A neoplasm derived from blood vessels, characterized by numerous prominent endothelial cells that occur singly, in aggregates, and as the lining of congeries of vascular tubes or channels. Hemangioendotheliomas are relatively rare and are of intermediate malignancy (between benign hemangiomas and conventional angiosarcomas). They affect men and women about equally and rarely develop in childhood. (From Stedman, 25th ed; Holland et al., Cancer Medicine, 3d ed, p1866).