Hyalinizing spindle cell tumor with giant rosettes and low-grade fibromyxoid sarcoma: an immunohistochemical and ultrastructural comparative investigation.

Hyalinizing spindle cell tumor with giant rosettes (HSCTGR) and low-grade fibromyxoid sarcoma (LGFMS) are 2 variants of fibrosarcoma, which share several clinicopathologic features. This study compares the light microscopic, immunohistochemical, and ultrastructural features of 2 examples of HSCTGR and 3 of LG FMS to determine the degree of overlap of these 2 tumors. HSCTGR were composed of bland spindle cells within hyalinized to myxoid stroma. Scattered throughout the lesions were characteristic rosette-like structures, formed by a central collagenous core surrounded by spindled neoplastic cells. LGFMS consisted of a mixture of fibrous and myxoid areas, composed of fibroblast-like cells arranged in a swirling, whorled growth pattern. Immunohistochemical analysis showed diffuse positivity for vimentin in all cases, while few scattered tumor cells stained for CD57, CD34, factor XIIIA, and actin. The extracellular matrix showed intense positivity for type IV collagen, laminin, and fibronectin, with the exception of myxoid areas of LGFMS and the central core of the giant rosettes in HSCTGR. Ultrastructurally, both tumor types were composed of cells with the features of fibroblasts, embedded in a collagenous stroma with irregular deposits of amorphous basal lamina-like substance. In conclusion, HSCTGR and LGFMS share similar immunophenotypic and ultrastructural features, and together with other fibrosing fibrosarcomas, like sclerosing epithelioid fibrosarcoma, may constitute a subset of fibrosarcomas formed by fibroblasts capable of producing large amounts of basal lamina-like material.