Extraosseous osteosarcoma arising from the small intestinal mesentery.

Summary Extraskeletal osteosarcoma (EOS) is a highly aggressive and exceedingly rare mesenchymal tumor. Due to the rare nature of the disease, the diagnosis can be difficult and is often confirmed only after diagnostic laparotomy and histopathology. We describe the clinical history, radiologic and histomorphologic presentation, and clinical management of a 61-year-old patient who presented with abdominal pain. Abdominal ultrasound and computerized tomography (CT) scan revealed a calcified intra-abdominal mass. Following an explorative laparotomy, histology showed a large extraosseous osteosarcoma of the small bowel mesentery. Therapy according to the Cooperative Sarcoma Study-96 (COSS-96) was commenced. Diagnosis, management, and outcome in the context of the current literature are discussed. To our knowledge, this is the first description of an extraosseous osteosarcomas in the small bowel mesentery in the literature.