Sternoclavicular Joint Involvement of Rheumatoid Arthritis Mimicking Malignancy: A Case Report

A 73-year-old woman with long-standing therapy-refractory rheumatoid arthritis presented with a persistent fever. Chest computed tomography revealed a mass at the right sternoclavicular joint, growing from 33 × 15 mm to 48 × 44 mm over 8 months, with osteolysis and clavicular erosions. Differential diagnoses were rheumatoid arthritis, spondyloarthritis, tuberculosis, and malignancy. Initial biopsy suggested lymphoid malignancy but was nondiagnostic due to necrosis. Rebiopsy indicated a benign lesion. Positron emission tomography/computed tomography ruled out cancer. The sternoclavicular joint is a fairly common location for rheumatic disease, but autoimmune-related lymphoproliferations are possible. Lesions with progressive growth, bone damage, and systemic symptoms (fever, weight loss, night sweats) require excluding malignancy. Accurate diagnosis depends on interdisciplinary collaboration and histopathologic confirmation.