Primary CNS tumors

CNS Tumor Classification - The Brainy Bunch

• Glial Tumors: Arise from the brain's supportive tissues (neuroglia).
• Non-Glial Tumors: Develop from other CNS structures like meninges, cranial nerves, and lymphoid tissue.

⭐ Most common primary adult CNS tumors: Glioblastoma (a type of astrocytoma), Meningioma, and Schwannoma.

Astrocytomas - Star-Powered Tumors

• Most common adult primary brain tumors; GFAP ⊕.
• Pilocytic Astrocytoma (WHO Grade I)
  • Children, often cerebellar.
  • Histology: Rosenthal fibers, eosinophilic granular bodies.
  • Good prognosis.
• Diffuse Astrocytoma (WHO Grade II)
  • Poorly defined, infiltrative tumors in adults.
• Anaplastic Astrocytoma (WHO Grade III)
  • Increased cellularity, pleomorphism, and mitotic activity.
• Glioblastoma (GBM, WHO Grade IV)
  • Most common & malignant.
  • Central necrosis surrounded by tumor cells (pseudopalisading).
  • Marked vascular proliferation.

⭐ High-Yield: Glioblastoma often arises in the cerebral hemisphere and can cross the corpus callosum, creating a "butterfly glioma" appearance on imaging.

Oligo & Ependy - The Other Gliomas

• Oligodendroglioma
  • Histology: Uniform "fried-egg" cells with delicate "chicken-wire" capillaries.
  • Genetics: 1p/19q co-deletion + IDH mutation is pathognomonic.
  • Location: Cerebral white matter, typically frontal lobes.
  • Presentation: Seizures are a common initial symptom.

• Ependymoma
  • Histology: Perivascular pseudorosettes (tumor cells around vessels); true rosettes are less common.
  • Location: Children → 4th ventricle (may cause hydrocephalus); Adults → spinal cord.

⭐ The 1p/19q co-deletion in oligodendrogliomas is a powerful prognostic marker, predicting a favorable response to chemotherapy.

Meningiomas & Schwannomas - The Outer Limits

• Meningioma

  • Benign tumor of arachnoid cap cells; extra-axial (external to brain parenchyma).
  • Often asymptomatic; may cause seizures or focal neurologic deficits due to compression.
  • Histology: Whorled pattern of spindle cells, psammoma bodies (laminated calcifications).
  • Imaging: Well-defined, dural-based mass with a characteristic “dural tail.”

• Schwannoma

  • Benign tumor of Schwann cells; S-100 positive.
  • Most common at cerebellopontine angle (CPA), affecting CN VIII (Vestibular Schwannoma).
  • Histology: Biphasic pattern with dense Antoni A and loose Antoni B areas.

⭐ Both tumor types are strongly associated with mutations in the NF2 gene on chromosome 22. Bilateral vestibular schwannomas are pathognomonic for Neurofibromatosis Type 2.

Medulloblastoma - Little Brain, Big Trouble

• What: Most common malignant brain tumor in children. A type of embryonal neuroepithelial tumor (PNET).
• Where: Exclusively in the cerebellum. In children, typically at the cerebellar vermis (midline).
• Presentation: Truncal ataxia, signs of ↑ ICP (hydrocephalus from 4th ventricle compression).
• Histo: Sheets of small, round, blue cells; Homer-Wright rosettes.
• Spread: High risk of CSF dissemination → drop metastases.

⭐ Homer-Wright rosettes are characteristic, showing tumor cells arranged in a circle around a central region filled with neurofibrillary material (neuropil).

High‑Yield Points - ⚡ Biggest Takeaways

• Glioblastoma is the most common and aggressive adult primary tumor, showing a butterfly appearance and pseudopalisading necrosis.
• Meningiomas are typically benign tumors from arachnoid cells, often presenting with psammoma bodies.
• Schwannomas are S-100 positive and classically involve the cerebellopontine angle (CN VIII).
• Oligodendrogliomas feature "fried egg" cells and delicate "chicken-wire" calcifications.
• Pilocytic Astrocytoma, the most common childhood tumor, contains Rosenthal fibers.
• Medulloblastoma is a malignant childhood tumor of the cerebellum that forms Homer-Wright rosettes.