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.
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 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. 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.
optic nerve/optic chiasm (25-30%)
- very common location in NF1
- hypothalamus/adjacent to the third ventricle
- other less common locations
- cerebral hemispheres: more frequent in adults
- cerebral ventricles
- velum interpositum
- spinal cord
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 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 heterogeneous variably enhancing mass
- usually seen in adults
- associated with von Hippel Lindau disease
- pleomorphic xanthoastrocytoma (PXA)
- cerebellar abscess: has a different clinical presentation and has no enhancing nodule
- WHO classification of CNS tumours
- WHO grading of CNS tumours
- VASARI MRI feature set
- diffuse astrocytoma grading
- grade I:
- grade II:
- grade III
- anaplastic astrocytoma
- anaplastic oligodendroglioma
- grade IV:
- glioblastoma vs cerebral metastasis
- radiation-induced gliomas
- gliomatosis cerebri (growth pattern)
- specific locations
- treatment response
- Stupp protocol
- glioma treatment response assessment in clinical trials
- multicentric glioblastoma
- multifocal glioblastoma
- prognostic genetic markers
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- 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