Cardiac hemangiosarcoma develops in the heart, most commonly in the right atrial appendage, where it arises from the same blood-vessel-lining cells as splenic hemangiosarcoma. The tumour often causes fluid to accumulate in the sac surrounding the heart (pericardial effusion), which can compress the heart and impair its ability to pump — a life-threatening condition called cardiac tamponade. This emergency frequently prompts the initial diagnosis. Cardiac hemangiosarcoma may occur as an isolated tumour or may be found alongside splenic hemangiosarcoma. Surgical access to the heart is more challenging than to the spleen, and complete tumour removal is often difficult, making the overall prognosis more guarded. Golden Retrievers, German Shepherd Dogs, and Labrador Retrievers are most commonly affected.