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)