“…The involvement of the fossa and extra-articular extension should be assessed for preoperative workup [2] . TSGCT is a benign tumor that arises from the tendon sheath in most cases, but, in a few cases, the diffuse type, which has destructive and aggressive properties, has been found to turn malignant, with lungs metastases [2 , 3] . There are no case reports or previous research showing the development of TSGCT, localized type, into malignant, and lungs metastases.…”