Hemangiosarcoma is a malignant cancer of the endothelial cells, the cells lining the blood vessels. In essence, it's a cancer of the circulatory system. It most frequently affects the spleen, heart, and liver, but can potentially develop anywhere in a dog's body.
It's a notoriously aggressive cancer with a propensity for rapid growth and metastasis. Unfortunately, it's often asymptomatic until it reaches advanced stages, leading to its reputation as a 'silent killer.'