Imaging features of benign synovial inflammation affecting the long digital extensor tendon sheath in a dog

A dog was evaluated for right pelvic limb lameness. Computed tomography and magnetic resonance imaging revealed an irregular, contrast-enhancing mass extending along the proximolateral right tibia, involving the long digital extensor tendon (LDET) ± cranial tibialis muscle. Pulmonary nodules, nonspecific hepatomegaly, and splenic nodules were also present. The primary differential diagnosis was soft tissue neoplasia. Surgical biopsy with histopathology revealed benign, chronic inflammation, and fibrosis. Idiopathic synovial inflammation should be included as a differential diagnosis for dogs with this combination of clinical and imaging characteristics.