Sarcomatoid mesothelioma of tunica vaginalis testis in the right scrotum of a dog.

A 12-year-old intact male Welsh Corgi was presented with enlargement of the right scrotum. Both testicles were surgically removed and histopathologically examined. On gross examination, white nodules were found in the epididymis and ductus deferens. Histopathologically, the nodules developed continuously from the tunica vaginalis testis of the right scrotum and consisted of spindle-shaped neoplastic cells that invaded the surrounding tissue. Immunohistochemically, the neoplastic cells were diffusely positive for vimentin, cytokeratin and Wilms tumor-1 (WT-1). Based on these findings, the tumor was diagnosed as sarcomatoid mesothelioma. The dog presented with respiratory distress 122 days after surgery and clinical examination found multiple metastatic lesions in the lung, abdominal lymph nodes and peritoneum. The dog died 144 days after surgery due to disease progression.