Retroperitoneal Solitary Fibrous Tumor: A “Patternless” Tumor

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