Solitary fibrous tumor. A cytologic-histologic study with clinical, radiologic, and immunohistochemical correlations.

Background: Solitary fibrous tumors (SFT) are rare neoplasms that most commonly involve the pleura, mediastinum, and lung. They are believed to be submesothelial in origin. Histologically, they are characterized by fibroblast-like cells and connective tissue in varying proportions. The "patternless pattern" and the hemangiopericytoma-like pattern are the most common arrangements. The majority of SFTs have been immunoreactive for CD34. Very little has been reported regarding the cytologic findings in these tumors.

Methods: The authors reviewed the radiographic findings and studied fine-needle aspiration biopsies performed on seven patients with SFT of the pleura, and examined subsequent histologic material. The cell blocks or smears of all cases were stained with a monoclonal antibody to CD34.

Results: The cytologic preparations showed varying degrees of cellularity. Smears were comprised of spindled cells in a bloody background with small amounts of collagen. The three malignant tumors had a greater number of cells, both dispersed and in clusters, with nuclear pleomorphism and prominent nucleoli. The cell blocks characteristically showed varying degrees of cellularity with thickened blood vessels and a hemangiopericytoma-like pattern. The cells from the four benign tumors tended to be smaller in size and grouped in more cohesive clusters than the malignant ones. CD34 stained the spindle tumor cells in the cell blocks or smears from five cases; one malignant and one benign case failed to stain. The corresponding histologic samples from these cases demonstrated the same CD34 staining pattern in all cases except one.

Conclusions: The differential diagnosis of SFT includes neurogenic tumors, mesotheliomas, sarcomatoid carcinoma, synovial sarcoma, hemangiopericytoma, and fibrosarcoma. CD34, when present, may prove equally helpful as a positive marker in cytology and in histologic preparations in the differential diagnosis of SFT.