Extramedullary Relapse of Multiple Myeloma Manifesting as Cutaneous Plasmacytosis and Myelomatous Pleural Effusion: A Rare Case Report

Extramedullary relapse of multiple myeloma (MM) involving both the skin and pleura is exceptionally uncommon and is associated with an aggressive clinical course and poor prognosis. We report a 42-year-old man with newly diagnosed ISS stage III MM who initially presented with extensive FDG-avid skeletal lesions, a large anterior mediastinal plasmacytoma, and pleural deposits on baseline ¹⁸ F-FDG PET/CT. Following induction and consolidation chemotherapy, the patient achieved a stringent complete response with disappearance of the monoclonal protein and bone marrow remission. However, subsequently developed rapidly progressive extramedullary relapse manifesting as multiple cutaneous chest wall plasmacytomas and myelomatous pleural effusion. Cytological and histopathological examination confirmed plasma cell infiltration. Follow-up FDG PET/CT examinations accurately delineated the extent of disease, guided biopsy from the metabolically active lesions, assessed treatment response, and demonstrated partial metabolic response following salvage chemotherapy, although the disease ultimately progressed despite multiple lines of therapy including VTD-PACE and venetoclax. The patient eventually succumbed to progressive disease. This case highlights the aggressive biology of extramedullary MM, the rarity of synchronous cutaneous and pleural involvement, and underscores the indispensable role of FDG PET/CT in staging, treatment planning, response assessment, and prognostication in patients with refractory extramedullary disease.