An Unusual Thoracic Presentation of Multiple Myeloma: Rib-Based Paraskeletal Plasmacytoma Mimicking a Lung Mass

Thoracic plasma cell neoplasms may be difficult to classify when they arise at the interface between the ribs, pleura, and lung.We report a 75-year-old male who presented with a five-month history of progressive right arm pain, functional limitation, and constitutional symptoms.Chest computed tomography revealed an osteolytic lesion centered in the right eighth rib, with a costal/extrapleural mass (6.0 × 3.5 cm) and, on MRI, extended to the adjacent vertebral body with a soft-tissue component and additional osteolytic lesions.Laboratory evaluation showed severe normocytic anemia, an increase in serum creatinine from a baseline of 0.7 mg/dL to 1.3 mg/dL, and an IgA-kappa monoclonal component.Biopsy of the rib lesion demonstrated a CD38-positive, kappa-restricted plasma cell neoplasm, while bone marrow showed 30% clonal plasma cells on trephine CD138 immunohistochemistry (15.9% by flow cytometry).The presence of clonal bone marrow involvement, severe anemia, and multifocal osteolytic disease established the diagnosis of active multiple myeloma presenting with a rib-based plasmacytoma and extrapleural paraskeletal soft-tissue component abutting the pleura, rather than a primary pulmonary or solitary bone plasmacytoma.Bortezomib, lenalidomide, and dexamethasone (VRd) induction was planned but could not be started because of competing surgical priorities and drug unavailability; the patient died 67 days after admission from progressive disease with refractory hypercalcemia.This case highlights the importance of integrating the anatomical origin of the lesion with histopathological, monoclonal protein, bone marrow, and systemic imaging findings to avoid diagnostic misclassification and guide appropriate treatment.