PURPOSE: Prognosis of extraskeletal osteosarcoma (ESOS) is reported to be poorer than that of skeletal osteosarcoma. This multicenter retrospective study aimed to evaluate factors influencing ESOS prognosis. PATIENTS AND METHODS: Members of the European Musculoskeletal Oncology Society (EMSOS) submitted institutional data on patients with ESOS. RESULTS: Data from 274 patients treated from 1981 to 2014 were coll...