Hemangiosarcoma is a form of cancer which originates in the endothelium, which is the lining of blood vessels and spleen. As might be expected of a tumor arising in the blood system they are highly malignant and can be found almost anywhere in the body since blood vessels are necessary in almost all body tissues. There is a strong predilection for the spleen, pericardium and heart. These tumors are most common in middle aged or older dogs which are medium sized or larger but can occur in any breed. German shepherds are reported to be more susceptible to this tumor than most dog breeds. Golden retrievers also seem to have a higher than normal incidence.

Because these tumors arise in internal organs there is often little warning that they are present prior to time they cause severe clinical signs of disease. A common estimate of the average time from discovery of the tumor until death occurs in affected dogs is six to eight weeks but death occurs more rapidly than this in a number of cases.

 Hemangiosarcoma is a malignant cancer of the cells that form blood vessels. Because these tumors start in blood vessels, they are frequently filled with blood. Consequently, when a blood-filled tumor ruptures, it can cause problems with internal or external
bleeding. Hemangiosarcoma is considered to be a very aggressive tumor and can spread rapidly to other organs.

Hemangiosarcoma is more common in dogs than in cats. It usually occurs in middle-aged to older dogs 9 to 11 years of age, and German shepherds appear to be predisposed to developing this cancer.

The most common primary location of this cancer in dogs is the spleen. Other primary locations include the heart, liver,
skin, and bone; however, it can start in any location where blood vessels are present. These tumors usually spread to the lungs, liver, spleen and heart.

This may occur without any warning, and the symptoms will depend upon where the tumor is located.