The sections show a variably hypercellular glial tumour within the cerebellar white matter. This shows a biphasic architecture.
One component consists of cells with pilocytic features. These have elongated hyperchromatic nuclei and bipolar processes which vary in thickness and are arranged in loose fasciculi. Rosenthal fibres are noted within this component.
The second component consists of cells with moderately pleomorphic round and oval vesicular nuclei. These are arranged in diffuse sheets. No mitotic figures are identified. Foci of vascular endothelial cell hyperplasia are identified. There is no necrosis. Tumour is sharply demarcated from the adjacent cerebellar cortex but merges gradually with cerebellar white matter.
Immunohistochemistry shows strong staining in tumour cells for GFAP and nestin. A small number of tumour cells show nuclear staining for p16. Nuclear staining for MGMT is seen in <15% of tumour cells indicating probable methylation. No staining for IDH-1 or BRAF-V600E is seen. The features are of pilocytic astrocytoma and, except for the absence of necrosis, are similar to those seen in the previous biopsies.
The topoisomerase labelling index shows regional variation - approximately 4-6%.
FINAL DIAGNOSIS: NF-1 associated pilocytic astrocytoma (WHO Grade I)