Dimethyl fumarate (DMF) is an oral disease modifying therapy (DMT) used to treat multiple sclerosis (MS). Recent reports of progressive multifocal leukoencephalopathy with use of DMF have prompted watchfulness for other potential risks with its long-term use. We present a case—the first to our knowledge—of extramedullary plasmacytoma (EMP) in a patient treated with DMF.
Patient is a 36-year-old woman with MS treated with DMF for five years before presenting with epistaxis. She underwent CT scan of paranasal sinuses that revealed a left maxillary sinus lesion. The lesion was excised and biopsied. Histopathology revealed Epstein-Barr virus (EBV) positive EMP with CD138-positive plasma cells. Additional testing including lymphocyte count, serum immunoglobulins, HIV and hepatitis C serology, skeletal survey and bone marrow biopsy were normal. The patient discontinued DMF at time of diagnosis and is awaiting proton radiation therapy to the lesion.
EMP is a rare soft tissue neoplasm caused by localized proliferation of monoclonal plasma cells without bone or systemic involvement. Typical features of EMP include age of onset > 50 years, male predominance, and head or neck location. The pathogenesis remains unclear; immune-alteration and chronic viral infections are hypothesized to play a role. EMP are seldom positive for EBV in immunocompetent hosts. Atypical features in our case—age, gender and EBV positivity—could suggest the tumor is secondary to immune alteration caused by long-term DMF exposure. Impaired T-cell mediated immune-surveillance can occur with DMF and may be a potential pathogenic mechanism for B-cell clonal proliferation and risk of plasma cell neoplasm.