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Pilocytic astrocytoma

Pilocytic astrocytomas are a low grade astrocytoma (WHO Grade I) found in young patients, and have a good prognosis. 

Demographics and clinical presentation

Pilocytic astrocytomas are tumours of young people, with 75% occurring in the first two decades of life, typically late in the first decade. There is no 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 of the tumour. 

Associations

There is a strong association with neurofibromatosis type 1 (NF1), especially affecting the optic nerves and chiasm. Pilocytic astrocytomas are seen in up to 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 and classification

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.

Pathology

The presence of eosinophillic Rosenthal fibres are a characteristic feature.

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Radiographic features

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
  • 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 a good prognosis following treatment (> 90% 5 year survival and > 70% 20 year survival).

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 diagnoses