Malignant Fat-Forming Solitary Fibrous Tumor of the Thigh

Fat-forming solitary fibrous tumor (SFT), previously known as lipomatous hemangiopericytoma, was described as an uncommon variant of extrapleural SFT in the 2013 World Health Organization (WHO) classification of soft tissue tumors. 1 Fat-forming SFT is an uncommon, slow-growing neoplasm and is characterized by the typical histologic features of SFT with a mature adipocytic component. The fat-forming variant of SFT exhibits benign biological behavior; however, in rare case, it can have malignant histologic features. Fewer than 20 malignant cases have been reported in the English literature. 2-5 Herein, we present a case of fat-forming SFT with malignant histologic features arising in the thigh, in addition to a review of the literature. An 84-year-old man visited our hospital complaining of a palpable mass in his left thigh. Eighteen years prior, he underwent radical cystectomy for urothelial carcinoma. Magnetic resonance imaging (MRI) revealed an 11-cm-sized soft tissue mass at the deep aspect of the proximal thigh with infiltration to the surrounding muscle. The tumor exhibited low signal intensity and no obvious intra-tumoral fat component could be identified on T1-weighted MRI (Fig. 1A). The tumor demonstrated high signal intensity on T2-weighted MRI, and showed peripheral enhancement with extensive necrosis and hemorrhage on contrast enhanced T1-weighted MRI with fat suppression. Wide excision was performed due to the suspicion of malignancy. Grossly, the tumor was relatively well circumscribed and had a yellowish-tan appearance with a glistering surface, extensive ne-crosis, and cystic degeneration (Fig. 1B). Microscopically, the tumor was encapsulated and had a partially infiltrative margin. The tumor demonstrated a SFT component characterized by a patternless architecture of oval to spindle cells surrounding the ectatic thin-walled branching vessels admixed with mature adi-pose tissue. The tumor had alternating hypercellular and hypo-cellular areas with extensive necrosis and hemorrhage. The tumor cells in the cellular area were pleomorphic and had numerous mitotic figures (up to 25 mitoses per 10 high power fields [ HPF ] and atypical mitosis) (Fig. 2A, B). The adipocytic components were located at the periphery of the tumor within the capsule, and comprised approximately 5% of the tumor. Fat lobules or scattered individual adipocytes were intermingled with pleomorphic stromal cells. Adipocytes had a clear cyto-plasm with a flat nucleus located at the periphery of the cells (Fig. 2C, D). No lipoblasts were observed in the adipocytic component. Immunohistochemical staining demonstrated that the neoplastic cells were positive for CD34, CD99, and bcl-2, but were negative for S-100 …