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.
- optic nerve / optic chiasm (25 - 30%)
- very common in patients with NF1
- hypothalamic / adjacent to third ventricle
- brainstem
- other less common locations
- cerebral hemispheres: more frequent in adults
- cerebral ventricles
- velum interpositum
- spinal cord
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
- 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)
