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PubMed This is a summary of 17 peer-reviewed journal articles Updated
Oncology

Biology & The 2021 WHO Classification: IDH, ATRX, and Astrocytoma

At a Glance

Under the 2021 WHO guidelines, astrocytomas are diagnosed by specific molecular markers rather than just microscopic appearance. A tumor is confirmed as an astrocytoma if it has an IDH mutation, ATRX loss, and a TP53 mutation, which generally indicates a more favorable prognosis than IDH-wildtype tumors.

In the world of brain tumors, the year 2021 marked a major shift. The World Health Organization (WHO) released a new classification system (CNS5) that moved away from looking at tumors solely under a microscope and toward understanding their molecular markers—the genetic code that tells us how a tumor will behave [1][2].

The New Definition of Astrocytoma

One of the biggest changes is the separation of Astrocytoma from Glioblastoma. Under the new rules:

  • Astrocytoma, IDH-mutant: These are now classified as grades 2, 3, or 4 based on their genetics [3].
  • Glioblastoma, IDH-wildtype: In adults, the term “Glioblastoma” is now used only for tumors that lack the IDH mutation [3][4].

This means if your tumor has an IDH mutation, it is an astrocytoma, even if it looks aggressive under a microscope. This is generally a positive finding, as IDH-mutant tumors often respond better to treatment and have a more favorable prognosis than those without the mutation [4][5].

The Molecular “Triad” of Astrocytoma

Doctors look for three specific markers—a “triad”—to confirm that a tumor is an astrocytoma rather than another type of brain tumor [6][7]:

  1. IDH Mutation: Think of this as the master key. It changes how the tumor cell processes energy. Its presence is the primary marker for an astrocytoma [7][8].
  2. ATRX Loss: In healthy cells, the ATRX protein helps maintain DNA stability. In many astrocytomas, this protein is “lost” or missing [6][9].
  3. TP53 Mutation: The TP53 gene is known as the “guardian of the genome” because it prevents damaged cells from growing. A mutation here allows the tumor cells to multiply [7][10].

Distinguishing Astrocytoma from Oligodendroglioma

You may see a test result for 1p/19q codeletion. This is a test to see if two specific pieces of chromosomes are missing.

  • If they are missing (codeleted), the tumor is an Oligodendroglioma [11].
  • If they are not missing, and the IDH/ATRX/TP53 markers are present, the diagnosis is confirmed as Astrocytoma [11][12].

Why Grade 4 is Different Now

In the past, a “Grade 4” tumor was only diagnosed if a pathologist saw certain aggressive signs under a microscope. Today, if a tumor has a specific genetic change called a CDKN2A/B homozygous deletion, it is automatically classified as Grade 4, regardless of how it looks [13][14]. This marker tells doctors the tumor is likely to behave more aggressively, allowing them to choose a more intensive treatment plan from the start [15][16]. Identifying this marker is now considered essential for accurate risk stratification [17].

Common questions in this guide

What is the difference between astrocytoma and glioblastoma under the new WHO guidelines?
Under the 2021 guidelines, the term glioblastoma is only used for adult tumors that do not have an IDH mutation. If your tumor has an IDH mutation, it is classified as an astrocytoma, even if it looks aggressive under a microscope.
What does it mean if my tumor has an IDH mutation?
An IDH mutation acts as the master key in how tumor cells process energy and is the primary genetic marker for an astrocytoma. Tumors with this mutation generally respond better to treatment and have a more favorable prognosis compared to tumors without it.
What is the molecular triad in an astrocytoma diagnosis?
The triad refers to three specific genetic markers used by doctors to confirm an astrocytoma diagnosis. These are an IDH mutation, the loss of the ATRX protein, and a mutation in the TP53 gene.
Why is the 1p/19q codeletion test important?
This test checks if specific pieces of chromosomes are missing to help distinguish between two types of brain tumors. If they are missing, the tumor is an oligodendroglioma, but if they are intact alongside the triad markers, it confirms an astrocytoma.
How does a CDKN2A/B deletion affect my tumor grade?
If an astrocytoma has a CDKN2A/B homozygous deletion, it is automatically classified as a Grade 4 tumor, regardless of how the cells look under a microscope. This genetic marker indicates a more aggressive tumor that requires a more intensive treatment plan from the start.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.According to the 2021 WHO criteria, what is the official name and grade of my tumor?
  2. 2.Can you walk me through the 'triad' of markers (IDH, ATRX, and TP53) in my pathology report?
  3. 3.Was my tumor tested for the CDKN2A/B deletion, and if so, what was the result?
  4. 4.How does the absence of 1p/19q codeletion affect my diagnosis and future treatment?
  5. 5.Does the presence of the IDH mutation change the way we plan for long-term monitoring?

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 (17)
  1. 1

    Classification of adult-type diffuse gliomas: Impact of the World Health Organization 2021 update.

    Whitfield BT, Huse JT

    Brain pathology (Zurich, Switzerland) 2022; (32(4)):e13062 doi:10.1111/bpa.13062.

    PMID: 35289001
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    Adult type diffuse gliomas in the new 2021 WHO Classification.

    Antonelli M, Poliani PL

    Pathologica 2022; (114(6)):397-409 doi:10.32074/1591-951X-823.

    PMID: 36534419
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    Implications of molecular classifications in glioma surgery.

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    Incidence and Survival of IDH-Wildtype Glioblastoma and IDH-Mutant Astrocytoma by Treatment and Sex: A Regional Study in Spain (2011-2021).

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    Medical sciences (Basel, Switzerland) 2025; (13(4)) doi:10.3390/medsci13040233.

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    Study of Molecular Markers in Glioma and Their Association with Clinicopathological Features.

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    Annals of African medicine 2025; (24(1)):28-36 doi:10.4103/aam.aam_127_23.

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    Molecular classification and stratification of adult diffuse gliomas: A tertiary care center study.

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    The oncologist 2025; (30(12)) doi:10.1093/oncolo/oyaf391.

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    Prognostic factors in progressive high-grade glial tumors treated with systemic approach: A single center experience.

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    New Molecular Considerations for Glioma: IDH, ATRX, BRAF, TERT, H3 K27M.

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    Frequency of false-positive FISH 1p/19q codeletion in adult diffuse astrocytic gliomas.

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    H3K27me3 immunostaining is diagnostic and prognostic in diffuse gliomas with oligodendroglial or mixed oligoastrocytic morphology.

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    Translational significance of CDKN2A/B homozygous deletion in isocitrate dehydrogenase-mutant astrocytoma.

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    Impact of CDKN2A/B Homozygous Deletion on the Prognosis and Biology of IDH-Mutant Glioma.

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    A novel grading system combining histological grade and CDKN2A homozygous and hemizygous deletion to predict prognosis in IDH-mutant astrocytoma.

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    Exploring Demographic, Clinical, Surgical, and Imaging Features in Relation to CDKN2A/B Status in Meningiomas.

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This page explains astrocytoma pathology terminology and WHO classifications for educational purposes. Your neuro-oncologist and pathologist are the best sources for interpreting your specific molecular testing results.

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