Retroperitoneal Solitary Fibrous Tumor: A(Patternless)Tumor

Solitary fibrous tumor is a rare type of mesenchymal, spindle-cell tumor reported mostly in the pleura. Retroperitonealoccurrence is rare and histopathological diagnosis is challenging.Case Presentation. A 55-year-old woman with nonspecificabdominal pain was found to have a retroperitoneal/pelvic mass adjacent to the upper rectum. The patient underwent surgicalresection in clear margins of this pelvic tumor, entering the total mesenteric excision surgical plane. Final histopathology revealeda solitary fibrous tumor and the case is presented herein.Discussion. Solitary fibrous tumor in the retroperitoneum is rarely foundin the literature and to the best of our knowledge less than a hundred cases are described so far. Histopathological diagnosis ismostly based on a “patternless pattern” on microscopic examination, which is a storiform arrangement of spindle cells combinedwith a “hemangiopericytoma-like appearance” and increased vascularity of the lesion. Surgery is the mainstay of treatment andrecurrence rates are generally low.