Extramedullary plasmacytoma in the pectoralis major muscle: an atypical case of the chest wall

Background Extramedullary plasmacytoma (EMP) is a rare plasma cell neoplasm arising outside the bone marrow, most commonly in the head and neck region. EMP involving the pectoralis major muscle is exceptionally rare, and progression to multiple myeloma may occur even without initial bone marrow involvement. Case presentation A 76-year-old male presented with a 2-month history of a right anterior chest mass, mild stabbing pain, and 3-kg weight loss. Imaging revealed a 46 × 47 × 45 mm soft tissue lesion in the right parasternal region involving the pectoralis major muscle without bone involvement. Incisional biopsy demonstrated malignant plasma cells with lambda light chain restriction, Ki67 45%, MUM1 and c-MYC positivity. The patient underwent radical external radiotherapy (40 Gy/20 fractions) with no immediate complications. Follow-up revealed elevated monoclonal proteins and, at 7 months, cranial MRI showed multiple lytic and blastic lesions. Multiple myeloma was confirmed by serum protein electrophoresis and multiparametric flow cytometry. The patient received chemoimmunotherapy with daratumumab, bortezomib, and dexamethasone, followed by lenalidomide maintenance. At the 13-month follow-up, the patient remained asymptomatic. Conclusion EMP of the pectoralis major muscle is extremely rare. Accurate diagnosis requires biopsy, immunohistochemistry, and close follow-up. Local radiotherapy is effective for solitary EMP, but systemic therapy is necessary if progression to multiple myeloma occurs. This case highlights the importance of multidisciplinary management and long-term surveillance.