MS may occasionally be present as a mass lesion indistinguishable clinically and radiographically from a brain tumor (153). Patients may present with headache, aphasia, disturbance in consciousness, or seizures. Neuroimaging often reveals unifocal or multifocal enhancing lesions with associated mass effect and edema. With neuroimaging, the presence of open-ring enhancement toward the cortical surface is more likely associated with demyelinating lesions (Figure 12) (154). These patients pose considerable diagnostic difficulty and often require brain biopsy to confirm a diagnosis. Pathologically, the biopsy specimen may be mistaken for a neoplasm given the hyper cellular nature of these lesions, and the association with bizarre astrocytic forms (i.e., Creutzfeld-Peters cells) limited necrosis (Figure 12). These features are a potential trap for the pathologist, and such cases are common causes of medicolegal litigation. The histological features detailed above (intimate admixture of macrophages and reactive astrocytes, discrete borders of myelin loss, and relative axonal preservation) should confirm the diagnosis of an inflammatory demyelinating disease.
Clinical follow-up has revealed that some of these patients will develop typical MS, whereas others will have recurring tumor-like lesions. Although some cases behave like the acute Marburg variant or have features suggestive of Balo's concentric sclerosis, there are other examples in which the course is monophasic and self-limited. Nonetheless, it is important for the neurologist to recognize that MS may present with clinical, radiographic, and even pathologic features suggestive of a primary brain malignancy.
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