MICROSCOPIC DESCRIPTION: Sections show brain cortex including grey and white matter. There is a moderately cellular tumour replacing white matter and extending into cortex. This is composed of sheets of oligodendroglial cells with round to oval nuclei, coarsely granular chromatin, inconspicuous nucleoli and prominent perinuclear cytoplasmic halos. There is prominent perineuronal, perivascular and sub-pial secondary structuring in cortex. There are thin capillaries with a "chicken-wire" arrangement and microcalcification is also noted. No necrosis or microvascular proliferation are seen. There is an occasional mitotic figure.
IMMUNOHISTOCHEMISTRY:
- GFAP negative in oligodendroglial tumour cells; positive in reactive astrocytes.
- NogoA positive in oligodendroglial tumour cells
- Nestin positive (low)
- IDH-1 R132H positive (mutated)
- ATRX positive (not mutated)
- MGMT negative (likely methylated)
- p53 negative p16
- CDKN2A positive
- Topoisomerase labelling index: Approximately 10%
DIAGNOSIS: IDH-1 mutated Oligodendroglioma (WHO Grade II).
Note: although this case predates the 2016 WHO classification of CNS tumours update and 1p19q codeletion status has not been assessed, the fact that ATRX is positive (not mutated) suggests that 1p19q would be co-deleted. Unfortunately, this is not enough (the "blue book" goes out of its way to stress this) and this tumour would be classified as oligodendroglioma NOS.