Malignant Mesenchymoma of the Heart Base in a Dog with Infiltration of the Pericardium and Metastasis to the Lung

A 9-year-old male rottweiler was presented with abdominal distension, ascites and respiratory distress and marked bulging in the perineal region. At necropsy examination the animal had profuse ascites and hydropericardium and a multinodular mass in the right auricle of the heart infiltrating the epicardium and pericardium and metastasizing to the caudal lobe of the left lung. Microscopically and immunohistochemically the tumour was composed of neoplastic cells with muscular, cartilaginous and adipose differentiation. A diagnosis of malignant mesenchymoma with leiomyosarcomatous (∼50%), rhabdomyosarcomatous (∼30%), chondrosarcomatous (25%) and liposarcomatous (5%) components was made. Metastatic malignant mesenchymoma has not been reported previously at this site in the dog.