summary Giant Cell Tumor of Tendon Sheath, also known as pigmented villonodular tumor of the tendon sheath, is a benign nodular tumor that is found on the tendon sheath of the hands and feet Diagnosis is made clinically with a firm, nodular mass that does not transilluminate with MRI studies showing decreased signal intensity on both T1-and T2-weighted MR imaging. Treatment is usually marginal excision of the mass. Epidemiology Incidence common second most common soft-tissue tumor seen in the hand, following ganglion cyst Demographics present in 3rd-5th decade of life Anatomic location it is most common on palmar surface of radial three digits near DIPJ Presentation Symptoms enlarging mass pain, worse with activity (or wearing shoes, for foot lesions) Physical exam firm, nodular mass that does not transilluminate Imaging Radiographs pressure-type bone erosion can be seen in up to 5% of patients on radiographs Ultrasound able to demonstrate relationship of lesion with adjacent tendon homogeneously hypoechoic, although some heterogeneity may be seen in echo-texture in a minority of cases most have some internal vascularity MRI MRI may be helpful diagnostically appearance of the focal form is generally decreased signal intensity on both T1-and T2-weighted MR imaging Histology Characterized by proliferating histiocytes, moderately cellular (sheets of rounded or polygonal cells) hemosiderin (brown color) may be present, but typically less than seen with PVNS multinucleated giant cells are common Differential Ganglion cyst cystic component Pigmented villonodular synovitis (PVNS) histologically identical involves larger joints Desmoid tumor fibroma/fibrosarcoma glomangioma Treatment Operative marginal excision 5-50% recurrence rate more common if tumor extends into joints and deep to the volar plate tendon involvement associated with high recurrence rate local recurrence is usually treated with repeat excision operative approach is dependant on location and extent of the tumor Prognosis No reports of metastasis in literature