[Extrapleural solitary fibrous tumor with uncommon histology: a clinicopathologic analysis of 7 cases].

Objective: To investigate the clinicopathologic characteristics, immunophenotypes, and differential diagnostic features of extra-pleural solitary fibrous tumor (SFT) with uncommon histology. Methods: Seven cases of extra-pleural SFT with uncommon histology were collected during January 2015 and December 2016 in Zhejiang Provincal People's Hospital; the clinical and radiologic features, histomorphology, immunophenotype and prognosis were analyzed. EnVision method was used for immunohistochemical staining of STAT6, CD34 and other differential diagnosis associated markers. Results: There were five male and two female patients, age from 23 to 54 years (mean=39 years). Three tumors were located in the soft tissue of head and neck, two in trunk subcutaneous soft tissue, one in sella region, and one in the kidney. Grossly the tumors ranged from 0.4 to 8.0 cm (mean=3.1 cm). Microscopically, all three head and neck cases resembled giant cell angiofibroma/giant cell subtype SFT, and one case showed sheet-like pattern of the multinucleated syncytial cells, creating a biphasic arrangement similar to myofibroma. Both truncal tumor resembled lipomatous type SFT, with one similar to dermatofibrosarcoma protuberans and the other to atypical spindle cell lipomatous tumor. The sella tumor showed morphology of a conventional SFT with high grade sarcomatous transformation. The renal tumor demonstrated a malignant SFT with entrapped benign renal tubules, mimicking a biphase synovial sarcoma or a malignant mixed epithelial and stromal tumor. By immunohistochemistry, all seven SFTs showed diffuse and strong nuclear reactivity to antibody against STAT6. Conclusions: Extra-pleural SFTs show a significant heterogeneity of morphology and biological behavior which could cause differential confusion.Careful attention to its characteristic histomorphology with the use of STAT6 immunohistochemistry can help distinguish this tumor from its many mimickers.