Extrapleural solitary fibrous tumor: clinicopathologic study of 17 cases and molecular analysis of the p53 pathway

Solitary fibrous tumor (SFT) occurring at various extrapleural sites is sometimes difficult to diagnose because of its histologic variability. Although a solitary fibrous tumor is usually a slow‐growing tumor with favorable prognosis, a small number of malignant cases have been reported. In the present study, we examined the clinical behavior, histologic, immunohistochemical and molecular features of 17 cases of extrapleural SFT. Four tumors were located in the pelvic cavity, two in the nasal cavity, two were confined to the pulmonary parenchyma, and there was one each in the meninges, kidney, mediastinum, retroperitoneum, temporal region, neck, groin, buttock and thigh. Histologically, all the tumors were characterized by the presence of areas consisting of a proliferation of bland spindle cells with variable amounts of thick, often hyalinized or keloid‐like intercellular collagen bundles. Highly cellular areas were observed in three tumors, frequent mitoses in two, and cellular pleomorph‐ism and tumor necrosis in one each. All 17 tumors showed immunoreactivity to CD34 and 15 (88%) to bcl‐2 protein. The labeling indices of p53, mdm2 protein and Ki‐67 were generally low. PCR‐SSCP and a subsequent sequence analysis of the p53 gene disclosed point mutation at codon 161 in exon 5 in one of the 13 cases analyzed. According to follow‐up information, none of the patients had developed local recurrence or distant metastasis. Our results suggest that most extrapleural SFTs behave in a benign fashion even in a higher histologic grade group, and it is difficult to predict their clinical outcome. Complete surgical excision in order to obtain clear margins and long‐term follow‐up is advisable for patients with an extrapleural SFT.