Giant cell tumor of the tendon sheath: A rare case in the left knee of a 15-year-old boy.

Localized forms of giant cell tumors are defined as giant cell tumors of the tendon sheath (GCTTS). GCTTS arises from the synovium of a joint, bursa or tendon sheath, and 85% of the tumors occur in the fingers. GCTTS in the knee is extremely rare. We report an unusual case of a 15-year-old boy who presented with an occult growing swelling and a 2-month history of infra-patellar pain in the left knee. Magnetic resonance imaging (MRI) demonstrated a well-circumscribed soft tissue mass in the infra-patellar fat pad posterior to the patella tendon. Excision biopsy was performed by surgical removal. Histopathological examination revealed that it was GCTTS. During the follow-up period, his recovery was propitious and there was no recurrence. Owing to its few and non-specific symptoms, and local recurrence varying from 9 to 44%, its proper, early diagnosis and appropriate treatment is necessary. The purpose for which we report the case is to emphasize the possibility of GCTTS where there is a mass with non-specific symptoms such as infra-patellar pain of the knee, and to avoid misdiagnosis where possible.