Cardiac myxosarcoma of the tricuspid valve in a dog

A case of cardiac myxosarcoma arising from the tricuspid valve is described in a 9-year-old dog that died of right-sided congestive heart failure. At necropsy, an oval-shaped, space-occupying mass of the right heart, measuring 5.5*3*2.5cm, was found attached to the septal leaflet of the tricuspid valve. The cut surface of the mass was variegated gray, yellow, and red with extensive areas of necrosis and hemorrhage. Histologically, the mass had an amorphous to finely fibrillar myxoid background rich in acid mucopolysaccharides, and was composed of pleomorphic and hyperchromatic cells containing a large number of mitotic figures. Immunohistochemically, the tumor cells showed a tendency toward smooth muscle differentiation.