Osteoclast-rich osteosarcoma.

Nine cases of primary osteoclast-rich osteosarcoma, an undifferentiated sarcoma with an overabundance of osteoclasts and a paucity of tumour osteoid, are reported. They comprise about 3% of all primary osteosarcomas and are histologically related to telangiectatic osteosarcoma, another undifferentiated tumour with abundant osteoclasts and scanty osteoid, perhaps being a predominantly solid variant of the latter. Many of the classical radiological features of an osteosarcoma are absent in these cases and differentiation from a benign lesion is sometimes difficult. Typically, an ill-defined margin surrounds a predominantly lytic lesion of the diaphysis or metaphysis of the femur or tibia of a young patient. A soft-tissue mass is not usually present and periosteal reaction is scanty.