| Summary: | Solitary fibrous tumor (SFT) is a rare spindle cell neoplasm mainly originated in the pleural cavity. We report here an unusual case of a large SFT in the retroperitoneum. A 27-year-old female complaining of a palpable mass in the right flank with dull pain was admitted to our hospital with the diagnosis of right retroperitoneal tumor. Computed tomography (CT) and magnetic resonance imaging (MRI) showed a large retroperitoneal tumor arising from latissimus dorsi muscle. Surgical findings revealed a partly encapsulated tumor and complete resection of tumor concomitantly with right kidney, 11th and 12th ribs, and diaphragm was performed. Pathological examination demonstrated the tumor to be composed of increased mitotic activity and cellularity of spindle cells with a collagenous matrix. Immunohistochemical staining was positive for CD34, vimentin, and basic fibroblast growth factor (bFGF) and negative for CD31, cytokeratin, desmin, S-100p, smooth muscle actin, Bcl-2, and insulin-like growth factor (IGF) with Ki-67 labeling index of 0.1%. Based on pathological features, diagnosis of SFT in the retroperitoneum was confirmed. To our knowledge, this is the first report of an SFT arising from latissimus dorsi muscle and it is important to include SFT in the differential diagnosis of retroperitoneal tumors that caused considerable diagnostic problems due to its unusual site of origin.
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