Residency · Residency · Neurology
Primary Brain Tumors: Classification and Initial Approach
Introduction
Primary brain tumors encompass a diverse group of neoplasms arising from CNS parenchyma or its coverings. The 2021 WHO Classification of CNS Tumors integrated molecular markers into diagnosis, fundamentally changing how tumors are classified and treated. The neurologist plays a central role in initial evaluation, symptom management, and coordination of multidisciplinary care.
Epidemiology
The annual incidence of primary CNS tumors is approximately 25 per 100,000 (including benign tumors). Meningiomas are the most common primary intracranial tumor (approximately 37%), while gliomas are the most common malignant primary brain tumor (approximately 25%). Glioblastoma (GBM) is the most common malignant glioma in adults. In children, pilocytic astrocytoma, medulloblastoma, and ependymoma predominate.
2021 WHO Classification: Key Updates
Integrated Diagnosis
Diagnosis now requires histology plus molecular markers in a layered diagnostic approach. Tumors with identical histology but different molecular profiles are classified as separate entities. IDH mutation status is the most important molecular divider for adult diffuse gliomas. The 1p/19q codeletion defines oligodendroglioma in the setting of IDH mutation.
Adult-Type Diffuse Gliomas
| Tumor Type | IDH Status | Key Molecular Markers | WHO Grade | Prognosis |
|---|---|---|---|---|
| Astrocytoma | IDH-mutant | ATRX loss, TP53 mutation; Grade 4 if CDKN2A/B homozygous deletion | 2, 3, or 4 | Intermediate |
| Oligodendroglioma | IDH-mutant | 1p/19q codeletion | 2 or 3 | Best among diffuse gliomas |
| Glioblastoma | IDH-wildtype | TERT promoter mutation, EGFR amplification, or +7/-10 | 4 | Worst |
Astrocytoma, IDH-mutant (grades 2, 3, and 4) is characterized by ATRX loss and TP53 mutation, with grade 4 assigned if CDKN2A/B homozygous deletion is present. Oligodendroglioma, IDH-mutant and 1p/19q-codeleted (grades 2 and 3) carries the best prognosis among diffuse gliomas. Glioblastoma, IDH-wildtype (grade 4) is diagnosed when at least one of three molecular features is present in a histologically lower-grade tumor: TERT promoter mutation, EGFR amplification, or +7/-10 chromosome changes.
Other Important Tumor Types
Meningiomas are WHO grade 1 in 80% of cases, grade 2 (atypical) in 15-20%, and grade 3 (anaplastic) in 1-3%. Ependymomas are now classified by molecular subgroups including RELA fusion and posterior fossa Group A/B. Medulloblastomas have four molecular subgroups: WNT, SHH, Group 3, and Group 4. CNS lymphoma refers to primary diffuse large B-cell lymphoma of the CNS.
Clinical Presentation
Symptom Patterns
Headache is present in approximately 50% of cases, typically progressive, worse in the morning, and worsened by Valsalva. Seizures are the presenting symptom in 20-40% of gliomas, more common with low-grade and cortical tumors. Focal neurological deficits depend on tumor location (hemiparesis, aphasia, visual field cuts). Cognitive and personality changes are especially prominent with frontal lobe or diffuse tumors. Signs of elevated ICP include headache, nausea, vomiting, papilledema, and CN VI palsy.
Location-Specific Syndromes
Frontal lobe tumors produce personality change, abulia, expressive aphasia (dominant hemisphere), and contralateral weakness. Temporal lobe tumors cause receptive aphasia (dominant hemisphere), memory impairment, complex partial seizures, and visual field cuts. Parietal lobe tumors produce hemisensory loss, neglect (non-dominant hemisphere), and apraxia. Posterior fossa tumors cause ataxia, cranial neuropathies, and obstructive hydrocephalus. Sellar and parasellar tumors produce visual field deficits (bitemporal hemianopia) and endocrinopathy.
Diagnostic Workup
Neuroimaging
MRI with gadolinium is the imaging modality of choice. Key sequences include T1 pre- and post-contrast, T2, FLAIR, DWI, perfusion, and spectroscopy. Enhancement patterns help narrow the differential: ring-enhancing suggests GBM, homogeneous enhancement suggests lymphoma or meningioma, and non-enhancing lesions suggest low-grade glioma. Advanced MRI techniques include perfusion (rCBV), spectroscopy (elevated choline, reduced NAA), and diffusion tensor imaging for surgical planning.
Additional Studies
CT chest/abdomen/pelvis should be performed if metastatic disease is suspected. Stereotactic biopsy or surgical resection provides tissue for diagnosis. Molecular profiling includes IDH1/2 mutation, 1p/19q codeletion, MGMT promoter methylation, TERT promoter, ATRX, CDKN2A/B, and EGFR amplification. Lumbar puncture with cytology is performed for suspected leptomeningeal disease or CNS lymphoma (only after excluding mass effect).
Initial Management
Acute Symptom Control
Dexamethasone (4-16 mg/day) reduces peritumoral vasogenic edema and provides rapid symptomatic improvement; it should be tapered as quickly as clinically feasible because long-term steroids cause significant morbidity. Levetiracetam is first-line for tumor-related seizures, but prophylactic antiseizure medications are not recommended in seizure-free patients (AAN guideline). Venous thromboembolism prophylaxis is essential because brain tumor patients are at high risk for DVT/PE.
Surgical Considerations
Maximal safe resection improves survival and quality of life in gliomas. Awake craniotomy with cortical mapping is used for tumors near eloquent cortex. Fluorescence-guided surgery with 5-ALA enhances completeness of resection for high-grade gliomas. Biopsy alone is performed when resection is not feasible (deep, diffuse, or multifocal tumors).
Multidisciplinary Tumor Board
The tumor board includes a neurologist, neurosurgeon, neuro-oncologist, radiation oncologist, neuropathologist, and neuroradiologist. Integrated molecular diagnosis guides treatment selection, and clinical trial eligibility should be assessed for all patients.
Clinical Pearls
The 2021 WHO classification mandates molecular testing; histology alone is no longer sufficient for glioma diagnosis. IDH mutation status is the single most important prognostic factor in adult diffuse gliomas. Prophylactic antiseizure medications should not be prescribed in brain tumor patients who have not had seizures. Dexamethasone provides rapid relief of vasogenic edema but should be minimized due to long-term complications.
References
- 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.
- Weller M, van den Bent M, Preusser M, et al. EANO guidelines on the diagnosis and treatment of diffuse gliomas of adulthood. Nat Rev Clin Oncol. 2021;18(3):170-186.
- Glantz MJ, Cole BF, Forsyth PA, et al. Practice parameter: anticonvulsant prophylaxis in patients with newly diagnosed brain tumors. Neurology. 2000;54(10):1886-1893.
- 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.