Diffuse-Type Tenosynovial Giant Cell Tumor of the Flexor Hallucis Longus Tendon Sheath: A Case Report

The synovium can be the primary site for the development of various benign or malignant tumors. Tenosynovial giant cell tumor (TGCT), also known as giant cell tumor of the tendon sheath, is a benign but locally aggressive synovial neoplasm occurring in localized and diffuse forms. The localized form predominantly affects the hand, whereas the diffuse form mainly involves large joints; involvement of the flexor hallucis longus tendon sheath is rare. Magnetic resonance imaging (MRI) is the principal modality for demonstrating the hemosiderin deposits characteristic of this lesion, although histopathology remains necessary for definitive diagnosis. Surgical resection remains the standard treatment. We report the case of a 24-year-old man with 10 months of right ankle pain, related to a diffuse-type TGCT involving the flexor hallucis longus tendon sheath and adjacent ankle joints. Radiographs initially showed soft-tissue filling of Kager's triangle. Computed tomography arthrography demonstrated synovial thickening but was nonspecific; MRI then revealed the characteristic findings, and histopathology confirmed the diagnosis. This report aims to raise awareness of this rare location and its associated diagnostic pitfalls.