Brain and CNS tumors: classification and an introduction to gliomas
Start with why primary brain cancer is relatively uncommon, then distinguish glioma, Glioblastoma（GBM）, meningioma, lymphoma, embryonal tumors and brain metastasis.
First, clarify one phrase: a brain tumor is not necessarily brain cancer
Distinguish these three terms first
Brain tumor: a tumor arising in the brain or associated intracranial tissues, which may be nonmalignant or malignant. CNS tumor is broader and also includes the spinal cord and other sites.
Primary brain/CNS tumor: a tumor originating in the brain, spinal cord or associated tissues such as the meninges.
Brain metastasis: another cancer that has spread to the brain.
Statistical scope: the US population of all ages, diagnosed in 2018–2022. The 6.86 figure is the average annual incidence adjusted to the 2000 US standard population. The denominator for 22.2% is all primary brain and other CNS tumors; for 52.2%, it is the malignant subset. Neither includes brain metastases, and neither represents incidence in Taiwan or an individual’s probability of disease. CBTRUS registry histology groupings are based on WHO 2016 and do not exactly match the integrated molecular diagnoses of WHO CNS5 used in this article. Statistical source
Primary CNS tumors (representative categories)
-
Gliomas, glioneuronal and neuronal tumors
- Adult-type diffuse gliomas: Astrocytoma, IDH-mutant; Oligodendroglioma, IDH-mutant and 1p/19q-codeleted; Glioblastoma, IDH-wildtype
- Pediatric-type diffuse gliomas
- Circumscribed astrocytic tumors
- Ependymal tumors (including ependymoma)
- Neuronal / glioneuronal tumors
- Meningioma
- Primary CNS lymphoma
- Medulloblastoma / embryonal tumors
- Germ-cell tumors
- Pituitary adenoma / pituitary neuroendocrine tumor (PitNET)
- Cranial / spinal nerve tumors such as schwannoma
Metastatic brain tumors (brain metastasis)
Spread from a primary cancer in another organ, for example:
- Lung cancer
- Breast cancer
- Melanoma
- Kidney cancer
- Other primary cancers
Why primary brain cancer is relatively uncommon: known context and limits of explanation
Most mature neurons no longer divide regularly, but the brain still contains distinct cell populations such as glial cells and neural stem / progenitor cells. This helps explain why a brain tumor is not simply a mature neuron becoming cancerous, but cannot by itself explain differences in cancer incidence between organs. The precise causes of most adult brain and spinal cord tumors remain unclear.
Low division activity in mature neurons is therefore a cell-biological background fact, not a single established causal explanation for the relative rarity of brain cancer. NINDS information on neurons; NCI background on causes and risks.
Glioma is an entire tumor family, not one cancer
Modern WHO CNS classification goes beyond which glial cell a tumor resembles under the microscope, integrating morphology + molecular genetic features into the diagnosis.
This is a classification concept map, not a complete diagnostic algorithm. Adult-type / pediatric-type describes major biological types and typical ages, not absolute age limits.
Astrocytoma, IDH-mutant
An adult-type diffuse glioma. It may be CNS WHO grade 2, 3 or 4.
Key point: grade 4 astrocytoma should not simply be equated with GBM. Modern WHO classification incorporates IDH status into tumor entity names.
Oligodendroglioma
The core molecular requirements for a formal diagnosis are IDH-mutant and
1p/19q-codeleted.
Usually classified as CNS WHO grade 2 / 3.
Glioblastoma, IDH-wildtype
One of the most important highly malignant diffuse glioma types in adults; CNS WHO grade 4.
Highly infiltrative and prone to recurrence; it should not be viewed simply as a sharply bounded ball.
Why is GBM particularly difficult to treat?
1. Infiltration, not just a single mass
GBM cells can infiltrate apparently normal brain tissue along white matter, perivascular spaces and other routes. The enhancing mass seen on MRI does not mark the full extent of all tumor cells.
2. Neurological function limits complete removal
When a tumor lies near eloquent structures for language, movement or vision, or near the brainstem, surgeons cannot freely widen the safety margin as they might with some peripheral organ tumors.
3. High intratumoral heterogeneity
Different molecular states and cell populations can coexist within the same tumor, causing different regions to respond differently to stress and treatment.
4. Recurrence is a central clinical problem
Residual infiltrating cells and treatment selection pressure can lead to recurrence. Treatment is therefore not just surgery, but usually integrates neurosurgery, radiotherapy and drug therapy.
Other important brain / CNS tumors
| Tumor | Main origin / location | Concept of malignancy | Typical features |
|---|---|---|---|
| Meningioma Meningioma |
Meningeal-associated cells | Most are nonmalignant | One of the most common primary nonmalignant CNS histologies. Even a benign tumor may compress the optic nerve, brainstem or cortex because of its location. |
| Ependymoma Ependymoma |
Ependymal lineage associated with the ventricles / central canal | Varies by subtype / molecular group | Distribution differs between children and adults; also occurs in the spinal cord. |
| Diffuse midline glioma H3 K27-altered |
Midline structures such as the brainstem, thalamus and spinal cord | Highly malignant | A pediatric-type diffuse high-grade glioma; location often limits surgical options. |
| Medulloblastoma Medulloblastoma |
Posterior fossa / cerebellar region; embryonal tumor | Malignant | More typical in children; may spread along CSF pathways. |
| Primary CNS lymphoma | Lymphocytes, often of B-cell lineage | Malignant | It is a lymphoma, not a glioma; the treatment approach is entirely different. |
| Germ-cell tumor | Common in the pineal / suprasellar region | Varies greatly by subtype | More typical in children, adolescents and young adults; some types are sensitive to radiotherapy / chemotherapy. |
| PitNET Pituitary neuroendocrine tumor |
Pituitary gland | Most are not typical malignant cancers | Clinical problems often arise from hormone excess / deficiency or optic chiasm compression. |
| Vestibular schwannoma | Schwann cells of the vestibular nerve sheath | Usually nonmalignant | Often presents with unilateral hearing loss, tinnitus or balance symptoms; large tumors can compress the brainstem. |
| Neuronal / glioneuronal tumors | Neuronal differentiation, or combined neuronal and glial differentiation | Many are low grade | Some are discovered in young patients through longstanding epilepsy. |
Particularly important in adults: brain metastasis
The cells of a metastatic brain tumor retain the identity of their organ of origin. For example, lung cancer spreading to the brain remains metastatic lung cancer pathologically; it does not suddenly become glioblastoma.
Primary cancer
For example, lung / breast / melanoma / kidney cancers.
Reaches the CNS through the bloodstream or other routes
Forms single or multiple brain metastases. Treatment must also account for the state of systemic disease.
Brain tumors are not primarily understood through Stage I–IV as lung cancer is
CNS WHO grade: the biological grade of the tumor itself
For CNS tumors, grade usually reflects pathological / molecular features and expected growth behavior. Grading rules are not identical across tumor entities.
Conceptually, from low proliferative potential to more aggressive behavior, but grades cannot be mechanically compared across different tumor entities.
Stage: the extent of spread commonly used for many other cancers
Adult primary brain / spinal cord tumors have no standard Stage I–IV clinical staging system. The NCI notes that treatment depends more on tumor type, location, residual amount and grade. Cancer registries can still record Summary Stage, which differs from conventional I–IV clinical staging.
Modern glioma diagnosis: genetic / epigenetic information is now part of the formal diagnosis
IDH1 / IDH2
One of the core classification points for adult diffuse gliomas. Whether it is
IDH-mutant directly changes the diagnostic entity.
1p/19q codeletion
Together with an IDH mutation, a core requirement for diagnosing oligodendroglioma.
H3 alterations
Critical in certain pediatric-type high-grade diffuse gliomas, such as H3 K27-altered and H3 G34-mutant.
ATRX / TP53
Can support the overall molecular pathological interpretation of certain astrocytic lineages, but must not be used in isolation from the full diagnostic context.
EGFR / TERT, etc.
May have classificatory or biological significance in integrated diagnosis of adult diffuse gliomas. Interpretation depends on WHO criteria and the combination of tests.
MGMT promoter methylation
is one of the predictive / prognostic biomarkers often considered clinically in glioblastoma, but its presence or absence is not a single switch determining all treatment.
There are many steps between something seen on MRI and a formal diagnosis
- Symptoms or incidental finding: neurological examination.
- Imaging: primarily MRI, assessing location, number and morphology.
- Tissue: obtained through surgery or biopsy when clinically feasible.
- Integrated diagnosis: histology + molecular findings.
- A multidisciplinary team assesses treatment.
What can imaging answer?
- Location, size, number, mass effect and edema.
- Imaging features such as enhancement, necrosis, hemorrhage or restricted diffusion.
- Helps plan surgery, biopsy and the differential diagnosis.
What can imaging not always answer?
- MRI alone usually cannot establish that a lesion is definitely a glioma.
- Metastasis, lymphoma, infection and inflammation / demyelination can sometimes mimic tumors.
- A formal WHO integrated diagnosis often requires tissue and molecular data.
Symptoms depend mainly on location + intracranial pressure + growth rate
Focal neurological function
Unilateral weakness, language disturbance, visual field changes, sensory abnormalities, balance / gait problems, etc.
Cortical irritation
A new seizure is an important presentation of some brain tumors. New-onset epilepsy in an adult particularly requires assessment of the cause.
Intracranial pressure / mass effect
Headache, nausea, vomiting, drowsiness or altered consciousness, but none of these symptoms by itself means brain cancer.
Remember the topic in seven statements
- Brain tumor ≠ brain cancer. Intracranial tumors include both benign and malignant tumors.
- Primary malignant CNS tumors are relatively uncommon, but their location means risk cannot be judged solely by pathological benignity or malignancy.
- Glioma is a family name, not a single fixed disease.
- Core entities of adult diffuse glioma include Astrocytoma, IDH-mutant, Oligodendroglioma, IDH-mutant and 1p/19q-codeleted, Glioblastoma, IDH-wildtype.
- GBM is a highly malignant glioma, but not all gliomas are GBM.
- Brain metastasis is very important in adults and is more common than primary brain tumors.
- Modern diagnosis focuses on location + histology + molecular profile + CNS WHO grade + resectability / residual tumor, not just how many centimeters or what stage.
Main sources
Biological background of neurons and neural stem / progenitor cells
Limits of interpreting diagnostic markers in adult IDH-wildtype gliomas (2025)
First seizure and failure to regain consciousness; Sudden unilateral weakness / difficulty speaking
Source prepared: 2026-10-04. This article was reformatted and its references checked on 2026-10-05, retaining its educational purpose of explaining disease classification. Content can be read offline; source links require a network connection.