Neim-15 an exception to the metastatic rule: a non-enhancing midbrain lesion due to metastatic fibromyxoid sarcoma with extensiveinflammatory infiltrate

Abstract Metastases from extra-CNS sites typically enhance on MR after the administration of gadolinium-based contrast agents due to disruption of the blood-brain barrier. Fibromyxoid sarcoma involving the brain is extremely rare, with only a few reported cases manifesting as well-circumscribed enhancing masses. Here, we report an extremely rare non-enhancing brain metastasis secondary to an already rare sarcoma. A 44-year-old woman with gluteal fibromyxoid sarcoma metastatic to cervical paraspinal muscles and adrenal gland treated with ipilimumab/nivolumab presented with diplopia and was found to have an 8 mm T2-hyperintense midbrain lesion without enhancement or mass effect. An inflammatory etiology associated with checkpoint inhibitor therapy was suspected, and she was treated with IV methylprednisolone 1 g daily for 3 days followed by an oral prednisone taper, IV immunoglobulin, and plasma exchange without improvement. The midbrain lesion steadily grew over 15 months of follow up, eventually involving the bilateral medial thalami and demonstrating diffusion restriction in the midbrain, but never showing gadolinium enhancement. The imaging was reminiscent of peripheral brainstem non-enhancing band-like lesions reported in leptomeningeal lung cancer, however CSF cytology was repeatedly normal, as was extensive infectious and inflammatory testing. Her neurologic disability continued to progress, with development of bilateral oculomotor nerve palsies, dysarthria, and depressed level of alertness, devolving into non-convulsive status epilepticus requiring intubation. She died six months after onset of neurologic symptoms. At autopsy, histologic and immunohistochemical evaluation revealed fibromyxoid sarcoma (MUC-4 positive, TAOK1-FUS fusion) centered on the periventricular aqueduct with accompanying disseminated single-cell infiltration throughout the midbrain. The background midbrain and periventricular parenchyma were notable for voluminous microglial activation and histiocytic infiltrate. In sum, this case represents a radiographically unique sarcoma brain metastasis which on pathologic exam showed robust inflammatory changes far out of proportion to the tumor present.