A case of Paget's disease of bone is presented in which sarcoma formation occurred in the lower end of the femur, and, apparently, at the same time in other areas. The histological picture of the tumor varies from polymorphonuclear sarcoma to spindle-cell sarcoma and fibrosarcoma. The tumor led to pathological fracture at the lower end of the femur, and to invasion of the cavity of the knee joint and of the popliteal vein, with sarcoma metastases in the lungs.
It seems that osteitis fibrosa is the predisposing cause of the tumor growth, because only those portions with very mature fibrous bone marrow show tumor formation, whereas the parts with looser bone marrow and younger bony changes, in the sense of Paget's disease, are not affected by tumor growth. It is questionable whether von Albertini's distinction between a presarcomatous and a real sarcoma formation can be made.