Pilocytic astrocytomas, also known as juvenile pilocytic astrocytomas, are low-grade, relatively well-defined astrocytomas that tend to occur in young patients. They are considered WHO grade I tumours in the current (2016) WHO classification of CNS tumours and correspondingly have a relatively good prognosis.
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.
The remainder of this article focuses on a general discussion of pilocytic astrocytomas, particularly those 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.
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.
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.
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.
- optic pathway (optic nerve, optic chiasm, hypothalamus, optic radiation)
- 25-30%, particularly common in NF1
- see: optic pathway glioma
- other less common locations
- cerebral hemispheres: more frequent in adults
- 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 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.
Immunohistochemistry reflects the astrocytic differentiation 6:
- GFAP: positive
- S100: positive
- OLIG2: positive
- IDH R132H mutation: negative
- p53 protein: negative or weak
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 is an uncommon complication.
- solid component: iso to hypointense compared to adjacent brain
- cystic component: ~fluid signal unless haemorrhage
- vivid contrast enhancement
- the cyst wall enhances in ~50% cases
- solid component: hyperintense compared to adjacent brain
- cystic component: high signal
- 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 and 10-year survival >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.
General imaging differential considerations include:
- typically arise from the midline (especially vermis and roof of the fourth ventricle) rather than cerebellar hemisphere
- usually seen in younger patients (2-6 years of age)
atypical teratoid/rhabdoid tumour
- larger heterogeneously enhancing mass
- usually seen in adults
- in children, usually associated with von Hippel-Lindau disease
- cyst wall usually does not enhance
- has a different clinical presentation
- no enhancing nodule
- pleomorphic xanthoastrocytoma (PXA)
- 1. Koeller KK, Rushing EJ. From the archives of the AFIP: pilocytic astrocytoma: radiologic-pathologic correlation. Radiographics. 24 (6): 1693-708. doi:10.1148/rg.246045146 - Pubmed citation
- 2. Beni-adani L, Gomori M, Spektor S et-al. Cyst wall enhancement in pilocytic astrocytoma: neoplastic or reactive phenomena. Pediatr Neurosurg. 2000;32 (5): 234-9. Pediatr Neurosurg (link) - Pubmed citation
- 3. Barkovich AJ. Pediatric neuroimaging. Lippincott Williams & Wilkins. (2005) ISBN:0781757665. Read it at Google Books - Find it at Amazon
- 4. Drevelegas A. Imaging of Brain Tumors with Histological Correlations. Springer. (2011) ISBN:3540876502. Read it at Google Books - Find it at Amazon
- 5. Koeller KK, Rushing EJ. From the archives of the AFIP: pilocytic astrocytoma: radiologic-pathologic correlation. Radiographics. 2004;24 (6): 1693-708. Radiographics (full text) - doi:10.1148/rg.246045146 - Pubmed citation
- 6. Louis DN, Perry A, Reifenberger G et-al. The 2016 World Health Organization Classification of Tumors of the Central Nervous System: a summary. Acta Neuropathol. 2016;131 (6): 803-20. doi:10.1007/s00401-016-1545-1 - Pubmed citation
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