Diffuse astrocytoma NOS ("protoplasmic" and gemistocytic components)
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A left frontal lobe mass, which involves the superior frontal gyrus cortex and subjacent white matter, demonstrates high T2 and low T1 signal, with some areas of partial FLAIR suppression. Within the centre of the lesion, corresponding to the areas of increased CT density, is irregular intrinsic high T1 material, with some signal loss on EPI suggestive of blood product. There is no significant restricted diffusion, and no elevated CBV. No convincing contrast enhancement. MRS demonstrates high myo-inositol, increased choline and somewhat depressed NAA. No lactate.
Slight high sulcal signal on FLAIR is attributable to the patient being intubated. The remainder of the brain is unremarkable. No abnormal vessels on MRA.
Features are those of a primary brain tumour. MRS, perfusion and morphology suggest a low (WHO II) or perhaps intermediate (WHO III) grade glioma with non-fibrillary components (e.g. gemistocytic, protoplasmic or oligodendroglial component).
The patient went on to have a craniotomy.
The sections show features of a moderately cellular astrocytic tumour. The tumour cells are mainly of the protoplasmic type. They have mildly enlarged round nuclei with short cytoplasmic processes. Scattered gemistocytes are noted, occupying less than 10% of the cell population. Prominent microcystic change is seen in the background.
Tumour cells extend into the cerebral cortex. Mitoses are inconspicuous. No microvascular proliferation or necrosis is present. The features are those of diffuse astrocytoma of predominantly the protoplasmic type. The topoisomerase index is 1%.
- IDH-1 negative
- MGMT negative (likely methylated)
- p53 and p16 positive
Diffuse astrocytoma (WHO Grade II) - mixed protoplasmic and gemistocytic.
Note: Until recently, protoplasmic astrocytomas were classified as a subtype low-grade astrocytoma, however, in the latest (2016) update to WHO classification of CNS tumours, protoplasmic astrocytomas no longer exists as a distinct entity.
Note: Although this tumour is likely to be an IDH wild-type molecular subtype, strictly speaking, to conclusively establish this, IDH would need to be sequenced to ensure that a non-IDH1 R132H mutation was present. In practice, an IDH1 R132H negative tumour in an individual over 55-years-of-age makes the possibility of this being IDH mutant remote (<1%), and sequencing is not felt to be necessary by many institutions, and not recommended by the WHO classification of CNS tumours (2016). In this age-group, however, sequencing is recommended, and without it, it should be considered a diffuse astrocytoma NOS (not otherwise specified).