Giant cell tumor of the tendon sheath in the knee of an 11-year-old girl.

Giant cell tumors are commonly found over the flexor tendon sheath of the hand and wrist. However, giant cell tumors in the knee joint are rare, especially in children. We report an interesting case of an 11-year-old girl who presented with a painful lump on her right knee that enlarged over time. Clinically, she had fullness over the anterolateral part of her knee. Magnetic resonance imaging revealed an encapsulated mass inferior to the patella. The tumor measured 3 x 3.5 x 1.5 cm. Histopathological findings confirmed that it was a tenosynovial giant cell tumor. Because of initial mild symptoms, there was a delay of 2 years from the initial symptoms until tumor excision. Her follow-up period was 35 months, and her health to date is excellent with no recurrence. We believe that reporting this rare case will help clinicians update their knowledge on possible causes of lumps in the knee, and avoid diagnostic delay. It could also prove to be beneficial in arriving at a diagnosis in future cases.