Understanding Brain Cancer

What Are the Brain Cancer Stages and Classifications?

Last updated and reviewed on June 16, 2026.

For most cancers, like breast cancer or colon cancer, doctors use a staging system numbered from Stage 1 to Stage 4 to describe how far the cancer has spread. Brain cancer works a little differently. While a TNM staging system does technically exist for brain tumors, it is rarely used in practice because the spread of a brain tumor to lymph nodes or distant organs (the things TNM staging measures) is extremely uncommon for primary brain tumors. What matters far more for brain tumors is the type of tumor, its grade, its molecular features, and where it is located, not whether it has spread to the lymph nodes.

Understanding how brain tumors are classified is important because classification directly drives treatment decisions and tells you and your doctor a great deal about what to expect.

Understanding the WHO Grading System

The most widely used system for classifying brain tumors is the World Health Organization (WHO) grading system. The WHO assigns each brain tumor a grade from 1 to 4 based on how the tumor cells look under a microscope and how fast they are likely to grow. As of the updated 2021 WHO Classification of Central Nervous System Tumors, molecular features (genetic markers of the tumor) are now incorporated alongside microscopic appearance to define tumor types and grades more precisely than ever before.

Grade 1: Cells look nearly normal. The tumor grows very slowly. Grade 1 tumors are often considered benign (not cancerous) and may be cured with surgery alone. Pilocytic astrocytoma, common in children, is a classic Grade 1 tumor.

Grade 2: Cells look slightly abnormal. The tumor grows slowly but has the potential to come back after treatment and sometimes progresses to a higher grade over time. Many low-grade gliomas fall into Grade 2.

Grade 3: Cells look clearly abnormal, and the tumor grows faster. Grade 3 tumors are malignant and require active treatment. Examples include anaplastic astrocytoma (IDH-mutant) and anaplastic oligodendroglioma.

Grade 4: Cells look very abnormal, and the tumor is growing rapidly. These are the most aggressive brain tumors. Glioblastoma (GBM) is the most common Grade 4 tumor in adults. Grade 4 tumors are highly malignant and are the most challenging to treat.

In addition to grade, the molecular features of the tumor are now central to its classification. For example, two tumors that look similar under the microscope may be classified differently based on whether they carry an IDH mutation, whether they have a 1p/19q codeletion, or whether the MGMT promoter is methylated. These features change both the tumor's name and its expected behavior.

What Are the Main Brain Tumor Classifications?

• Gliomas (IDH-mutant and IDH-wild type): Since the 2021 WHO update, gliomas are primarily classified based on whether they carry a mutation in the IDH gene (IDH-mutant) or do not (IDH-wild type). IDH-mutant gliomas tend to have a better prognosis. IDH-wild type gliomas, particularly Grade 4 IDH-wild type astrocytoma, which is the molecular definition of glioblastoma, are more aggressive.
• Gliomas are further subdivided as: Astrocytoma, IDH-mutant (Grade 2, 3, or 4): Arises from astrocytes. The higher the grade, the more aggressive the tumor.
• Oligodendroglioma, IDH-mutant and 1p/19q codeleted (Grade 2 or 3): Arises from oligodendrocytes. Defined by both IDH mutation and the loss of both the 1p and 19q chromosome arms. Generally associated with a better prognosis and good response to PCV chemotherapy.
• Glioblastoma, IDH-wild type (Grade 4): The most common and most aggressive malignant brain tumor in adults. Always IDH-wild type. May also have EGFR amplification, TERT promoter mutation, or chromosome 7 gain with chromosome 10 loss.
• Diffuse Hemispheric Glioma, H3.3 K27M-mutant and other pediatric diffuse gliomas: Distinct tumor types more common in children, defined by specific molecular markers.
• Meningioma (Grade 1, 2, or 3): Meningiomas arise from the meninges the three-layered tissue covering the brain and spinal cord. Most are Grade 1 (benign, slow-growing) and can often be cured with surgery. Grade 2 (atypical) and Grade 3 (malignant) meningiomas are more aggressive and more likely to recur.
• Medulloblastoma: Medulloblastoma is the most common malignant brain tumor in children. It arises in the cerebellum and is classified into four molecular subgroups (WNT-activated, SHH-activated, Group 3, and Group 4), each with distinct characteristics, behaviors, and treatment responses. WNT-activated medulloblastoma has the best prognosis; Group 3 medulloblastoma tends to be the most aggressive.
• Ependymoma: Ependymomas arise from cells lining the ventricles and central canal of the spinal cord. They can occur in the brain or spinal cord. In children, spinal ependymoma has a better outlook than intracranial (brain) ependymoma. Molecular subgroups have recently been identified that have prognostic significance.
• Primary CNS Lymphoma: A lymphoma that arises within the brain, eyes, spinal cord, or meninges. It is a fast-growing tumor, but it is often highly responsive to high-dose methotrexate-based chemotherapy.

Questions to Ask Your Doctor About Your Classification

Coming to your appointments prepared with questions is one of the most powerful things you can do. Your diagnosis can feel overwhelming, and it is completely appropriate to ask for detailed explanations in plain language.

Questions to ask your doctor about your tumor type and classification:

• What specific type of brain tumor do I have?

• What is the grade, and what does that mean for how aggressive it is?

• What molecular tests were done on my tumor? What were the results of IDH testing, MGMT methylation, 1p/19q codeletion, and other markers?

• How does my tumor's molecular profile affect my treatment options and prognosis?

• Should I get a second opinion on my pathology from a neuropathologist at a major brain tumor center?

• Will my case be reviewed by a multidisciplinary team that includes a neurosurgeon, neuro-oncologist, radiation oncologist, and neuropathologist?

• Are there clinical trials available for my specific tumor type and molecular profile?

Sources:

1. Louis DN, et al. The 2021 WHO Classification of Tumors of the Central Nervous System. Neuro-Oncology. 2021;23(8):1231–1251.https://pubmed.ncbi.nlm.nih.gov/34185076/

2. American Cancer Society. Brain and Spinal Cord Tumor Grades. https://www.cancer.org/cancer/types/brain-spinal-cord-tumors-adults/detection-diagnosis-staging/staging.html

3. National Cancer Institute. Adult Central Nervous System Tumors Treatment (PDQ) – Patient Version. https://www.cancer.gov/types/brain/patient/adult-brain-treatment-pdq

4. Wen PY, et al. Glioblastoma in adults: a Society for Neuro-Oncology (SNO) and EANO consensus review. Neuro-Oncology. 2025. https://europepmc.org/article/med/40827022

5. National Comprehensive Cancer Network (NCCN). NCCN Clinical Practice Guidelines: Central Nervous System Cancers. 2024. https://pmc.ncbi.nlm.nih.gov/articles/PMC11889715/