Pilocytic astrocytoma

A peripherally enhancing fluid-filled cavity in the right parietal lobe has a rim that is mildly irregular with mild nodularity, enhancing thin septations and susceptibility artefact. The internal fluid demonstrates layering. There is no diffusion restriction and no elevation of cerebral blood volume (CBV). 

Mild adjacent FLAIR signal hyperintensity at the superior and inferior aspects. There is no large volume of edema. Minimal mass effect. The posterior horn of the right lateral ventricle is effaced.

There is extensive superficial siderosis throughout the brain, most pronounced in the posterior fossa.

Conclusion:

Overall, the findings are most suggestive of a long-standing lesion with chronic or intermittent hemorrhage over time. A low grade/indolent cystic neoplasm (ie. ganglioglioma, PXA, pilocytic) are thought more likely than recurrent hemorrhage into a neuroepithelial cyst. 

Sections show a variably cellular tumor with a fibrillary background. Tumor cells have elongated bipolar to round nuclei with inconspicuous nucleoli. Frequent Rosenthal fibers are seen. There are focal aggregates of thin-walled, hyalinised blood vessels with the adjacent parenchyma containing hemosiderin deposition consistent with a cyst wall. No mitoses, necrosis or microvascular proliferation are identified.

IMMUNOHISTOCHEMISTRY:

• GFAP: Positive
• Nestin: Positive
• NogoA: Negative
• IDH1 R132H: Negative (non-mutated)
• ATRX: Intact (non-mutated)
• p53: Equivocal
• p16: Negative
• BRAFV600E: Negative
• H3K27M: Negative
• H3K27me3: Intact
• Topoisomerase proliferation index: 1%

FINAL DIAGNOSIS: Pilocytic astrocytoma (WHO grade I).