Abstract
A giant cell tumor of the tendon sheath (GCTTS) is a benign, soft tissue (synovial membrane) tumor that frequently involves the fingers. However, the localized form of the GCTTS can be rarely seen in large joints such as knees, ankles, and shoulders. GCTTS may occur due to questionable etiology that includes inflammation, trauma, or chromosomal abnormalities. This is a case of a 30-year-old female, who presented with a painful mass in her right knee. Examination revealed a palpable, tender, slightly mobile mass lateral to the patella. MRI demonstrated a cystic lesion that underwent excision. Data from the reported cases imply that local recurrence was observed in 10%-20% of patients due to incomplete primary resection. Physicians should consider the likelihood of a GCTTS when an infra-articular knee prominence is found. Early diagnosis and appropriate treatment can help with rapid clinical improvement.