Adult Diffuse Gliomas (2021 WHO)

Adult diffuse gliomas are the most common primary malignant brain tumors. The 2021 WHO Classification of Tumors of the Central Nervous System reorganized their categorization around molecular features as much as histology, recognizing that two tumors with similar microscopic appearance can have radically different biology and prognosis. The integrated diagnosis combines histologic features (cell type, grade) with critical molecular markers (IDH mutation, 1p/19q codeletion, CDKN2A/B status, MGMT methylation). This page covers the modern classification and pathology of adult diffuse gliomas.

2021 WHO Adult Diffuse Glioma Classification

Three families of adult diffuse gliomas (each defined molecularly):

  1. Astrocytoma, IDH-mutant: grades 2, 3, or 4.
  2. Oligodendroglioma, IDH-mutant and 1p/19q-codeleted: grades 2 or 3.
  3. Glioblastoma, IDH-wildtype: grade 4 by definition.

Astrocytoma, IDH-Mutant

Diagnostic Criteria

  • Diffusely infiltrating astrocytic tumor.
  • IDH1 R132H mutation (most common; also IDH1 other codons or IDH2 mutations).
  • Loss of ATRX (ATRX-mutant) and TP53 nuclear positivity are supportive.
  • Absence of 1p/19q codeletion (which would make it oligodendroglioma).

Histologic Features

  • Grade 2 astrocytoma, IDH-mutant: low cellular density, mild atypia, no mitoses, no microvascular proliferation, no necrosis.
  • Grade 3: increased cellularity, mitotic activity (Ki-67 typically > 5%), nuclear atypia.
  • Grade 4 astrocytoma, IDH-mutant: microvascular proliferation OR necrosis OR CDKN2A/B homozygous deletion. Note: presence of CDKN2A/B deletion qualifies as grade 4 regardless of microscopy.

Molecular Workup

  • IDH1 R132H IHC; if negative in patient < 55, sequence for less common IDH mutations.
  • ATRX (nuclear loss).
  • p53 IHC.
  • CDKN2A/B status (FISH, NGS).
  • 1p/19q (FISH, NGS) — to exclude oligodendroglioma.
  • MGMT promoter methylation (for treatment guidance).

Clinical-Pathologic

  • Younger patients (30s-50s).
  • Often present with seizures.
  • Better prognosis than IDH-wildtype glioblastoma.
  • Median survival: grade 2 ~10-15 years; grade 3 ~5-10 years; grade 4 ~3-5 years.
  • Treatment: surgery + radiation + temozolomide. PCV chemotherapy also used. Vorasidenib (IDH inhibitor) approved 2024 for grade 2 IDH-mutant glioma.

Oligodendroglioma, IDH-Mutant and 1p/19q-Codeleted

Diagnostic Criteria

  • IDH mutation.
  • 1p/19q codeletion (BOTH chromosomes; partial deletions are not codeletion).
  • Both features are required for the diagnosis.

Histologic Features

  • “Fried egg” cells: round nuclei with perinuclear halos (artifact of formalin fixation but characteristic).
  • “Chicken wire” vasculature: delicate branching capillaries.
  • Often microcalcifications.
  • Mucinous matrix may be present.

Grading

  • Grade 2: relatively low cellularity, mild atypia, no mitoses.
  • Grade 3 (anaplastic): mitoses, increased cellularity, microvascular proliferation, sometimes necrosis. Grade 4 oligodendroglioma is not recognized in 2021 WHO.

Clinical-Pathologic

  • Slow-growing, frontal lobes common.
  • Often calcified on imaging.
  • Best prognosis among adult diffuse gliomas: median survival 10-15+ years.
  • Excellent response to PCV (procarbazine + lomustine + vincristine) and temozolomide chemotherapy.
  • Vorasidenib also relevant for IDH-mutant tumors.

Glioblastoma, IDH-Wildtype

Diagnostic Criteria (2021 WHO)

An adult diffuse astrocytic glioma is glioblastoma, IDH-wildtype if it has IDH-wildtype status AND at least one of:

  • Microvascular proliferation.
  • Necrosis.
  • TERT promoter mutation.
  • EGFR amplification.
  • +7/-10 chromosome combination.

The 2021 criteria explicitly recognize that some histologically “lower-grade” IDH-wildtype tumors are biologically glioblastomas based on molecular features.

Histologic Features

  • High cellular density with nuclear pleomorphism.
  • Mitotic activity.
  • Microvascular proliferation (glomeruloid).
  • Necrosis (often with pseudopalisading — tumor cells lined up around necrotic foci).
  • Infiltrative growth pattern (extends beyond visible tumor on imaging).

Molecular Features

  • IDH-wildtype.
  • TERT promoter mutation: ~70%.
  • EGFR amplification: ~40%.
  • PTEN loss: common.
  • CDKN2A/B homozygous deletion: common.
  • +7/-10: common.
  • MGMT promoter methylation: ~40-45% (predicts temozolomide response).
  • NF1 mutation: subset.
  • PDGFRA amplification: subset.

Historical Transcriptomic Subgrouping (TCGA, 2010)

Important context: the Classical / Proneural / Mesenchymal / Neural categories below are historical TCGA transcriptomic subtypes. They are not part of the current WHO CNS5 (2021) integrated diagnosis of glioblastoma, IDH-wildtype — they are research-oriented groupings that are still cited in the literature but do not drive routine diagnostic categorization or treatment selection. The “Neural” subtype in particular has been re-evaluated in later analyses as likely reflecting non-tumor brain contamination and is not considered a robust biological category.

  • Classical (EGFR amplification + CDKN2A deletion).
  • Proneural (PDGFRA amplification; IDH-mutant tumors historically grouped here are now classified separately as IDH-mutant astrocytoma under WHO CNS5).
  • Mesenchymal (NF1 alteration, increased macrophage infiltration).
  • Neural (now generally regarded as non-robust / contamination artifact in later re-analyses).

Current adult glioblastoma diagnosis follows WHO CNS5 molecular criteria: IDH-wildtype + at least one of EGFR amplification, TERT promoter mutation, or +7/−10 chromosomal pattern, with histologic GBM features or any of these molecular criteria sufficient even without classic histology (so-called “molecular GBM”).

Imaging

  • Ring-enhancing mass with central necrosis.
  • Surrounding vasogenic edema.
  • Often crosses midline (butterfly glioma — corpus callosum involvement).
  • Spectroscopy: elevated choline, lipid/lactate peaks; reduced NAA.

Clinical

  • Most common primary malignant brain tumor in adults.
  • Median age: 60s.
  • Poor prognosis: median survival ~12-15 months with optimal treatment.
  • Treatment: maximal safe resection + concurrent radiation/temozolomide (Stupp protocol) + adjuvant temozolomide. Tumor treating fields (Optune) added for selected patients.
  • Bevacizumab for recurrence.

Other Considerations

Gliosarcoma

Glioblastoma with mesenchymal differentiation (spindled sarcoma-like areas). Same prognosis as glioblastoma.

Giant Cell Glioblastoma

Glioblastoma with prominent multinucleated giant cells. Similar prognosis.

Epithelioid Glioblastoma

Distinct subtype with BRAF V600E in many cases; younger patients; can mimic metastasis. Targeted BRAF therapy possible.

Gemistocytic Astrocytoma

Histologic pattern within astrocytoma; gemistocytic cells with abundant eosinophilic cytoplasm. May behave more aggressively within IDH-mutant astrocytoma.

Modern Treatment Considerations

  • Surgical resection: extent of resection is prognostic.
  • Radiation: standard adjuvant.
  • Temozolomide (Stupp): standard for glioblastoma and high-grade IDH-mutant.
  • PCV: oligodendroglioma chemotherapy.
  • Vorasidenib: oral IDH1/IDH2 inhibitor; approved 2024 for IDH-mutant grade 2 glioma.
  • BRAF/MEK inhibitors for BRAF V600E tumors.
  • NTRK inhibitors for NTRK-fused tumors.
  • Bevacizumab for recurrent GBM.
  • Tumor treating fields (Optune).
  • Clinical trials: immunotherapy, CAR-T, targeted agents.

🔍 Did You Know?

The 2024 approval of vorasidenib for IDH-mutant grade 2 glioma is the first targeted therapy approved specifically for low-grade gliomas. Vorasidenib is an oral, brain-penetrant inhibitor of mutant IDH1 and IDH2 enzymes — it blocks production of 2-hydroxyglutarate (2-HG), the oncometabolite that IDH-mutant tumors produce and that contributes to tumorigenesis through DNA hypermethylation and other effects. The INDIGO trial (2023) showed that vorasidenib significantly delays disease progression in residual or recurrent grade 2 IDH-mutant gliomas after surgery, with manageable side effects. The approval marks a transition: for the first time, the molecular diagnosis of “IDH-mutant glioma” — once a research subdivision — directly determines specific oral targeted therapy. The treatment paradigm for IDH-mutant gliomas may shift from “watchful waiting after partial resection” (the historical approach for indolent grade 2 lesions) to “targeted therapy at diagnosis.” Patients now require IDH mutation status as part of their integrated diagnosis to know whether they can benefit from this therapy. The lesson: modern neuro-oncology requires molecular diagnostics before treatment decisions. The 2021 WHO classification anticipated this; vorasidenib’s approval makes it clinical reality.

Pitfalls and Pearls

  • 2021 WHO adult diffuse gliomas: astrocytoma IDH-mutant, oligodendroglioma IDH-mutant/1p19q-codeleted, glioblastoma IDH-wildtype.
  • IDH1 R132H IHC: first-pass screen; sensitive in adults.
  • IDH negative in < 55 year old: sequence for less common IDH mutations.
  • 1p/19q codeletion: required for oligodendroglioma; FISH or NGS.
  • CDKN2A/B homozygous deletion: grade 4 IDH-mutant astrocytoma.
  • TERT promoter mutation, EGFR amplification, +7/-10: molecular features of IDH-wildtype GBM.
  • Pseudopalisading necrosis: histologic hallmark of glioblastoma.
  • Fried-egg cells + chicken-wire vasculature: oligodendroglioma.
  • Butterfly glioma: corpus callosum involvement of GBM.
  • MGMT methylation: predicts temozolomide response in GBM.
  • Vorasidenib: approved 2024 for IDH-mutant grade 2 glioma.
  • Stupp protocol: standard for GBM (concurrent + adjuvant TMZ + RT).
  • PCV: oligodendroglioma chemo.
  • BRAF V600E: epithelioid GBM subset; targeted therapy possible.

References

  1. Louis DN, Perry A, Wesseling P, et al. The 2021 WHO Classification of Tumors of the Central Nervous System: a summary. Neuro Oncol. 2021;23(8):1231-1251.
  2. Stupp R, Mason WP, van den Bent MJ, et al. Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma. N Engl J Med. 2005;352(10):987-996.
  3. Mellinghoff IK, van den Bent MJ, Blumenthal DT, et al. Vorasidenib in IDH1- or IDH2-mutant low-grade glioma. N Engl J Med. 2023;389(7):589-601.
  4. Cancer Genome Atlas Research Network. Comprehensive, integrative genomic analysis of diffuse lower-grade gliomas. N Engl J Med. 2015;372(26):2481-2498.
  5. Verhaak RG, Hoadley KA, Purdom E, et al. Integrated genomic analysis identifies clinically relevant subtypes of glioblastoma. Cancer Cell. 2010;17(1):98-110.
  6. Stupp R, Taillibert S, Kanner A, et al. Effect of tumor-treating fields plus maintenance temozolomide vs maintenance temozolomide alone on survival in patients with glioblastoma. JAMA. 2017;318(23):2306-2316.