Giant cell tumor of bone in an aged Fischer 344 rat

A giant cell tumor (GCT) was detected on the distal end of the femur in a 98-wk-old male Fischer 344 rat. The yellowish white mass had expanded, compressing adjacent muscle tissues. The tumor had an osteolytic and relatively homogeneous appearance and was composed of multinuclear giant cells scattered in a mass of mononuclear stromal cells. No osteoid tissue formation was observed. The tumor cells were strongly immunoreactive for ED-1 and some were also positive for alpha-smooth muscle actin, suggesting that the tumor originated from the monocyte/macrophage lineage showing myofibroblastic differentiation. This is the first report concerning spontaneous GCT of bone in a rat.