutation; produces oncometabolite 2-hydroxyglutarate; present in virtually all oligodendrogliomas |
| Whole-arm 1p/19q codeletion | Defining alteration; results from unbalanced translocation t(1;19)(q10;p10); co-occurs obligatorily with IDH mutation |
| TERT promoter mutation (C228T or C250T) | Present in the vast majority of oligodendrogliomas; shared with IDH-wildtype glioblastoma but in a distinct molecular context |
| CDKN2A deletion (9p loss) | Associated with progression to grade 3 |
| EGFR amplification | Absent in oligodendroglioma; presence argues strongly against this diagnosis |
Macroscopic Pathology
Oligodendrogliomas are typically gelatinous, grey, poorly circumscribed masses within the cerebral hemispheres. Characteristic features include:
- Cortical infiltration with blurring of the grey-white junction
- Dystrophic calcification in up to 90% of cases, ranging from microscopic foci to gross gritty depositions
- Focal haemorrhage and small cysts in some cases
- Grade 2 tumours: non-enhancing on MRI
- Grade 3 tumours: may show focal contrast enhancement; occasional necrosis
Microscopic Pathology
Characteristic Architectural Patterns
1. Perinuclear Haloes ("Fried-Egg" Appearance)
- Tumour cells possess round, centrally placed nuclei with finely granular chromatin surrounded by a clear halo of vacuolated cytoplasm.
- This appearance is an artefact of formalin fixation and is most prominent in formalin-fixed paraffin-embedded material.
- It is unreliable in frozen sections or intraoperative smear preparations.
2. "Chicken-Wire" Vasculature
- A delicate anastomosing network of thin-walled capillaries courses between tumour cell islands, producing a pattern resembling chicken wire.
- This is architecturally distinct from the glomeruloid microvascular proliferation of high-grade astrocytomas and glioblastoma.
Grading Features
| Feature | Grade 2 | Grade 3 (Anaplastic) |
|---|---|---|
| Cellularity | Moderate | High |
| Nuclear atypia | Mild to moderate | Significant |
| Mitotic activity | Low | Readily identified, often brisk |
| Necrosis | Absent | May be present |
| Microvascular proliferation | Absent | May be present |
| Calcification | Common (up to 90%) | Common |
| Perineuronal satellitosis | May be present in cortex | May be present |
- Grade 3 is assigned when there is increased mitotic activity combined with high cellularity and nuclear anaplasia; necrosis, when present, also supports grade 3.
- There is no single universally agreed mitotic threshold, but the overall morphological picture determines grading.
- Grade 3 cases may arise de novo but more often progress from grade 2 tumours.
Ancillary Testing
Immunohistochemistry
| Marker | Expected Result | Comment |
|---|---|---|
| IDH1 R132H (clone H09) | Positive (cytoplasmic) in ~90% | Negative result requires IDH1/IDH2 sequencing |
| GFAP | Variable, often focally positive | Less robust than in astrocytomas |
| Olig2 | Positive (nuclear) | Present in most gliomas; not specific |
| p53 | Negative or scattered | Strong diffuse positivity favours astrocytoma |
| ATRX | Retained (normal nuclear expression) | Loss strongly favours IDH-mutant astrocytoma |
| Ki-67 (MIB-1) | Low in grade 2; elevated in grade 3 | Supports grading; not a standalone criterion |
$$\text{Ki-67 index} = \frac{\text{Ki-67 positive nuclei}}{\text{total nuclei counted}} \times 100\%$$
Molecular / Cytogenetic Testing
| Test | Method | Expected Finding |
|---|---|---|
| 1p/19q codeletion | FISH, array CGH, or NGS | Whole-arm loss of 1p and 19q (not focal/partial) |
| IDH1/IDH2 mutation | IHC then sequencing if negative | R132 (IDH1) or R172 (IDH2) hotspot mutations |
| TERT promoter mutation | Sequencing | C228T or C250T present in majority |
| ATRX status | IHC ± sequencing | Retained in oligodendroglioma |
| TP53 mutation | Sequencing | Usually wildtype |
| MGMT promoter methylation | Methylation-specific PCR or pyrosequencing | Commonly methylated; predictive of alkylator chemosensitivity |
- Technical note: 1p/19q codeletion requires demonstration of whole-arm loss of both 1p and 19q simultaneously.
- Isolated or focal (interstitial) deletion of one arm does not satisfy the diagnostic criterion.
Clinical Significance of 1p/19q Codeletion
Diagnostic Role
The 1p/19q codeletion is the defining molecular alteration distinguishing oligodendroglioma from IDH-mutant astrocytoma:
| Feature | Oligodendroglioma | IDH-mutant Astrocytoma |
|---|---|---|
| 1p/19q codeletion | Present (whole-arm) | Absent |
| ATRX | Retained | Lost |
| TP53 | Usually wildtype | Usually mutated |
| IDH | Mutated | Mutated |
| TERT promoter | Mutated | Usually wildtype (except grade 4) |
This discrimination is clinically critical because oligodendrogliomas have markedly superior prognosis and distinct chemosensitivity compared to IDH-mutant astrocytomas.
Prognostic Role
1p/19q codeletion confers significantly improved survival compared to 1p/19q-intact gliomas of equivalent histological grade:
| Tumour Type | Approximate Median Overall Survival |
|---|---|
| Oligodendroglioma, grade 2 | 15-20 years |
| Oligodendroglioma, grade 3 (anaplastic) | 10-15 years |
| IDH-mutant astrocytoma, grade 2-3 | 5-10 years |
| IDH-mutant astrocytoma, grade 4 | 2-4 years |
| IDH-wildtype glioblastoma | ~6-15 months |
Progression from grade 2 to grade 3 typically occurs over 5 or more years.
Predictive Role, Chemosensitivity
- 1p/19q codeletion is the strongest predictive biomarker for response to PCV chemotherapy (procarbazine, lomustine/CCNU, vincristine).
- Landmark phase III trials (EORTC 26951 and RTOG 9402) demonstrated that 1p/19q-codeleted anaplastic oligodendrogliomas treated with radiotherapy plus PCV had dramatically superior outcomes compared to radiotherapy alone, with benefits persisting at 10+ years of follow-up.
- Temozolomide is also used clinically (particularly for grade 2 oligodendrogliomas and in some grade 3 settings), but the randomised evidence base for PCV in codeleted anaplastic oligodendroglioma is more robust.
- For grade 2 tumours in patients with residual disease or aged >40 years, current practice supports surgery followed by radiotherapy and chemotherapy.
The biological basis for chemosensitivity in codeleted tumours is incompletely understood but may relate to impaired DNA damage repair pathways associated with loss of 1p/19q gene products.
Differential Diagnosis
| Diagnosis | Key Distinguishing Features |
|---|---|
| IDH-mutant astrocytoma | ATRX loss, TP53 mutation, intact 1p/19q; elongated or irregular nuclei; fibrillary background |
| Clear cell ependymoma | Perivascular pseudorosettes; EMA positive; no 1p/19q codeletion; RELA or YAP1 fusions |
| Central neurocytoma | Intraventricular location; Syn+, GFAP−; no IDH mutation; bland round nuclei with neuropil islands |
| Dysembryoplastic neuroepithelial tumour (DNET) | Cortical location; specific glioneuronal element; no IDH mutation; benign behaviour |
| Pilocytic astrocytoma | KIAA1549-BRAF fusion; biphasic architecture; Rosenthal fibres; WHO grade 1; no IDH mutation |
| Metastatic clear cell carcinoma | Cytokeratin and EMA positive; clinical history; no IDH mutation or 1p/19q codeletion |
- The morphological features of perinuclear haloes and chicken-wire vasculature, while suggestive, are not pathognomonic, molecular confirmation is mandatory.
- Clear cell ependymoma in particular can closely mimic oligodendroglioma histologically.
Diagnostic Pitfalls
-
Frozen section / intraoperative smear: Perinuclear haloes are a formalin fixation artefact and do not appear reliably in frozen sections or smear preparations, do not exclude oligodendroglioma intraoperatively on this basis.
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Morphology-only diagnosis: Classic oligodendroglioma histology does not guarantee 1p/19q codeletion. A tumour with round nuclei and chicken-wire vasculature but ATRX loss and TP53 mutation should be classified as IDH-mutant astrocytoma pending full molecular workup.
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Partial 1p or 19q loss: Focal (interstitial) deletions detected by FISH must not be equated with whole-arm codeletion. Whole-arm loss must be confirmed, ideally with complementary methodology (array CGH or NGS) if FISH is equivocal.
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IDH IHC false negatives: The H09 antibody detects only IDH1 R132H. A negative immunostain does not exclude IDH mutation, less common IDH1 variants and all IDH2 mutations require sequencing.
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Grade assessment in small biopsies: Tumour heterogeneity may cause grading to be unreliable in stereotactic needle biopsies. The highest-grade area present should determine the overall grade assigned.
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TERT mutation as a standalone criterion: TERT promoter mutations are present in the majority of oligodendrogliomas but also in IDH-wildtype glioblastoma. TERT mutation alone cannot establish the diagnosis, IDH mutation and 1p/19q codeletion remain mandatory.
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Chicken-wire versus microvascular proliferation: The delicate capillary network of oligodendroglioma is architecturally distinct from the glomeruloid endothelial proliferation of high-grade astrocytic tumours. Correct identification is essential for both diagnosis and grading.