Extrapleural solitary fibrous tumor: A clinicopathologic study of 24 cases

Solitary fibrous tumors (SFTs), rare in extrapleural sites, can present dif ficulties in diagnosis at these locations. From the files of the M. D. Ande rson Cancer Center, we accessed 24 cases of extrapleural SFT (14 females, 1 0 males, ages 19 to 85 yr) obtained for clinical, histologic, immunohistoch emical, and follow-up findings. Tumor locations included the head and neck (n = 12), the abdomen (n = 10), and the extremities (n = 2). Tumors were 2 to 25 cm in greatest dimension (mean, 8.2 cm) and were well circumscribed o r encapsulated. Histologic features were typical of their pleural counterpa rts, e.g., bland spindle cells with some hypercellular areas and myxoid to hyalinized backgrounds. A hemangiopericytic vascular pattern was present in 19 cases and prominent in 11. Mitotic activity ranged from 0 to 4 counts p er 10 high-power fields. Necrosis was seen in two cases. Focally increased cellularity was seen in seven cases and margins were positive in another se ven cases. Spindle cells were positive for vimentin (19 of 19) and CD34 (18 of 20), and negative for cytokeratin (0 of 19). Fibroblastic differentiati on was present in the three cases studied by electron microscopy. Flow cyto metry in three cases revealed diploid cell populations. Follow-up for 19 ca ses (9 to 99 mo) revealed no evidence of metastasis, although one patient h ad residual tumor after an incomplete excision, and one patient died of unr elated causes. Histologic findings such as mitotic counts, necrosis, cellul arity, and marginal status were not associated with outcome. We conclude th at extrapleural SFTs are clinically and histologically similar to their ple ural counterparts. Although the length of clinical follow-up was short (mea n follow-up, 41.4 mo), recognizing these lesions is important because they typically follow an indolent clinical course if completely excised. Althoug h CD34 is nonspecific, it is usually positive hi SFTs and may aid in their diagnosis.