CSF1 expression has been found in musculoskeletal tumors including TSGCT and leiomyosarcoma by ISH or IHC [5, 6, 10, 11], although no study has focused on the diagnostic utility of CSF1 IHC for the differential diagnosis between TSGCTs and giant cell-rich lesions including giant cell tumor of bone, aneurysmal bone cyst, and giant cell reparative granuloma. Here, CSF1 is linked to tenosynovial giant cell tumor, diffuse type.