Fibroma of tendon sheath arising from the radio-ulnar joint

A report of a 65-year-old male with a tumor arising from synovial tissue of the radio-ulnar joint. On magnetic resonance imaging, the tumor was demons trated as a heterogeneous and lobulated mass with a low signal intensity bo th in T1-and T2-weighted images. Histological findings of the tumor were id entical to those of fibroma of the tendon sheath. In the peripheral villous synovial tissue, several small and fibrous nodules were observed, and thei r histological features were identical to those of the main tumor. Immunohi stochemically, the tumor cells showed diffuse and intense reactivity to vim entin, muscle actin and S-100. These results indicated that the tumor might be a fibromatous analog of synovial chondromatosis.