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Posterior fossa tumours in children
In a child's posterior fossa the three main tumours are told apart by location: medulloblastoma in the vermis and roof of the fourth ventricle, ependymoma from the floor of the fourth ventricle extending through the foramina, and pilocytic astrocytoma in the cerebellar hemisphere with a cyst and nodule — alongside the pontine glioma, which is not operated on.
Why it matters in the exam
A child with morning headache, vomiting and ataxia and an MRI showing a posterior fossa mass with hydrocephalus — a classic scenario. Medulloblastoma is the commonest malignant brain tumour of childhood; the commonest histology is "classic", and molecular grouping sets the prognosis: WNT — the best, SHH — intermediate and dependent on TP53, and Group 3 — the worst, with MYC. Good prognosis is also linked to near-complete resection (residual under 1.5 square centimetres), absence of CSF metastases and age above 3. Treatment: maximal resection, craniospinal irradiation (avoided under age 3) and chemotherapy; cisplatin causes hearing loss and vincristine neuropathy.
Posterior fossa ependymoma arises from the floor of the fourth ventricle and squeezes through the foramina of Luschka and Magendie into the cisterns — which makes complete removal difficult; histology shows perivascular pseudorosettes. Complete resection is the key prognostic factor, followed by focal radiation; chemotherapy has a small role. Pilocytic astrocytoma is a cyst with an enhancing mural nodule in the cerebellar hemisphere, with Rosenthal fibres and a KIAA1549-BRAF fusion; complete resection usually cures. Diffuse midline glioma with H3 K27M in the pons is diagnosed on MRI, is not resected and is treated with palliative radiation. Persistent hydrocephalus after resection is predicted by the CPPRH rule, in which age under two years and metastases carry the highest weight.
Key points
- Medulloblastoma — vermis; WNT best, Group 3 worst.
- Ependymoma — floor of the fourth ventricle; pseudorosettes; complete resection decides.
- Pilocytic — cyst and nodule; cure by resection.
- Pontine DMG — radiation, no surgery.
- Cisplatin — deafness; vincristine — neuropathy.
Summary table
| Tumour | Location and imaging | Histology or molecular | Treatment and prognosis |
|---|---|---|---|
| Medulloblastoma | Vermis, roof of the fourth ventricle, solid lesion | Classic; WNT, SHH, Group 3, Group 4 | Resection, craniospinal radiation, chemo; WNT best |
| Ependymoma | Floor of the fourth ventricle, extension through the foramina | Perivascular pseudorosettes | Complete resection and focal radiation |
| Pilocytic astrocytoma | Cerebellar hemisphere, cyst with nodule | Rosenthal fibres, KIAA1549-BRAF | Complete resection — usually curative |
| Pontine glioma (DMG) | Expanded pons, no surgery | H3 K27M | Palliative radiation; poor prognosis |
| AT/RT | Child under 3 | Loss of SMARCB1 (INI1) | Aggressive treatment; poor prognosis |
In the full version for subscribers
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Sources
- WHO Classification of Tumours: Central Nervous System Tumours, 5th edition (2021) — Embryonal tumours; Ependymal tumours; Circumscribed astrocytic gliomas
- Children's Oncology Group and SIOP-Europe treatment frameworks for medulloblastoma, current protocols — Risk stratification
- Canadian Preoperative Prediction Rule for Hydrocephalus (Riva-Cambrin), 2009 — Persistent hydrocephalus after posterior fossa tumour resection
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.