Tumefactive demyelination

High T2 signal within the right optic nerve (best seen on coronal images). Neither this scan nor the history of optic neuritis were available at the time the patient presented. 

Sizable non-enhancing lesion in right occipital lobe with minor positive mass effect. 

There is a large region of FLAIR/T2 hyperintensity within the right parietooccipital lobe. The more anteromedial component of the T2 signal change extends into the spenium of the corpus callosum and crosses the midline. Ill-defined peripheral nodular enhancement is demonstrated along the margins of the lesion with no enhancing component demonstrated within the corpus callosum. Despite it's size minimal mass effect is exerted on the adjacent parenchyma. 

The MR spectroscopy (not shown) reveals elevated choline peaks and depressed NAA peaks. There are large lipid/lactate peaks. There is reduction in the regional perfusion (not shown). Restricted diffusion is shown only in the very peripheral portion of the lesion.

A region of ill-defined T2 hyperintensity within the central pons is of uncertain significance. 

Conclusion:

The imaging features are non-specific but favor tumefactive demyelination. The alternative diagnosis is of a high-grade diffuse glioma although this is thought less likely. 

The patient underwent a biopsy via burrhole.

Paraffin sections show small fragments of cerebral cortex and white matter. The cortex appears unremarkable. Luxol Fast Blue staining shows several areas with reduced myelin density which are sharply demarcated from adjacent white matter with intact myelin. Within these demyelinated areas there are large numbers of CD68+ monocyte-macrophages with strong surface expression of MHC Class II. Granular LFB+ myelin debris is noted within the cytoplasm of these cells. Activation of microglia with strong MHC Class II expression is also noted. Small caliber blood vessels are surrounded by narrow cuffs of T lymphocytes (CD4>CD8). No viral inclusion bodies are identified. There is no evidence of tumor. The features are of inflammatory demyelination.

FINAL DIAGNOSIS: inflammatory demyelination.

Since the previous investigation, there has been significant progressive FLAIR signal change extending into the left posterior parietal and occipital lobes. There is avid, nodular enhancement along the peripheral margin of the FLAIR signal abnormality both in the parietal and occipital lobes. The enhancement pattern is distinctly limited to the typical "advancing front" (open ring enhancement) of active demyelinating plaque. The enhancing nodularity demonstrates restricted diffusion. Previous right occipital biopsy site noted. The volume of FLAIR signal abnormality related to the posterior right parietal and occipital lobes has substantially reduced. The small region of nodular enhancement visible in the more anterior component of the FLAIR signal abnormality has completely resolved. T2 signal abnormality in the tail of the corpus callosum remains visible. The remainder of the brain is within normal limits, with no intra or extraaxial collection, mass or region of abnormal contrast enhancement.