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# Medulloblastoma and other embryonal tumours

- Lesion

Medulloblastoma is the commonest malignant brain tumour of children: small blue cells in the cerebellum that can spread through the CSF.

Updated Nov 12, 2026

## Key points

- Medulloblastoma: CNS WHO grade 4; sheets of small blue cells with scant cytoplasm, many mitoses, and Homer Wright (neuroblastic) rosettes.
- Midline vermis tumour causes truncal ataxia and hydrocephalus; spread by CSF to the spinal cord (drop metastases).
- WNT group: CTNNB1 mutation, nuclear beta-catenin, older children, best prognosis.
- SHH group: PTCH1, SUFU, SMO or TP53; desmoplastic-nodular histology, infants and adults; Gorlin syndrome.
- Group 3: MYC amplification, worst prognosis; group 4: the commonest group.
- Atypical teratoid/rhabdoid tumour: infants, rhabdoid cells, loss of nuclear INI1 (SMARCB1).

## What it is

Embryonal tumours are highly malignant small round blue cell tumours of the CNS, mostly in children. Medulloblastoma arises in the cerebellum, usually the midline vermis, and spreads through the CSF. It is classified into four molecular groups, WNT, SHH, group 3 and group 4, which differ in age, site and outcome. Atypical teratoid/rhabdoid tumour affects infants, is marked by loss of SMARCB1 (INI1) and carries a poor prognosis. Despite their aggressiveness, many medulloblastomas respond to radiation and chemotherapy.

## Structure

- Gross: soft grey-pink well-demarcated or infiltrative mass in the vermis, often filling the fourth ventricle.
- Variants: desmoplastic-nodular (reticulin-free nodules), extensive nodularity (infants), large cell-anaplastic (aggressive).
- Neuronal markers such as synaptophysin are positive.

## Medulloblastoma molecular groups

| Group | Genetic hallmark | Typical patient | Prognosis |
| --- | --- | --- | --- |
| WNT | CTNNB1 mutation, monosomy 6 | Older children, teenagers | Excellent |
| SHH | PTCH1, SUFU, SMO; TP53 in some | Infants and adults; Gorlin syndrome | Intermediate; poor with TP53 |
| Group 3 | MYC amplification | Infants and young children | Poor |
| Group 4 | Isochromosome 17q, MYCN | Children | Intermediate |

## Markers

- Synaptophysin
- Beta-catenin (nuclear in WNT)
- GAB1, YAP1 (SHH)
- INI1 loss (AT/RT)
- Ki-67 high

## Clinical notes

- Children present with morning headache, vomiting, gait ataxia and papilledema.
- Craniospinal radiation is part of treatment beyond infancy, with long-term cognitive and endocrine effects.

## Memory aid

WNT wins, SHH in the middle, group 3 is grim; and the blue cells ride the CSF down the spine.

## Test yourself

Think of the answer, then tap the question to see it.

## What is a Homer Wright rosette?

A ring of tumour cells around a central tangle of fibrillary processes without a lumen, typical of medulloblastoma and other neuroblastic tumours.

## Which medulloblastoma group has the best prognosis?

The WNT group, with CTNNB1 mutation and nuclear beta-catenin.

## Which gene is lost in atypical teratoid/rhabdoid tumour?

SMARCB1 (INI1), shown by loss of nuclear staining.

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## Connections

## Related 7

- [CNS tumours: an overview](https://durgical.com/en/general-pathology/cns-tumours-an-overview/)
- [Ependymal, choroid plexus and glioneuronal tumours](https://durgical.com/en/general-pathology/ependymal-choroid-plexus-and-glioneuronal-tumours/)
- [Suprasellar and pineal tumours: craniopharyngioma](https://durgical.com/en/general-pathology/suprasellar-and-pineal-tumours-craniopharyngioma/)
- [Intraocular tumours: uveal melanoma and retinoblastoma](https://durgical.com/en/general-pathology/intraocular-tumours-uveal-melanoma-and-retinoblastoma/)
- [Tumours of infancy and childhood](https://durgical.com/en/general-pathology/tumours-of-infancy-and-childhood/)
- [Differentiation and anaplasia: how a tumour cell looks under the microscope](https://durgical.com/en/general-pathology/differentiation-and-anaplasia-how-a-tumour-cell-looks-under-the-microscope/)
- [Tumour suppressor genes: RB, TP53, APC, BRCA and the two-hit hypothesis](https://durgical.com/en/general-pathology/tumour-suppressor-genes-rb-tp53-apc-brca-and-the-two-hit-hypothesis/)

## Related conditions 2

- [Childhood brain tumours (medulloblastoma, pilocytic astrocytoma, craniopharyngioma)](https://durgical.com/en/conditions/childhood-brain-tumours/)
- [Hydrocephalus](https://durgical.com/en/conditions/hydrocephalus/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Childhood Medulloblastoma and Other Central Nervous System Embryonal Tumors Treatment (PDQ®)–Health Professional Version](https://www.cancer.gov/types/brain/hp/child-cns-embryonal-treatment-pdq) (National Cancer Institute, 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/medulloblastoma-and-other-embryonal-tumours/