Histologic sections showed a well-circumscribed, nonencapsulated spindle cell tumor with a histologic pattern consisting of variably cellular areas composed of bland spindle cells separated by fine collagen. Cytologic atypia, necrosis, and mitotic figures were not identified. There were occasional dilated thin-walled vessels, although most were collapsed. The spindle cells were positive for CD34 and negative for S100 protein.
Solitary fibrous tumor is an unusual neoplasm of mesenchymal differentiation that was originally described in the pleura but has increasingly been found in a variety of cutaneous and subcutaneous locations, including the scalp, neck, cheek, back, and thighs, with sizes ranging from 1 to 4 cm in diameter.1- 5 The tumor may derive from CD34+ dendritic interstitial cells, which are found in a wide variety of body sites, including the dermis.6 Histologically, it is a well-circumscribed but nonencapsulated neoplasm composed of bland, fine spindle cells separated by fine collagen. The cells may be arranged in hypercelluar and hypocellular areas with a variety of patterns, including storiform, herringbone, sclerosing, palisading, and myxoid. Dilated thin-walled vessels are frequently seen. Solitary fibrous tumors are positive for CD34, bcl-2, and vimentin and negative for S100 protein, epithelial membrane antigen, actin, keratin, and desmin.
Slow-Growing Nodule on the Thigh—Diagnosis. Arch Dermatol. 2006;142(7):921-926. doi:10.1001/archderm.142.7.921-h