Pilocytic astrocytoma

A pilocytic astrocyoma (PA) (also termed a juvenile pilocytic astrocytoma (JPA)) is a type of low grade relatively well defined astrocytoma (WHO Grade I). It is found in young patients and has a good prognosis. 

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 more common posterior fossa tumours, positive mass effect with signs of raised intracranial pressure, especially when hydrocephalus is present predominate. 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 eosinophillic Rosenthal fibres are a characteristic feature, and hyalinization of blood vessels a common features. 

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 15 - 20% of all patients with NF1 and typically manifest in early childhood. Approximately 1/3 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 depending on the study. Safe enough to say that the location within the cerebellum is not very helpful, with many tumours involving both the vermis and the cerebellar hemisphere.

In general they typically they arise from midline structures.

• optic nerve / optic chiasm (25 - 30%)
  • very common location in NF1
• hypothalamic / 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 astroctyomas range in appearance.

• large cystic component with a brightly enhancing mural nodule : 67%
  • non enhaning cyst wall : 21%
  • enhancing cyst wall : 46%
• heterogenous, 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 characterisitics include

• T1 : iso to hypo intense solid component compared to adjacent brain
• T2 : hyper intense solid component compared to adjacent brain

Treatment and prognosis

They are slow growing well circumscribed tumours with an overall good prognosis following treatment (> 90% 5 year survival and > 70% 20 year survival). 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 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 heterogenous 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 : different clinical presentation and no enhancing nodule

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