Pilocytic astrocytoma

Last revised by Henry Knipe on 17 Nov 2023

Pilocytic astrocytomas, also known as juvenile pilocytic astrocytomas, are circumscribed astrocytic gliomas that typically occur in young patients. The majority of sporadic pilocytic astrocytomas arise from the cerebellum, whereas in the setting of neurofibromatosis type 1 (NF1), they often involve the optic chiasm and pathway. In adults, they are typically supratentorial 11.

Pilocytic astrocytomas are considered WHO grade 1 tumors in the current WHO classification of CNS tumors and correspondingly have a relatively good prognosis.

These tumors have a range of imaging appearances, 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.

Optic pathway gliomas and spinal cord pilocytic astrocytomas are discussed separately. The remainder of this article focuses on a general discussion of pilocytic astrocytomas, particularly those in the cerebellum. 

Pilocytic astrocytomas are tumors of young people, with ~70% occurring in the first two decades of life, typically late in the first decade (9-10 years) 11. They do, however, also occasionally arise in adults 11.

Although they account for ~2.5% (range 0.6-5.1%) of all intracranial neoplasms (~4% of all glial tumors), they are the most common primary brain tumor of childhood (~15%) 7 and the second most common pediatric posterior fossa tumor (~30%) after medulloblastomas 10.

There is a strong association with NF1. NF1-associated tumors have a tendency to affect the optic nerves and chiasm (see: optic pathway glioma). The association between NF1 and pilocytic astrocytomas is so strong that up to 20% of all patients with NF1 will develop these tumors, typically in early childhood. Conversely, approximately one-third of pilocytic astrocytomas involving the optic nerves have associated NF1.

Presentation depends on location. In posterior fossa tumors, 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.

By far the most common location is the cerebellum, with the optic pathway being the next most common, particularly in patients with NF1. In adults, most tumors are supratentorial 11.

The distribution within the cerebellum varies with many tumors involving both the vermis and the cerebellar hemisphere.

In general, pilocytic astrocytomas typically arise from midline structures.

  • cerebellum

    • 60%

  • optic pathway (optic nerve, optic chiasm, hypothalamus, optic radiation)

  • other less common locations

    • brainstem

    • cerebral hemispheres: more frequent in adults 11

    • cerebral ventricles

    • velum interpositum

    • spinal cord: see spinal pilocytic astrocytoma

The term pilocytic refers to the elongated hair-like projections from the neoplastic cells 4. The presence of eosinophilic Rosenthal fibers is a characteristic feature and hyalinization of blood vessels is also common. The histological features are, however, fairly heterogeneous even within the one tumor, with some areas mimicking diffuse astrocytomas and even oligodendrogliomas 6

Immunohistochemistry reflects astrocytic differentiation 6

Pilocytic astrocytoma, as well as pleomorphic xanthoastrocytomas, frequently have BRAF alterations (present in ~70% of cases). Importantly they, along with other pediatric low-grade gliomas, lack IDH mutations and TP53 mutations 6,7

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. Hemorrhage is an uncommon complication.

  • T1

    • solid component: iso- to hypointense compared to adjacent brain

    • cystic component: fluid signal unless hemorrhagic

  • T1 C+ (Gd)

    • vivid contrast enhancement

    • the cyst wall enhances in ~50% of cases

  • T2

    • solid component: hyperintense compared to adjacent brain 

    • cystic component: high signal

  • T2*/GRE/SWI: signal loss if calcification or hemorrhage present

They are slow-growing well-circumscribed tumors with an overall good prognosis in children following treatment (5-year and 10-year survival >95%) 6. Cystic tumors have an even better prognosis while fibrillary variants tend to do worse.

Surgical resection, if complete, is usually curative. Some surgeons advocate that only the nodule needs to be resected to achieve a cure, as the cyst walls are non-neoplastic, even if enhancing 2.

Adults have a poorer prognosis, with a ~50% 5-year survival 11,12.

It is also worth noting that historically some patients with neurofibromatosis type 1 and high-grade cerebellar tumors diagnosed as pilocytic astrocytomas, actually represented high-grade astrocytoma with piloid features.

In 1931, Harvey Cushing was the first who described this tumor based on his studies of 76 cases of cerebellar astrocytomas 5. Pilocytic means "hair-like" and is derived from the Latin word pilus for hair.

General imaging differential considerations include:

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Cases and figures

  • Figure 1: histology - Rosenthal fibers with H & E stain
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  • Case 1
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  • Case 2
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  • Case 3
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  • Case 4: with hypertrophic olivery degeneration
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  • Case 5
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  • Case 6
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  • Case 7
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  • Case 8
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  • Case 9: predominantly cystic
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  • Case 10
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  • Case 11
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  • Case 12: with hemorrhage
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  • Case 13: brainstem
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  • Case 14: brainstem
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  • Case 15: midbrain
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  • Case 16: suprasellar
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  • Case 17: third ventricle
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  • Case 18: third ventricle
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  • Case 19: periventricular
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  • Case 20: trigone
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  • Case 21: supratentorial
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  • Case 22: supratentorial
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  • Case 23: craniocervical junction
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  • Case 24
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