Malignant Fat-Forming Solitary Fibrous Tumor of the Thigh
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 (). 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 necrosis, and cystic degeneration (). 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 adipose tissue. The tumor had alternating hypercellular and hypocellular 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) (). 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 cytoplasm with a flat nucleus located at the periphery of the cells (). 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 protein, keratin, epithelial membrane antigen, CD31, desmin, and CD68. Mature adipose tissue between SFT components was positive for S-100 protein (). Based on these histologic and immunohistochemical findings, the tumor was diagnosed as a malignant fat-forming SFT of the thigh. The patient was followed postoperatively for 14 months, and there was no evidence of recurrence or metastasis.