Unusual Mesenchymal Tumors of the Lower Gastrointestinal Tract: When You Hear Hoofbeats in the Night, Do Not Forget the Zebras

Introduction: Little information about clinical presentation of mesenchymal tumors of the lower gastrointestinal (GI) tract due to their extreme heterogeneity is available for clinical management. Usually, small solitary asymptomatic polyps are accidently found during a screening colonoscopy performed for hematochezia, abdominal pain, constipation, diarrhea, and bowel obstruction. In this case series, we illustrate our experience with mesenchymal tumors of the lower GI tract, which are a group of unusual and quite challenging lesions. Case Presentation: We retrospectively collected mesenchymal tumors of the lower GI tract in our institution (Fondazione IRCSS Ca' Granda - Ospedale Maggiore Policlinico di Milano) during the last 10 years. We reviewed the histological slides, and, when necessary, we performed immunohistochemical analyses to better characterize the tumors. A total of 99 cases were identified: 45 GISTs, 42 lipomas, 4 leiomyomas, 3 Kaposi sarcomas, 1 schwannoma, 1 ganglioneuroma, 1 hemangioma, 1 inflammatory fibroid polyp, and 1 challenging case of spindle cell melanoma. We focused on the most rare entities excluding therefore all GISTs and lipomas from re-evaluation. Conclusion: Mesenchymal tumors of the lower GI tract represent a highly heterogeneous group of lesions encompassing GISTs, lipomas, smooth muscle tumors (leiomyoma and leiomyosarcoma), GI schwannomas, inflammatory fibroid polyps, solitary fibrous tumors, and other unusual spindle cell tumors. Immunohistochemistry and, in selected cases, molecular biology remain a useful tool which, in addition to a meticulous study of the morphology, helps the pathologist in the tangled jungle of differential diagnosis.