Angiosarcoma: A type of cancer that begins in the lining of blood vessels. This type of tumor tends to be aggressive, recur locally, and spread widely. It can originate anywhere in the body but is well known to arise in skin, soft tissue, liver, breast, spleen, bone, lung and heart. Factors predisposing to angiosarcoma include chronic lymphedema (as from a radical mastectomy), radiotherapy, foreign materials (such as Dacron, shrapnel, steel, and plastic) in the body, and environmental agents (such arsenic solutions used to spray grapevines, the radiation-emitting contrast agent Thorotrast, and vinyl chloride in the plastic industry).