Grossly, the tumors are sharply defined, smooth, lobulated masses of homogeneous density. Due to compression of the surrounding structures, they may contain adherent lung parenchyma or soft tissue at the edges. This should not be mistaken for invasion. On cut sections, they show a lobulated or whorled appearance and must be differentiated from a leiomyoma. They can contain cystic cavities, calcification, hemorrhage, and necrosis. Histologically, most characteristic for this tumor is the “patternless pattern” in which spindle cells are haphazardly arranged. Occasionally, there is a storiform or pallisading pattern, and this must be differentiated from other soft-tissue tumors. The spindle cells are intermixed with varying amounts of collagen, focal myxoid changes, or hyalinization of fibrous tissue. Tissue sampling will confirm the diagnosis. In some cases, fine-needle aspiration biopsy, rather than cytology alone, can be sufficient.9Occasionally, the diagnosis can be established only after resection. Confirmation should be gained through immunostaining that is positive for CD34; almost 80 to 100% will express this marker. The staining will also be negative for cytokeratin, S-100, and vimentin. In contrast, the sarcomatous form of diffuse malignant mesothelioma is positive for cytokeratin, and negative for CD34.10 Thus, immunostaining for CD34 is an essential step in differentiating SFTP from other mesothelial tumors arising in the pleura.