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 demonstrated as a heterogeneous and lobulated mass with a low signal intensity both in T1- and T2-weighted images. Histological findings of the tumor were identical to those of fibroma of the tendon sheath. In the peripheral villous synovial tissue, several small and fibrous nodules were observed, and their histological features were identical to those of the main tumor. Immunohistochemically, the tumor cells showed diffuse and intense reactivity to vimentin, muscle actin and S-100. These results indicated that the tumor might be a fibromatous analog of synovial chondromatosis.