A tumor composed of spindle cells with a rich vascular network, which apparently arises from pericytes, cells of smooth muscle origin that lie around small vessels. Benign and malignant hemangiopericytomas exist, and the rarity of these lesions has led to considerable confusion in distinguishing between benign and malignant variants. (From Dorland, 27th ed; DeVita Jr et al., Cancer: Principles & Practice of Oncology, 3d ed, p1364).
