Solitary fibrous tumors (SFTs) are rare spindle cell tumors that usually arise in the pleura, although they also occur in extrapleural sites. SFTs usually have a favorable clinical course; however, a small number of metastasizing cases have been reported. We evaluated the detailed clinicopathological findings of 20 extrapleural and 16 pleural SFTs. Among them, 6 cases of the histologically malignant variant (4 extrapleural and 2 pleural tumors) were identified. According to the follow-up data, none of the cases, including the 6 malignant tumors, developed metastases in this series. There were no significant differences in the histologic findings between the extrapleural and pleural SFTs. Tumor necrosis was frequently associated with the malignant variant. Immunohistochemically, malignant SFTs revealed more frequent p53 expression (4/6) compared with ordinary SFTs (2/30) (P=0.0027). The MIB-1 labeling index (LI) was significantly higher in malignant SFTs (mean 13.48) than in ordinary SFTs (mean 7.27) (P=0.0023). Our results indicate that tumor necrosis, p53 expression and a high MIB-1 LI are additional parameters of the malignant variant of SFTs, although they do not necessarily have predictive value for biological behavior. Further studies are required to find reliable molecular biomarkers for predicting the clinical outcome of patients with this unique tumor.

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