Hemangiosarcoma is a rapidly growing, highly invasive variety of cancer that occurs almost exclusively in dogs, and only rarely in cats, horses, mice,[1] or humans (vinyl chloride toxicity).

It is a sarcoma arising from the lining of blood vessels; that is, blood-filled channels and spaces are commonly observed microscopically.

Other sites the tumor may occur include bone, kidneys, the bladder, muscle, the mouth, and the central nervous system.

Some episodes of collapse are a result of ventricular arrhythmias, which are relatively common in dogs with splenic or cardiac HSA.

A retrospective study published in 1999 by Ware, et al., found a five times greater risk of cardiac hemangiosarcoma in spayed vs. intact female dogs and a 2.4 times greater risk of hemangiosarcoma in neutered dogs as compared to intact males.

[6] Hemangiosarcoma can cause a wide variety of hematologic and hemostatic abnormalities, including anemia, thrombocytopenia[7] (low platelet count), disseminated intravascular coagulation (DIC); presence of nRBC, schistocytes, and acanthocytes in the blood smear; and leukocytosis with neutrophilia, left shift, and monocytosis.

Cytologic aspirates may be inconclusive with studies reporting various specificity, and negative results may not correlate with absence of disease, as one study concludes "cytological diagnosis of splenic neoplasia is reliable, but a negative result cannot be used to exclude the possibility of splenic neoplasia.

A 2012 paper published the University of Pennsylvania Veterinary School, in dogs treated with a compound derived from the Coriolus versicolor (commonly known as "Turkey Tail") mushroom showed a favorable response to the agent compared to historical controls of dogs treated with splenectomy and Doxorubicin .

However, as noted in the 2012 study, the Turkey Tail mushroom extract did not cause any systemic adverse reactions, even when combined with Doxorubicin.:.

[22] A preventive vaccine based on several proteins commonly found in dog cancers, including hemangiosarcomas, is also being developed.