Answer: Solitary fibrous tumor

Histology: Since it is “a diagnosis in vogue”[1], it is important that solitary fibrous tumor be presented! In early literature, these tumors were described as pleural-based lesions believed to be the benign localized counterpart of “fibrous” mesotheliomas, an assertion supported by tissue culture studies, which resulted in growth of mesothelial-like sheets. Subsequent ultrastructural and early immunohistochemical, studies supported a fibroblastic origin for the proliferating cells[2]. The clinical behavior of the pleural-based tumors was well-delineated in a landmark paper by England and others [3], who described 223 cases of pleural-based tumors from AFIP archives and offered criteria for separating malignant from benign examples which included the presence of >4 mitosis/10 high power fields, prominent nuclei and nuclear pleomorphism in malignant lesions. They were also known to elaborate insulin-like growth factors and were sometimes associated with hypoglycemia. Solitary fibrous tumors have now been described in many sites, including the soft tissues [4-33].

Grossly, regardless of their location, solitary fibrous tumors are typically well-marginated tumors.

Microscopically, solitary fibrous tumors characteristically display a “patternless pattern” of spindled fibroblast-like cells and collagen in varying proportions arranged in a disorderly pattern. The individual cells have spindled to plump, ovoid nuclei and eosinophilic cytoplasm. They may also appear hemangiopericytoma-like with closely packed cells with amphophilic cytoplasm arranged around staghorn-shaped vessels.

On immunohistochemical staining, solitary fibrous tumors consistently express CD34, regardless of their malignant potential [29, 34, 35] and lack muscle markers (actins and desmin), keratins, and S100 protein.

Discussion: The differential diagnosis of solitary fibrous tumors revolves around fibroblastic lesions with abundant collagen and lesions with a hemangiopericytoma-like vascular pattern. Separating them from fibromatoses is straightforward as fibromatoses do not express CD34 and are infiltrative. The associated collagen is less wiry and more solid. Low-grade fibrosarcomas are also CD34 negative. This then leads to other CD34 positive lesions and HPC-like lesions as the principal contenders in the differential diagnosis. They can be quite difficult to distinguish from hemangiopericytomas, which usually have only weak CD34 expression. The criteria for malignancy are similar for hamangiopericytoma and solitary fibrous tumor although the former are known to occasionally display unpredicable behavior. Other lesions having a hemangiopericytoma-like vascular pattern include synovial sarcoma and mesenchymal chondrosarcoma, both of which are readily distinguished by application of the appropriate immunohistochemical panel. Synovial sarcoma is essentially always CD34 negative and mesenchymal chondrosarcoma has a characteristic juxtaposition of mature-appearing cartilage and primitive cells.