The patient went on to have a biopsy and subtotal resection.
All sections show a diffusely infiltrating glial neoplasm. The majority of the tumour is composed of cells with uniform small round nuclei surrounded by a clear perinuclear halo. These areas are associated with delicate branching capillary vasculature, microcalcification, and focal microcystic architecture. Where the tumour diffusely infiltrates into the neocortex, there are prominent Sherer's secondary structures in the form of perineuronal, perivascular, and subpial aggregation of cells. In addition, there are focal nodular areas of marked hypercellularity where the cells have greater variation in nuclear size and shape and much more mitotic activity (up to 5 mitoses in a single high power field). Some of these cells have visible cytoplasm with either short delicate processes or micro-gemistocytic morphology. The hypercellular regions are also associated with and at least some endothelial proliferation and focal tumour necrosis. There are much larger confluent areas of acute necrosis associated with haemorrhage that more likely represents acute hemorrhagic infarction of part of the tumour. GFAP immunohistochemistry stains the mini gemistocytes as well as reactive astrocytes in the cortex. MIB-1 labelling in significantly higher in the hypercellular nodules.
Final Diagnosis: Anaplastic oligodendroglioma NOS (WHO grade III).
Note: This case predates the recent (2016) revision WHO classification of CNS tumours, and thus molecular markers (IDH mutation and 1p19q co-deletion) are not available. As such, this tumour would now be classified as an anaplastic oligodendroglioma NOS.