Pilocytic astrocytoma
- Lesion
Pilocytic astrocytoma is a slow, well-defined childhood tumour, often a cyst in the cerebellum, that can often be cured by surgery.
What it is
Unlike diffuse gliomas, circumscribed astrocytic tumours grow as well-defined masses. Pilocytic astrocytoma, CNS WHO grade 1, is the commonest brain tumour of children, typically in the cerebellum as a cyst with a mural nodule, or in the optic nerve and hypothalamus, where it is associated with neurofibromatosis type 1. Its main driver is activation of the MAPK pathway, most often by a KIAA1549-BRAF fusion. Related tumours are pleomorphic xanthoastrocytoma (BRAF V600E) and subependymal giant cell astrocytoma of tuberous sclerosis.
Structure
- Gross: cyst with a solid nodule, or a solid fusiform enlargement of the optic nerve.
- Micro: elongated cells with long fine processes, Rosenthal fibres (corkscrew, eosinophilic) and eosinophilic granular bodies; few mitoses.
- GFAP strongly positive; IDH not mutated.
Circumscribed astrocytic tumours
| Tumour | Where | Driver | Association |
|---|---|---|---|
| Pilocytic astrocytoma | Cerebellum, optic pathway, hypothalamus, brainstem | KIAA1549-BRAF fusion, NF1 | Neurofibromatosis type 1 (optic glioma) |
| Pleomorphic xanthoastrocytoma | Superficial temporal lobe | BRAF V600E, CDKN2A loss | Epilepsy in young people |
| Subependymal giant cell astrocytoma | Wall of lateral ventricle near foramen of Monro | TSC1 or TSC2 loss (mTOR active) | Tuberous sclerosis |
Clinical notes
- Cerebellar tumours cause ataxia and hydrocephalus with headache and vomiting.
- Complete resection is often curative; BRAF and MEK inhibitors are used for unresectable tumours.
Test yourself
Think of the answer, then tap the question to see it.
What is the classic imaging appearance of a cerebellar pilocytic astrocytoma?
A cyst with an enhancing mural nodule.
Which genetic alteration drives most cerebellar pilocytic astrocytomas?
A KIAA1549-BRAF fusion that activates the MAPK pathway.
Which syndrome is associated with optic pathway pilocytic astrocytoma?
Neurofibromatosis type 1.
Connections
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 Astrocytomas, Other Gliomas, and Glioneuronal/Neuronal Tumors Treatment (PDQ®)–Health Professional Version (National Cancer Institute, 2026).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.