Small Benign Storiform Fibrous Tumor (Fibrous Histiocytoma) of the Conjunctival Substantia Propria in a Child: Review and Clarification of Biologic Behavior.

PURPOSE: A case of a small benign storiform fibrous tumor of the conjunctival substantia propria is described to clarify the category of fibrous histiocytoma. In addition, a comparison of the various spindle cell tumors of the conjunctival substantia propria is explored. METHODS: The patient underwent a complete tumor excision, and the specimen was analyzed by histopathologic and immunohistochemical investigations. RESULTS: A cellular mass, composed solely of spindle cells in a storiform pattern without a component of histiocytic cells, was found beneath an undisturbed nonkeratinizing squamous epithelium and was separated from the epithelium by a grenz zone of uninvolved collagen. The lesion was sharply demarcated but not encapsulated. The Masson trichrome stain revealed scant deposition of intercellular collagen. The reticulin stain displayed numerous and delicate wiry fibers between the tumor cells and encircling capillaries. The Alcian blue stain demonstrated faint positivity in the interstitium. Immunohistochemistry revealed positivity for vimentin, factor XIIIa, smooth muscle actin, CD10, and CD45. Negative stains were obtained for CD34, CD56, S100, desmin, and Ki67. CONCLUSIONS: The broad term of fibrous histiocytoma should be reserved for deep fibroblastic spindle cell tumors (e.g., those of the orbit) that display an aggressive behavior. More benign superficial spindle cell tumors of the dermis are now preferentially characterized as dermatofibromas. It is suggested that equally benign epibulbar tumors should no longer be designated as fibrous histiocytomas but rather as benign storiform fibrous tumors. Tumors completely composed of polygonal histiocytoid (epithelioid) cells that are CD34+ should be excluded from the benign storiform fibrous tumor category. Positive smooth muscle actin and factor XIIIa staining in conjunction with negative staining for CD34 and desmin in the current spindled tumor cells are findings consistent with those of cutaneous dermatofibromas. Both the epibulbar and dermal spindle cell lesions have displayed an indolent and nonaggressive behavior. Microscopically they contain a high proportion of dendrocytic stellate cells that are either factor XIIIa+ or XIIIa-. Given the anatomic differences between the dermis and conjunctiva, the term dermatofibroma is inappropriate for the current tumor; instead the term benign storiform fibrous tumor has been proposed for superficial tumors of the conjunctiva.