Below information was taken from “American Academy of Orthopaedic Surgeons” http://orthoinfo.aaos.org/ which mentioned the description of GCT:
Giant cell tumors are named for the way they look under the microscope. Many “giant cells” are seen. They are formed by fusion of several individual cells into a single, larger complex. Many bone tumors and other conditions (including normal bone) contain giant cells. Giant cell tumor of bone is given its characteristic appearance by the constant finding of a large number of these cells existing in a typical background. Most bone tumors occur in the flared portion near the ends of long bone (metaphysis), but giant cell tumor of bone occurs almost exclusively in the end portion of long bones next to the joints (epiphysis). Giant cell tumors of bone most frequently occur around the knee joint in the lower end of the thighbone (femur) or the upper end of the shinbone (tibia). Other common locations include the wrist (lower end of the lower arm bone), the hip (upper end of the thighbone), the shoulder (upper end of the upper arm bone), and lower back (connection of the spine and pelvis). In rare cases, this tumor may spread to the lungs.