Extracardiac intrapericardial myxosarcoma causing right ventricular outflow tract obstruction in a dog

A 13-year-old male castrated pomeranian cross was referred for evaluation of episodes of collapse and a suspected cardiac mass. The presence of a mass at the base of the heart within the pericardial space was confirmed by echocardiography. Additional diagnostics included computed tomography, ultrasound-guided fine-needle aspirate, and thoracic radiographs. The mass was surgically debulked and diagnosed as myxosarcoma via histopathology. This case report describes the diagnostic imaging, laboratory findings, and short-term positive clinical outcome of a dog with a myxosarcoma in a previously undescribed location.