Pathology
The patient went on to have a biopsy.
Histology
Sections show a moderately cellular glial tumour. The tumour is composed of relatively uniform piloid cells with oval nuclei and fine processes. In some areas, there is a slightly microcystic appearance. No mitoses or necrosis is seen. There are a few Rosenthal fibres and eosinophilic granular bodies.
Immunohistochemistry
BRAF V600E: equivocal
S100: positive
Ki6: 2-3%
p16 CDKN2A: positive (not lost)
p53: mosaic (not mutated)
ATRX: positive (not mutated)
IDH1 R132H: negative (not mutated)
OLIG2: positive
GFAP: positive
Final diagnosis
Pilocytic astrocytoma (CNS WHO grade 1)