Giant cell tumor, malignant: A type of bone tumor characterized by massive destruction of bone near the end (epiphysis) of a long bone. The site most commonly struck by this tumor is the knee -- the far end of the femur and the near end of the tibia. The tumor is often coated by new bony growth. It causes pain and restricts movement. Treatment is by surgery, usually followed by chemotherapy.
There is no evidence that the tumor cells themselves are capable of bone destruction; instead, the tumor cells stimulate the formation of cells that function like osteoclasts and resorb bone.