Orbital hemangiopericytoma: A case report E. Omoscharka * , K. Lankachandra * University of Missouri, Dept. of Pathology, Kansas City, USA Objective: Hemangiopericytomas are rare vascular tumors arising from Zimmerman's pericytes, which ubiquitously surround blood capillaries and post-capillary venules. They rarely occur in the orbit, accounting for only 1 % of all orbital neoplasms. In most cases the primary presentation is progressive proptosis. Method: A 26-year-old female was first seen for the evaluation of a right orbital mass which has been causing proptosis. Associated symptoms included peripheral vision loss in the right eye, persistent headache with occasional balance problems. A palpable, 3.2 × 2.5 cm well circumscribed mass with a smooth surface, resting on the inferior orbital wall was completely removed through right anterior orbitotomy. H&E stained histologic sections were studied. In addition, tissue sections were subjected to immunohistochemical stains for CD34, CD99, BCL2, SMA, desmin, EMA and S-100. Results: The mass was composed of spindle cells with very low mitotic activity. Staghorn vascular channels were evident, and in several areas the tumor cells invaded the pseudocapsule. The tumor cells were positive for CD34, CD99 and bcl-2 supporting the histopathologic diagnosis of benign hemangiopericytoma.