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Spindle cell tumors are mesenchymal tumors having mesodermal origin. A hemangiopericytoma arises from pericytes. Another view is that it originates from pluripotent perivascular cells. In 1942, Stout and Murray described tumors, which were composed of capillary blood vessels with one or more layers of rounded cells arranged about them. These differed from capillary hemangioma because of the presence of perivascular cells and they named it hemangiopericytoma based on the morphological similarities with pericytes (Stout AP et al.,1942). Pericytes were first described by Zimmermann in 1923.
Hemangiopericytoma is an unusual tumor representing only about 1% of all vascular neoplasms. It occurs in both genders with equal frequency in the head and neck region but is seen less frequently in the orbital region. It is more commonly found in the nasal cavity. It is responsible for about 1-3% of orbital lesions.
Dorfman first reported solitary fibrous tumours in 1994. After 1994 more cases of solitary fibrous tumors were detected. These were earlier labeled as hemangiopericytoma or hemangiopericytoma like tumors. Furusato et al (2011) published a study of 41 cases designated as hemangiopericytoma, fibrous histiocytoma and giant angiofibroma. They found that these tumors have overlapping morphologic and histochemical features and suggested solitary fibrous tumor be used as an all-encompassing terminology. According to Nappi et al (2012), a monomorphic population of compact polygonal or bluntly fusiform cells and stag horn stromal vascular pattern classically characterize a hemangiopericytoma. On the other hand solitary fibrous tumors are composed of variable pleomorphic spindle cells, admixed with collagen and arranged haphazardly in a patternless manner (Gengler C, 2006).
The distinction between solitary fibrous tumors and hemangiopericytoma can be subtle.
According to Gengler C et al (2006), the ramifying vascular pattern is not specific for hemangiopericytoma and can be seen in a variety of soft tissue tumors, like solitary fibrous tumor, fibrous histiocytoma, and other variants of solitary fibrous tumors. He no longer considered hemangiopericytoma a specific entity but rather as a growth pattern. He reclassified them as fibrous variant, giant cell variant, cellular variant, fat forming variant, and fibrous histiocytoma like variant.
Despite this resemblance, there are individual clinical features and distinctions on histochemical analysis. According to Westra WH et al (1994), a solitary fibrous tumor shows strong and consistent reactivity for CD34, with ropy collagen fibrils in contrast to a hemangiopericytoma, which hardly shows any focal reactivity to CD34 (Goldsmith et al 2001). Both these tumors show negative reactivity for immunodeterminants of epithelial and neuromyogenic differentiation (Keratin, cytokeratin SMA, S100). GenglerC et al (2006) reports reactivity against CD99 and Bcl-2 could be positive in both hemangiopericytoma and solitary fibrous tumors.