Pilocytic astrocytoma

This cases illustratescase illustrates a large cystic mass lesion arising from the left inferior cerebellar hemisphere with an enhancing solid component and associated obstructive hydrocephalous. In the patient's age group and with these image features, haemangioblastoma and pilocytic astrocytoma must be considered.

Haemangioblastoma typically occur in the young adult, and although they are the most common posterior fossa mass in a young adult, they are nonetheless uncommon in absolute terms, accounting for only 1-2.5% of all intracranial tumours. They have a peak incidence around 30-60 years of age. One imaging feature that makes this case less likely a haemangioblastoma is the presence of enhancement in the cystic component wall.  

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. 

This case was first considered to favour a pilomyxoid astrocytoma (PMA) are diagnosis by the the histopathology study. However, after the BRAF genetic tests, the diagnosis was confirmed to be a recently described variantpilocytic astrocytoma due the absence of pilocytic astrocytoma with unique clinical and histopathologic characteristics, being considered a genetic fusion or mutations. WHO grade II tumour. PMA was originally reported as arising in the hypothalamus or optic chiasm of an infant or young child, however it is also know that they may also occur in the posterior fossa and in the spinal cord ¹. 

MICROSCOPIC DESCRIPTION: Paraffin sections show a moderately hypercellular glial tumour involving cerebellum. Tumour cells are a mixture of plump piloid cells and cells with round and oval nuclei and delicate processes. Both cell types show moderate nuclear pleomorphism. No mitotic figures are identified. There is no microvascular proliferation and no necrosis is seen. Small numbers of Rosenthal fibres are identified. A moderate amount of extracellular Alcian Blue positive myxomatous material is noted throughout the tumour. Tumour gradually merges with surrounding cerebellar parenchyma. Gliosis and patchy loss of Purkinje cells is noted in the cerebellar cortex. 

IMMUNOHISTOCHEMISTRY:

• GFAP positive
• Nestin positive (moderate)
• ATRX positive (not mutated)
• IDH-1 R132H negative (not mutated)
• MGMT positive (likely unmethylated)
• p53 negative
• p16 negative
• Topoisomerase labelling index: Approximately 2%.
• The features are of pilomyxoid astrocytoma (WHO Grade II).

DIAGNOSIS: "Posterior cranial fossa tumour": Pilomyxoid astrocytoma (WHO Grade II)