A mesenchymal tumor of perivascular cell origin, composed of dilated vascular spaces (some with a staghorn or bifed shape) throughout the entire tumor. The tumor cells are surrounded by reticulin, are negative for muscle, nerve sheath, and epithelial markers, and are positive for CD34. The biologic behavior of hemangiopericytoma cannot always be predicted from the morphologic features. The 5- and 10-year survival rates of malignant hemangiopericytomas have been reported to be 40% and 29% respectively (partially adapted from S. Sternberg's Diagnostic Surgical Pathology, 3rd ed., 1999). -- 2004