Giant expansile humeral lesion with soap-bubble appearance mimicking a primary bone tumor: Multiple myeloma as the final diagnosis.

A man in his 60s presented with progressive right arm swelling and functional limitation. Plain radiographs demonstrated a large long-segment expansile lytic lesion of the humerus with a multiloculated "soap-bubble" appearance, raising concern for a primary bone tumor or metastatic disease. Magnetic resonance imaging showed a large marrow-based expansile lesion with cortical thinning, heterogeneous T2/STIR hyperintensity, patchy enhancement, and no fluid-fluid levels. The patient was lost to follow-up and re-presented 2 years later with symptomatic anemia, hypercalcemia, marked hyperglobulinemia, and a reversed albumin-globulin ratio. Repeat magnetic resonance imaging demonstrated interval enlargement with cortical breach and intra-articular extension into the elbow joint. Staging CT demonstrated disseminated lytic skeletal disease involving the axial and appendicular skeleton. Serum protein electrophoresis demonstrated an IgG kappa paraprotein, and bone marrow biopsy confirmed diffuse clonal plasma cell infiltration consistent with multiple myeloma. This case highlights that dominant expansile long-bone lesions with tumor-like morphology may represent plasma cell neoplasia and can closely mimic primary bone malignancy or metastatic disease on imaging.