Pilocytic astrocytoma
Updates to Article Attributes
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.
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.
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
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.
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.
-
optic nerve/optic chiasm (25-30%)
- very common location in NF1
- hypothalamus/adjacent to third ventricle
- brainstem
- other less common locations
- cerebral hemispheres: more frequent in adults
- cerebral ventricles
- velum interpositum
- spinal cord
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 rare complication.
MRI
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:
-
medulloblastoma
- 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
-
ependymoma
- tends to fill the fourth ventricle and protrude out of the foramen of Luschka and foramina of Magendie
- large cystic component less common
-
haemangioblastoma
- usually seen in adults
- associated with von Hippel Lindau disease
- ganglioglioma
- pleomorphic xanthoastrocytoma (PXA)
- cerebellar abscess: has a different clinical presentation and has no enhancing nodule
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