[Bone tumors].

The primary round cell tumors of the skeletal system focus on the differential diagnosis of Ewing sarcoma. It is known that this tumor can be defined on the basis of translocation 11; 22. Therefore, in most cases, the differential diagnosis to other "round cell" tumors of the skeletal system no longer causes great problems. Besides the differential diagnosis of chondrogenic tumors, the diagnosis of osteoid-forming tumors is very problematic histologically. Focus of attention is the diagnosis of highly malignant osteosarcoma. This must be differentiated from the low malignant, intramedullary osteosarcomas, as well as from benign, osteoid-forming tumors and -like tumors. Especially the tumors located at the border between highly malignant osteosarcoma and benign osteoblastoma are difficult as far as the differential diagnosis is concerned. These cases are so rare that they are dealt with only at a casuistic level. Therefore, with regard to their classification, there have been almost no changes recently.