The patient went on to have a resection.

Histology

Sections show the cerebellum infiltrated by a glial tumor of only moderate cellularity.  The tumor cells have elongated nuclei and fine processes.  Rosenthal fibers and eosinophilic granular bodies are widespread.  In some areas, the stroma is much looser and somewhat microcystic.  There are no mitoses, necrosis or microvascular proliferation.

Immunohistochemistry

• GFAP: positive

• OLIG2: positive

• IDH1 R132H: negative (not mutated)

• ATRX: positive (not mutated)

• p53: mosaic (wildtype)

• p16: positive

• BRAF V600E: negative

• S100: positive

• H3K27M: negative

• Ki67: <1%

Final diagnosis

Pilocytic astrocytoma (CNS WHO grade 1).