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PubMed This is a summary of 19 peer-reviewed journal articles Updated
Neurology

The Biology of Glioblastoma: The 2021 Standards

At a Glance

Under the 2021 WHO classification, a true glioblastoma is defined by its genetics, specifically an IDH-wildtype status. Tumors with an IDH mutation are now called astrocytomas. Genetic testing for markers like EGFR and TERT is essential to confirm a Grade 4 diagnosis and personalize treatment.

In recent years, the way doctors define and treat glioblastoma has undergone a massive paradigm shift. In 2021, the World Health Organization (WHO) updated the classification of brain tumors, moving away from just looking at cells under a microscope and toward a deep understanding of their genetic “instruction manuals.” This means that two tumors that look identical to a pathologist may actually be two completely different diseases based on their DNA [1][2].

The IDH “Switch”

The most important term in your pathology report is IDH status. IDH (isocitrate dehydrogenase) is an enzyme involved in cell metabolism.

  • IDH-Wildtype (The “Natural” State): In biology, “wildtype” refers to the normal, unmutated version of a gene. Think of it like the standard factory settings on a computer. In the context of brain tumors, IDH-wildtype is the defining characteristic of true Glioblastoma [3][4]. It is more aggressive and follows the standard glioblastoma treatment path [5].
  • IDH-Mutant: If the IDH gene has a mutation (an error in the code), the tumor is now classified as an Astrocytoma, IDH-mutant—even if it is Grade 4 [6][7]. These tumors typically occur in younger patients and have a different, often more favorable, clinical outlook than glioblastoma [8].

Under the 2021 rules, if a tumor is IDH-mutant, it is no longer called Glioblastoma. The term “Glioblastoma” is now strictly reserved for IDH-wildtype, Grade 4 tumors [1].

The “Molecular Glioblastoma”

One of the most significant changes in the new classification is the concept of a molecular glioblastoma. Previously, if a tumor looked like a Grade 2 or Grade 3 tumor under the microscope (meaning the cells weren’t moving or dividing as rapidly), it was treated less aggressively.

We now know that some of these “lower-grade” looking tumors have the genetic “engine” of a glioblastoma. If an IDH-wildtype tumor has any of the following three genetic markers, it is automatically “upgraded” to CNS WHO Grade 4 Glioblastoma, regardless of what it looks like under the microscope [1][9][10]:

  1. EGFR Amplification: The tumor has too many copies of the Epidermal Growth Factor Receptor, which acts like a stuck gas pedal for cell growth [11].
  2. TERT Promoter Mutation: A mutation that allows the tumor cells to become “immortal” by constantly rebuilding their DNA telomeres (the caps at the ends of chromosomes) [12][13].
  3. +7/-10 (Chromosome 7 Gain/Chromosome 10 Loss): The tumor has gained an extra copy of Chromosome 7 and lost a copy of Chromosome 10. This specific combination is a signature of glioblastoma [14][15].

Why This Matters for You

This reclassification ensures that you receive the right intensity of treatment from day one. If your tumor has these molecular markers, your care team will treat it as a Grade 4 glioblastoma because it is expected to behave like one [16][17].

By identifying these drivers—such as the PI3K/AKT/mTOR pathway, which often helps these tumors grow—scientists are also able to design targeted therapies and clinical trials that are specific to your tumor’s genetic makeup [18][19]. Understanding your molecular profile is the key to personalizing your care.

Common questions in this guide

What does IDH-wildtype mean on my pathology report?
IDH-wildtype means the tumor cells have the normal, unmutated version of the IDH gene. In brain tumors, this wildtype status is the defining characteristic of true glioblastoma and indicates it will be treated with standard glioblastoma protocols.
Why is my IDH-mutant Grade 4 tumor no longer called glioblastoma?
Under the 2021 WHO classification, the term glioblastoma is strictly reserved for IDH-wildtype tumors. If a tumor has an IDH mutation, it is classified as an IDH-mutant astrocytoma, which generally has a different clinical outlook.
What is a molecular glioblastoma?
A molecular glioblastoma is a tumor that looks like a lower grade under a microscope but has specific genetic markers that make it behave aggressively. These genetic markers automatically upgrade the diagnosis to a Grade 4 glioblastoma so it can be treated appropriately.
Which genetic markers upgrade a brain tumor to a Grade 4 glioblastoma?
An IDH-wildtype tumor is upgraded to a Grade 4 glioblastoma if it has EGFR amplification, a TERT promoter mutation, or a specific combination of gaining Chromosome 7 and losing Chromosome 10.
Why is genetic and molecular testing important for my glioblastoma treatment?
Understanding your tumor's specific genetic makeup helps doctors predict how the tumor will behave. This molecular profile ensures you receive the right intensity of treatment and helps identify targeted therapies or clinical trials that may work best for your specific cancer.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.My report says I have an 'Astrocytoma, IDH-mutant, Grade 4'—why is this no longer called Glioblastoma?
  2. 2.If my tumor looked like a Grade 2 or 3 under the microscope, which molecular marker triggered the upgrade to 'molecular glioblastoma'?
  3. 3.Was my diagnosis determined using the 2021 WHO Classification standards?
  4. 4.Can you explain the significance of TERT promoter mutation and EGFR amplification in my specific case?
  5. 5.How does my IDH-wildtype status change the way we plan my treatment compared to an IDH-mutant tumor?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (19)
  1. 1

    Molecular GBM versus Histopathological GBM: Radiology-Pathology-Genetic Correlation and the New WHO 2021 Definition of Glioblastoma.

    Agarwal A, Edgar MA, Desai A, et al.

    AJNR. American journal of neuroradiology 2024; (45(8)):1006-1012 doi:10.3174/ajnr.A8225.

    PMID: 38438167
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    TERT promoter mutation status is necessary and sufficient to diagnose IDH-wildtype diffuse astrocytic glioma with molecular features of glioblastoma.

    Fujimoto K, Arita H, Satomi K, et al.

    Acta neuropathologica 2021; (142(2)):323-338 doi:10.1007/s00401-021-02337-9.

    PMID: 34148105
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    IDH-Mutant Low-grade Glioma: Advances in Molecular Diagnosis, Management, and Future Directions.

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    Implications of molecular classifications in glioma surgery.

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    Journal of neuro-oncology 2025; (171(3)):559-569 doi:10.1007/s11060-024-04883-0.

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    Beyond the World Health Organization classification of central nervous system tumors 2016: what are the new developments for gliomas from a clinician's perspective?

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    Current opinion in neurology 2020; (33(6)):701-706 doi:10.1097/WCO.0000000000000871.

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    Genetic and epigenetic landscape of IDH-wildtype glioblastomas with FGFR3-TACC3 fusions.

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    Diffuse astrocytic glioma, IDH-Wildtype, with molecular features of glioblastoma, WHO grade IV: A single-institution case series and review.

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    The Prognostic Significance of Epidermal Growth Factor Receptor Amplification and Epidermal Growth Factor Receptor Variant III Mutation in Glioblastoma: A Systematic Review and Meta-Analysis with Implications for Targeted Therapy.

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    Prospective genomically guided identification of "early/evolving" and "undersampled" IDH-wildtype glioblastoma leads to improved clinical outcomes.

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    Molecular Correlates of Long Survival in IDH-Wildtype Glioblastoma Cohorts.

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This page explains the 2021 WHO classification for glioblastoma for educational purposes only. Always consult your neuro-oncologist or pathologist to interpret your specific molecular testing and pathology results.

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