Pilocytic astrocytoma

Pilocytic astrocytomas (PA), also known as juvenile pilocytic astrocytomas (JPA), are low-grade, relatively well-defined astrocytomas (WHO Grade I). They tend to occur in young patients and have a relatively good prognosis.

They range in appearance on imaging, with the majority presenting as a large cystic lesion with a brightly enhancing mural nodule. Calcification can be present in around one fifth of cases. They typically arise from midline structures, and around 60% are in the cerebellum. 


Pilocytic astrocytomas are tumours of young people, with 75% occurring in the first two decades of life, typically late in the first decade (9-10 years). There is no recognised gender predisposition.

Although only accounting for between 0.6-5.1% of all intracranial neoplasms (1.7-7% of all glial tumours) they are the most common primary brain tumour of childhood, accounting for 70-85% of all cerebellar astrocytomas.

Clinical presentation

Presentation depends on location. In the posterior fossa tumours there is predominantly a mass effect with signs of raised intracranial pressure, especially when hydrocephalus is present. Bulbar symptoms or cerebellar symptoms may also be present.


The term pilocytic refers to the the elongated hair-like projections from the neoplastic cells 4. The presence of eosinophilic Rosenthal fibres is a characteristic feature, and hyalinisation of blood vessels is also common. Pilocytic astrocytoma, as well as pleomorphic xanthoastrocytomas, frequently have BRAF alterations, but lack IDH mutations 6


There is a strong association with neurofibromatosis type 1 (NF1). NF1 associated tumours have a tendency to affect the optic nerves and chiasm and may also have a better prognosis. Pilocytic astrocytomas are seen in up to 20% of all patients with NF1 and typically manifest in early childhood. Approximately one-third of pilocytic astrocytomas involving the optic nerves have associated NF1.


By far the most common location is the cerebellum (60%). The distribution within the cerebellum varies with many tumours involving both the vermis and the cerebellar hemisphere.

In general they typically arise from midline structures.

Radiographic features


Pilocytic astrocytomas range in appearance:

  • large cystic component with a brightly enhancing mural nodule: 67%
    • non enhancing cyst wall: 21%
    • enhancing cyst wall: 46%
  • heterogeneous, mixed solid and multiple cysts and central necrosis: 16%
  • completely solid: 17%

Enhancement is almost invariably present (~95%). Up to 20% may demonstrate some calcification. Haemorrhage (case 1) is a rare complication.


Signal characteristics include:

  • T1: iso to hypointense solid component compared to adjacent brain
  • T2: hyperintense solid component compared to adjacent brain

Treatment and prognosis

They are slow growing well circumscribed tumours with an overall good prognosis following treatment (5-year survival >90%; 20-year survival >70%). Cystic tumours have even better prognosis while fibrillary variants tend to do worse.

Surgical resection, if complete, is usually curative. Some surgeons advocate that only the nodule need be resected to effect cure, as the cyst walls are non neoplastic, even if enhancing.

Differential diagnosis

General imaging differential considerations include:

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