[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Central nervous system](https://durgical.com/en/general-pathology/areas/cns/) › Pilocytic astrocytoma

# 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.

Updated Nov 12, 2026

## Key points

- Cerebellar cystic mass with an enhancing mural nodule in a child.
- Biphasic pattern: compact areas of bipolar hair-like (piloid) cells with Rosenthal fibres and loose microcystic areas with eosinophilic granular bodies.
- Vascular proliferation can occur and does not mean higher grade here.
- KIAA1549-BRAF fusion in cerebellar tumours; NF1 loss in optic pathway gliomas.
- Subependymal giant cell astrocytoma: tuberous sclerosis, near the foramen of Monro, causes hydrocephalus; responds to mTOR inhibitors.
- Pleomorphic xanthoastrocytoma: superficial temporal tumour of young people with bizarre and lipid-laden cells; BRAF V600E.

## 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](https://durgical.com/en/conditions/childhood-brain-tumours/) | Cerebellum, optic pathway, hypothalamus, brainstem | KIAA1549-BRAF fusion, NF1 | Neurofibromatosis type 1 (optic glioma) |
| [Pleomorphic xanthoastrocytoma](https://durgical.com/en/conditions/epilepsy-and-seizures/) | Superficial temporal lobe | BRAF V600E, CDKN2A loss | Epilepsy in young people |
| [Subependymal giant cell astrocytoma](https://durgical.com/en/conditions/tuberous-sclerosis-complex/) | 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.

## Memory aid

Pilocytic: hairs, a cyst and a nodule in a child's cerebellum, and Rosenthal fibres like pink corkscrews.

## 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.

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## Connections

## Related 6

- [CNS tumours: an overview](https://durgical.com/en/general-pathology/cns-tumours-an-overview/)
- [Adult diffuse gliomas](https://durgical.com/en/general-pathology/adult-diffuse-gliomas/)
- [Familial tumour syndromes of the nervous system](https://durgical.com/en/general-pathology/familial-tumour-syndromes-of-the-nervous-system/)
- [Ependymal, choroid plexus and glioneuronal tumours](https://durgical.com/en/general-pathology/ependymal-choroid-plexus-and-glioneuronal-tumours/)
- [Tumours of infancy and childhood](https://durgical.com/en/general-pathology/tumours-of-infancy-and-childhood/)
- [Oncogenes: RAS, MYC, HER2, BCR-ABL and the table to know](https://durgical.com/en/general-pathology/oncogenes-ras-myc-her2-bcr-abl-and-the-table-to-know/)

## Related conditions 4

- [Childhood brain tumours (medulloblastoma, pilocytic astrocytoma, craniopharyngioma)](https://durgical.com/en/conditions/childhood-brain-tumours/)
- [Neurofibromatosis (types 1 and 2)](https://durgical.com/en/conditions/neurofibromatosis/)
- [Tuberous sclerosis complex](https://durgical.com/en/conditions/tuberous-sclerosis-complex/)
- [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 Astrocytomas, Other Gliomas, and Glioneuronal/Neuronal Tumors Treatment (PDQ®)–Health Professional Version](https://www.cancer.gov/types/brain/hp/child-astrocytoma-glioma-treatment-pdq) (National Cancer Institute, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/pilocytic-astrocytoma/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/pilocytic-astrocytoma/