A rare, slow-growing tumor that usually forms on or under the skin of the arms, legs, and trunk. The tumor has blood vessels that branch out like a tree. Retiform hemangioendotheliomas can spread to nearby tissue and often come back after treatment. They usually do not spread to other parts of the body. They occur in young adults and sometimes children. Retiform hemangioendotheliomas are a type of vascular tumor.