Primary extraskeletal osteosarcoma of the mesentery: A case report

•Extraskeletal osteosarcoma is a rare malignant soft tissue tumor without attachment to the bone.•This is the first report of a single incisional laparoscopic resection.•Its diagnosis should be taken into consideration also when a soft tissue mass of the mesentery is found. Extraskeletal osteosarcoma is a rare malignant soft tissue tumor without attachment to the bone. To the best of our knowledge, we present here the first report of a primary extraskeletal osteosarcoma of the mesentery in Japan. A 46-year-old female underwent a health examination, with no complaint. Following an ultrasonography a solid mass was detected in the abdominal cavity. Computed tomography showed a 38 × 25 mm heterogeneously enhancing mass. The latter was characterized by the presence of mottled calcifications and a cystic portion. The tumor was resected with a single incisional laparoscopic curative resection. Histopathological examination revealed the presence of a primary extraskeletal osteosarcoma arising from the mesentery. Postoperative course was uneventful. The patient did not receive chemotherapy during follow-up. She was recurrence free 10 months post-surgery. Patients with extraskeletal osteosarcoma generally have a poor prognosis. A tumor size