Grossly, an SFTP is described as a firm, encapsulated, glistening white/gray mass that can be pedunculated or sessile. Malignant SFTPs tend to be >8 cm and are rarely pedunculated. Histologically, the tumor consists of elongated spindle cells within connective tissue, distributed in a random fashion. Malignant tumors are characterized by more than four mitotic figures per 10 high-power fields, cytologic atypia, increased cellularity, and presence of necrosis. Malignant tumors also have associated areas of hemorrhage, myxomatous changes, pleural effusion, and tumor invasion. Immunostaining of an SFTP is positive for CD34, CD99, and bcl-2. CD34 is a transmembrane glycoprotein, and a marker of hematopoietic stem cells and endothelial cells. CD34 can be absent in high-grade tumors. CD99 is another transmembrane glycoprotein and is associated with apoptosis, adhesion, and maintenance of cell morphology. Bcl-2 is an antiapoptotic molecule that is located in the mitochondrial membrane or the nuclear envelope and is overexpressed in certain tumors. Unlike mesothelioma, an SFTP does not stain for cytokeratin and epithelial membrane antigen. Additionally, these tumors do not stain for S-100, carcinoembryonic antigen, or smooth muscle actin. Cytogenetic analysis may be helpful in distinguishing an SFTP from mesothelioma and sarcoma, and in evaluating whether a tumor is malignant or benign. Cytogenetic studies in SFTPs have found mutations including trisomy 8, trisomy 21, translocations, and complex karyotypes. One study showed that all malignant SFTPs had mutations of p53, a tumor suppressor gene. Further studies in cytogenetics are needed to further elucidate the role of genetic mutations on tumor formation.