A 65-year-old man presented with a right femur shaft fracture after a fall. The radiograph showed diffuse sclerosis and cortical thickening in his pelvis (panel A) and both femurs, consistent with Paget disease. After orthopedic surgery, he was sent for hematology consult because of anemia. The blood work showed hemoglobin of 8.8 g/dL, white blood cell count of 5.6 × 10⁹/L, platelets 186 × 10⁹/L, alkaline phosphatase 503 U/L, and immunoglobulin A λ paraproteins. Bone marrow aspiration and biopsy demonstrated monoclonal plasmacytosis with 50% marrow involvement (panel B; original magnification ×40, Wright-Giemsa stain), numerous large multinucleated osteoclasts (panel C; original magnification ×20, hematoxylin and eosin stain, negative for CD61 [panel D; original magnification ×40, immunohistochemical stain]) within the Howship lacunae (panel E; original magnification ×20, hematoxylin and eosin stain, white arrow), diffusely thickened trabecular bone with irregular cement line (panel F; original magnification ×20, hematoxylin and eosin stain, black arrow), and grade 3 fibrosis (panel G; original magnification ×10, trichrome stain). The diagnosis of plasma cell myeloma and Paget disease of bone was made.

Coexistence of myeloma and Paget disease is extremely rare. In both diseases, osteoclastic activity is enhanced; although the former may lead to lytic bone lesions, the latter causes compensatory osteoblastic overactivity and disorganized bone formation, which is thicker but weaker. The radiologic features of multiple myeloma could be masked if associated with Paget disease. Our case is distinguished from osteosclerotic myeloma by the absence of clinical (polyneuropathy, organomegaly, endocrinopathy, skin changes), typical radiologic (irregular spiculated sclerosis), and pathologic (focally thickened trabecular bone with entrapped plasma cells) features.