Oligoastrocytoma NOS (anaplastic)

An ill-defined, mainly hypodense and non-contrast enhancing lesion is demonstrated in the left frontal lobe medially, adjacent to the falx and measures 2.5 x 2.9 x 3.7cm (AP x lat x cc). Two curvilinear hyperdensities are shown within the lesion, as well as several punctate hyperdensities, possibly calcific or small hemorrhagic foci.

A further region of ill-defined low attenuation is shown supero-posteriorly in subcortical and partially cortical location and measures 2.2 x 1.5 x 2.3cm. Associated mass effect with sulci effacement.

No other abnormality is noted. The appearance of the ventricles, basal cisterns and cerebral volume is unremarkable and appropriate for age.

Conclusion:

Left frontal mass with coarse calcification, glial series tumor such as oligodendroglioma / mixed oligo-astrocytoma most likey, MRI/MRA recommended as next investigation.  Atypical thrombosed AVM reasonable to consider but MRI tumor study next appropriate investigation.

There is a left frontal cortical and sub-cortical mass lesion measuring 28mm x 30 mm x 65 mm involving the left superior frontal gyrus. Within the mass lesion there are punctate foci of heterogeneous calcification,  minimal patchy contrast enhancement and no definite restricted diffusion. Spectroscopy demonstrates raised choline and decreased NAA with mildly increased left frontal perfusion (CBV).   

No remote intra- or extra-axial mass lesions, acute hemorrhages or collections. No acute infarcts.

Conclusion:

Left frontal mass lesion likely representing a glial series tumor, possibly with oligodendroglial components.

MICROSCOPIC DESCRIPTION:

The sections show a moderately hypercellular glial tumor composed of astrocytic and oligodendroglial components. The astrocytic component is predominantly protoplasmic in type with a smaller component of fibrillary astrocytes. Both the astrocytic and oligodendroglial components show moderate nuclear pleomorphism. Moderate numbers of minigemistocytes and gliofibrillary oligodendrocytes are identified.

There is moderate mitotic activity and foci of microvascular proliferation with endothelial cell hyperplasia are noted in the oligodendroglial component. In addition a very small necrotic focus is seen in the oligodendroglial component. This incorporates thin-walled necrotic blood vessels.

IMMUNOHISTOCHEMISTRY:

• GFAP              positive in astrocytic cells; negative in oligodendroglial cells. (not shown)
• Nogo-A          positive in both astrocytic and oligodendroglialcells (not shown)
• Nestin            positive
• IDH-1             positive
• p53                positive (not shown)
• p16                negative
• MGMT           negative (likely methylated)
• Toposiomerase labeling index:  Regional variation  2-10%

COMMENT:

Despite the presence of a small necrotic focus in the oligodendroglioma component of this mixed oligoastrocytic tumor, a histopathological diagnosis of anaplastic oligoastrocytoma (WHO Grade III) is preferred to that of glioblastoma with an oligodendroglioma component (WHO Grade IV) because each component, in isolation, appears histologically of low grade, IDH-1 is mutated and the topoisomerase labeling index is low, certainly much lower than would be expected in glioblastoma with an oligodendroglioma component. In addition, MGMT is likely to be methylated which is an added positive prognostic indicator.

Chromosome 1p/19q deletion analysis might be considered to further characterize the tumor.

FINAL DIAGNOSIS:

Brain tumor: High-grade anaplastic oligoastrocytoma favoring WHO Grade III.

NOTE: This case predates the 2016 WHO classification of CNS tumor revision. As no 1p19q co-deletion status is available a formal diagnosis cannot be reached and the NOS is therefore used (not otherwise specified) - which is recognized in the current classification for cases where molecular information is unavailable. It should also be noted, that under the new classification both an astrocytic and oligodendroglial component needs to be identified, each with its own molecular markers. True oligoastrocytomas are therefore going to be rare, and this case would most likely be classified as either an astrocytoma or an oligodendroglioma.