Mediastinal Extramedullary Plasmacytoma in the Absence of Classical Myeloma Markers: A Case Report.

Multiple myeloma (MM) typically presents with bone pain, pathological fractures, anemia, renal impairment, or detectable monoclonal gammopathy. In rare instances, MM may manifest with extramedullary plasmacytomas, and mediastinal involvement can challenge diagnostic expectations. We report the case of a 69-year-old man presenting with severe shoulder pain and progressive dyspnea who was found to have a large anterior mediastinal mass, bilateral pleural effusion, and multiple bone lesions. Routine laboratory evaluation revealed no anemia, renal dysfunction, or detectable monoclonal protein on serum protein electrophoresis. However, serum-free light chain assay revealed markedly elevated lambda chains. Biopsy confirmed plasmacytoma. The final diagnosis was light chain MM with mediastinal extramedullary plasmacytoma. This case underscores the importance of considering plasmacytoma in the differential diagnosis of mediastinal masses, even when classical laboratory markers of myeloma are absent, and highlights the pivotal role of histopathology and advanced laboratory testing in establishing a timely diagnosis.