Background: Solitary fibrous tumor (SFT) is a rare neoplasm frequentlyinvolving the pleura. Benign and malignant forms of the tumor occur, the benign variant being three tofour times more common than the malignant. Case Report: We present herein a rare case of large malignantsolitary fibrous tumor (SFT) originating from the peritoneum. An abdominal computed tomography scan revealeda well-defined solid tumor with mixed density. An abdominal ultrasonography (US) revealed a well-circumscribedsolid tumor containing a partially cystic lesion. T1-weighted abdominal magnetic resonance imaging demonstrateda hypo- to isointensity, which was a hypo- to hyperintensity on T2-weighted images. Liposarcoma originatingfrom the retroperitoneum was suggested, and the patient underwent a complete resection of the tumor aswell as the left kidney because tumor invasion of the upper left kidney was suspected. Immunohistochemically,the spindle-shaped cells were positive for CD34, and the diagnosis was SFT originating from the peritoneum.At the 14-month follow-up evaluation, no recurrence or metastasis was detected. Conclusions: This casegave us some difficulty, and the correct diagnosis of the peritoneal mass was valuable. To diagnose themalignant potential of this type of tumor accurately may have value to direct the appropriate therapeuticoperations after surgery and postoperative progress observation.

Keywords: Antigens, CD34 - metabolism, Follow-Up Studies, Neoplasms, Fibrous Tissue - surgery, Peritoneal Neoplasms - surgery, Time Factors, Tomography, X-Ray Computed