Vascular invasion (also known as angioinvasion) describes the movement of tumour cells into a blood vessel such as a capillary, artery, or vein. Vascular invasion is almost always associated with malignant (cancerous) tumours although some benign (non-cancerous) tumours can also (very rarely) show vascular invasion. Vascular invasion is important because it increases the risk that tumour cells will metastasize (spread) to other parts of the body.