Histology
MICROSCOPIC DESCRIPTION: The sections show features of a moderately cellular glial tumour. There is a proliferation of neoplastic oligodendroglial cells. They have round nuclei and perinuclear haloes. They extend into the cortex. Rare mitoses are seen, being 1 per 10 high-power fields. No microvascular proliferation or necrosis is present. The background shows a chicken-wire pattern of blood vessels and with scattered deposits of dystrophic calcification. The features are those of oligodendroglioma.
The tumour cells are focally p53 positive. The topoisomerase index is about 3-4%. IDH1-R132H immunostain is positive (mutated). MGMT is negative (likely methylated). ATRX shows no loss of staining (non-mutated).
DIAGNOSIS: Oligodendroglioma, IDH-mutant (WHO Grade II).
NB: ATRX status and 1p19q co-deletion status are usually inversely related. In this case, ATRX being intact suggests that 1p19q is co-deleted, further supporting the diagnosis, even in light of the 2016 WHO classification of CNS tumour update. Unfortunately, the classification goes out of its way to explicitly state that ATRX is not a replacement for 1p19q codeletion and as such strictly speaking, this tumour must be classified as an oligodendroglioma NOS.