Hemangiopericytoma. An asymptomatic 73-year-old man was found to have a large retroperitoneal mass on routine physical examination. This was confirmed by computer tomography scan; at surgery, the mass did not involve any abdominal organs. Immunoperoxidase stains were positive for vimentin and negative for actin, desmin, neurofilament, keratin, and S-100 protein. Electron microscopy showed a tumor composed of pericytes, illustrated at low magnification in cross-section (A) and at higher magnification in longitudinal section (B). Note the dispersed nuclear chromatin pattern and the paucity of cellular organelles in the cytoplasm. Focal strips of basal lamina beneath extended cytoplasmic processes (arrowhead) and cytoplasmic glycogen pools (G) characterize these neoplastic pericytes. Original magnifications: (A), ×3,500; (B), ×13,000. (Courtesy of Ann M. Dvorak, MD, Department of Pathology, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215.)