56-year-old male with left-sided weakness.

A 56-year-old male with a 3-year history of multiple myeloma that was treated with tandem autologous stem cell transplantations. He presented in July 2006 with a left partial hemiparesis and ataxia. MRI of the brain revealed an enhancing large right frontoparietal lesion with heterogeneously hyperintense nodular septations on T2WI, with no significant surrounding edema. The differential diagnosis of this lesion placed a high-grade glioma at the top of the list. The patient underwent surgical resection of the lesion. Pathology confirmed the diagnosis of a relapse of his multiple myeloma. The specimen demonstrated a monomorphic neoplasm arranged in sheets and trabeculae floating in a background of a proteinaceous substance. Immunohistochemistry showed the neoplastic cell was a CD138 positive plasma cell. The nature of the background substance is still undetermined and is a very unusual appearance for multiple myeloma. Flow cytometry revealed a monoclonal plasma cell population with an immunophenotype identical to the initial diagnostic bone marrow aspirate. A completely intraparenchymal manifestation of multiple myeloma is rare in itself but the unusual radiographic and histological appearance of this case is extraordinary.