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Pilocytic astrocytoma

Changed by Frank Gaillard, 15 May 2017
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Pilocytic astrocytomas (PA), also known as juvenile pilocytic astrocytomas (JPA), are low-grade, relatively well-defined astrocytomas (WHO Grade I). They that tend to occur in young patients and. They are considered WHO grade I tumours in the current (2016) WHO classification of CNS tumours and correspondingly have a relatively good prognosis.

TheyThese tumours have a range in appearance onof 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-fifth of cases. They typically

The majority of pilocytic astrocytomas arise from midline structuresthe cerebellum. Optic pathway gliomas and spinal cord pilocytic astrocytomas are discussed separately. 

The remainder of this article focuses on a general discussion of pilocytic astrocytomas, and around 60% areparticularly those in the cerebellum. 

Epidemiology

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.

Associations

There is a strong association with neurofibromatosis type 1 (NF1). NF1 associated tumours 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 tumours, typically in early childhood. Conversely, approximately one-third of pilocytic astrocytomas involving the optic nerves have associated NF1.

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.

Pathology

Location

By far the most common location is the cerebellum, with optic pathway being the next most common, particularly in patients with neurofibromatosis type 1. The distribution within the cerebellum varies with many tumours 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
    • cerebral ventricles
    • velum interpositum
    • spinal cord, see: spinal pilocytic astrocytoma
Microscopic appearance

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

Immunophenotype

Immunohistochemistry reflects the astrocytic differentiation 6: 

Genetics

Pilocytic astrocytoma, as well as pleomorphic xanthoastrocytomas, frequently have BRAF alterations but(present in ~70% of cases). Importantly they lack IDH mutations and TP53 mutations6. 

Associations

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.

Location

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

General

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 rarean uncommon complication.

MRI

Signal characteristics include:

  • T1
    • solid component:iso to hypointense solid component compared to adjacent brain
    • cystic component: ~fluid signal unless haemorrhage
  • T1 C+
    • vivid contrast enhancement
    • the cyst wall enhances in ~50% cases
  • T2: hyperintense
    • solid component: hyperintense compared to adjacent brain
    • cystic component: high signal
  • T2*:
    • signal loss if calcification or haemorrhage present

Treatment and prognosis

They are slow growing well-circumscribed tumours with an overall good prognosis following treatment (5-year survival >90%; 20and 10-year survival >70;95%)6. 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 a cure, as the cyst walls are non-neoplastic, even if enhancing 2.

Differential diagnosis

General imaging differential considerations include:

  • -<p><strong>Pilocytic astrocytomas (PA)</strong>, also known as <strong>juvenile pilocytic astrocytomas (JPA)</strong>, are low-grade, relatively well-defined <a href="/articles/astrocytic-tumours">astrocytomas</a> (<a href="/articles/who-classification-of-cns-tumours-1">WHO Grade I</a>). They tend to occur in young patients and have a relatively good prognosis.</p><p>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. </p><h4>Epidemiology</h4><p>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.</p><p>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.</p><h4>Clinical presentation</h4><p>Presentation depends on location. In the <a href="/articles/posterior-fossa-tumours">posterior fossa tumours</a> 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.</p><h4>Pathology</h4><p>The term pilocytic refers to the elongated hair-like projections from the neoplastic cells <sup>4</sup>. The presence of eosinophilic <a href="/articles/rosenthal-fibres">Rosenthal fibres</a> is a characteristic feature, and hyalinization of blood vessels is also common. Pilocytic astrocytoma, as well as <a href="/articles/pleomorphic-xanthoastrocytoma">pleomorphic xanthoastrocytomas</a>, frequently have <a href="/articles/braf-1"><em>BRAF</em></a> alterations but lack IDH mutations <sup>6</sup>. </p><h5>Associations</h5><p>There is a strong association with <a href="/articles/neurofibromatosis-type-1">neurofibromatosis type 1 (NF1)</a>. NF1 associated tumours have a tendency to affect the <a href="/articles/optic-nerve">optic nerves</a> 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.</p><h5>Location</h5><p>By far the most common location is the <a href="/articles/cerebellum">cerebellum</a> (60%). The distribution within the cerebellum varies with many tumours involving both the vermis and the cerebellar hemisphere.</p><p>In general, they typically arise from midline structures.</p><ul>
  • -<li>
  • -<a href="/articles/optic-nerve">optic nerve</a>/<a href="/articles/optic-chiasm">optic chiasm</a> (25-30%)<ul><li>very common location in NF1</li></ul>
  • +<p><strong>Pilocytic astrocytomas</strong>, also known as <strong>juvenile pilocytic astrocytomas</strong>, are low-grade, relatively well-defined <a href="/articles/astrocytic-tumours">astrocytomas</a> that tend to occur in young patients. They are considered WHO grade I tumours in the current (2016) <a href="/articles/who-classification-of-cns-tumours-1">WHO classification of CNS tumours</a> and correspondingly have a relatively good prognosis.</p><p>These tumours 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.</p><p>The majority of pilocytic astrocytomas arise from the cerebellum. <a href="/articles/optic-pathway-glioma">Optic pathway gliomas</a> and <a href="/articles/spinal-pilocytic-astrocytoma">spinal cord pilocytic astrocytomas</a> are discussed separately. </p><p>The remainder of this article focuses on a general discussion of pilocytic astrocytomas, particularly those in the cerebellum. </p><h4>Epidemiology</h4><p>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.</p><p>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.</p><h5>Associations</h5><p>There is a strong association with <a href="/articles/neurofibromatosis-type-1">neurofibromatosis type 1 (NF1)</a>. NF1 associated tumours have a tendency to affect the optic nerves and chiasm (see: <a href="/articles/optic-pathway-glioma">optic pathway glioma</a>). The association between NF1 and pilocytic astrocytomas is so strong that up to 20% of all patients with NF1 will develop these tumours, typically in early childhood. Conversely, approximately one-third of pilocytic astrocytomas involving the optic nerves have associated NF1.</p><h4>Clinical presentation</h4><p>Presentation depends on location. In the <a href="/articles/posterior-fossa-tumours">posterior fossa tumours</a>, 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.</p><h4>Pathology</h4><h5>Location</h5><p>By far the most common location is the cerebellum, with optic pathway being the next most common, particularly in patients with <a href="/articles/neurofibromatosis-type-1">neurofibromatosis type 1</a>. The distribution within the cerebellum varies with many tumours involving both the vermis and the cerebellar hemisphere.</p><p>In general, pilocytic astrocytomas typically arise from midline structures.</p><ul>
  • +<li>cerebellum<ul><li>60%</li></ul>
  • -<li>
  • -<a href="/articles/hypothalamus">hypothalamus</a>/adjacent to the <a href="/articles/third-ventricle">third ventricle</a>
  • +<li>optic pathway (optic nerve, optic chiasm, hypothalamus, optic radiation)<ul>
  • +<li>25-30%, particularly common in NF1</li>
  • +<li>see: <a href="/articles/optic-pathway-glioma">optic pathway glioma</a>
  • +</li>
  • +</ul>
  • -<li><a href="/articles/brainstem">brainstem</a></li>
  • +<li>brainstem</li>
  • -<li>spinal cord</li>
  • +<li>spinal cord, see: <a href="/articles/spinal-pilocytic-astrocytoma">spinal pilocytic astrocytoma</a>
  • +</li>
  • -</ul><h4>Radiographic features</h4><h5>General</h5><p>Pilocytic astrocytomas range in appearance:</p><ul>
  • +</ul><h5>Microscopic appearance</h5><p>The term pilocytic refers to the elongated hair-like projections from the neoplastic cells <sup>4</sup>. The presence of eosinophilic <a href="/articles/rosenthal-fibres">Rosenthal fibres</a> is a characteristic feature, and hyalinization of blood vessels is also common. The histological features are, however, fairly heterogeneous even within the one tumour, with some areas mimicing diffuse astrocytomas and even oligodendrogliomas <sup>6</sup>. </p><h5>Immunophenotype</h5><p><a href="/articles/immunohistochemistry">Immunohistochemistry</a> reflects the astrocytic differentiation <sup>6</sup>: </p><ul>
  • +<li>
  • +<a href="/articles/glial-fibrillary-acid-protein-gfap">GFAP</a>: positive</li>
  • +<li>
  • +<a href="/articles/s100">S100</a>: positive</li>
  • +<li>
  • +<a href="/articles/olig2">OLIG2</a>: positive</li>
  • +<li>
  • +<a href="/articles/isocitrate-dehydrogenase-idh">IDH R132H mutation</a>: negative</li>
  • +<li>
  • +<a href="/articles/p53">p53 protein</a>: negative or weak</li>
  • +</ul><h5>Genetics</h5><p>Pilocytic astrocytoma, as well as <a href="/articles/pleomorphic-xanthoastrocytoma">pleomorphic xanthoastrocytomas</a>, frequently have <a href="/articles/braf-1"><em>BRAF</em></a> alterations (present in ~70% of cases). Importantly they lack <a href="/articles/isocitrate-dehydrogenase-idh">IDH mutations</a> and <a href="/articles/tp53">TP53 mutations</a> <sup>6</sup>. </p><h4>Radiographic features</h4><h5>General</h5><p>Pilocytic astrocytomas range in appearance:</p><ul>
  • -</ul><p>Enhancement is almost invariably present (~95%). Up to 20% may demonstrate some calcification. Haemorrhage (case 1) is a rare complication.</p><h5>MRI</h5><p>Signal characteristics include:</p><ul>
  • +</ul><p>Enhancement is almost invariably present (~95%). Up to 20% may demonstrate some calcification. Haemorrhage is an uncommon complication.</p><h5>MRI</h5><ul>
  • +<li>
  • +<strong>T1</strong><ul>
  • +<li>solid component:<strong> </strong>iso to hypointense compared to adjacent brain</li>
  • +<li>cystic component: ~fluid signal unless haemorrhage</li>
  • +</ul>
  • +</li>
  • +<li>
  • +<strong>T1 C+</strong><ul>
  • +<li>vivid contrast enhancement</li>
  • +<li>the cyst wall enhances in ~50% cases</li>
  • +</ul>
  • +</li>
  • -<strong>T1:</strong> iso to hypointense solid component compared to adjacent brain</li>
  • +<strong>T2</strong><ul>
  • +<li>solid component: hyperintense compared to adjacent brain </li>
  • +<li>cystic component: high signal</li>
  • +</ul>
  • +</li>
  • -<strong>T2:</strong> hyperintense solid component compared to adjacent brain</li>
  • -</ul><h4>Treatment and prognosis</h4><p>They are slow growing well-circumscribed tumours with an overall good prognosis following treatment (5-year survival &gt;90%; 20-year survival &gt;70%). Cystic tumours have even better prognosis while fibrillary variants tend to do worse.</p><p>Surgical resection, if complete, is usually curative. Some surgeons advocate that only the nodule need be resected to effect a cure, as the cyst walls are non-neoplastic, even if enhancing <sup>2</sup>.</p><h4>Differential diagnosis</h4><p>General imaging differential considerations include:</p><ul>
  • +<strong>T2*:</strong><ul><li>signal loss if calcification or haemorrhage present</li></ul>
  • +</li>
  • +</ul><h4>Treatment and prognosis</h4><p>They are slow growing well-circumscribed tumours with an overall good prognosis following treatment (5-year and 10-year survival &gt;95%) <sup>6</sup>. Cystic tumours have even better prognosis while fibrillary variants tend to do worse.</p><p>Surgical resection, if complete, is usually curative. Some surgeons advocate that only the nodule need be resected to effect a cure, as the cyst walls are non-neoplastic, even if enhancing <sup>2</sup>.</p><h4>Differential diagnosis</h4><p>General imaging differential considerations include:</p><ul>
  • -<a href="/articles/atypical-teratoidrhabdoid-tumour">atypical teratoid/rhabdoid tumour</a>: larger heterogeneous variably enhancing mass</li>
  • +<a href="/articles/atypical-teratoidrhabdoid-tumour">atypical teratoid/rhabdoid tumour</a><ul><li>larger heterogeneously enhancing mass</li></ul>
  • +</li>
  • -<li>associated with <a href="/articles/von-hippel-lindau-disease-5">von Hippel Lindau disease</a>
  • +<li>in children, usually associated with <a href="/articles/von-hippel-lindau-disease-5">von Hippel-Lindau disease</a>
  • +</li>
  • +<li>cyst wall usually does not enhance</li>
  • +</ul>
  • +<li>
  • +<a href="/articles/cerebellar-abscess">cerebellar abscess</a><ul>
  • +<li>has a different clinical presentation</li>
  • +<li>no enhancing nodule</li>
  • -<li>
  • -<a href="/articles/cerebellar-abscess">cerebellar abscess</a>: has a different clinical presentation and has no enhancing nodule</li>

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