Epidemiological and clinicopathologic characteristics, and prognostic factors of patients with extra-skeletal osteosarcoma: a Surveillance, Epidemiology, and End Results population study.

Background: Extra-skeletal osteosarcoma (ESOS) has an extremely low incidence and unclear prognosis. We studied ESOS via Surveillance, Epidemiology, and End Results data to characterize it and build a prognostic nomogram. Methods: The baseline information and survival outcome of patients were illustrated according to different primary tumor sites. The independent prognostic factors for ESOS were analyzed using univariate and multivariate Cox regression analyses. A nomogram was constructed using these prognostic factors to predict the survival of patients. The Kaplan-Meier method was performed to estimate survival, and both the log-rank test and the Wilcoxon-Breslow-Gehan test were used to compare the survival. Results: A total of 4,567 patients with osteosarcoma who met the inclusion criteria were enrolled, including 4,317 patients with osteosarcoma of bone and joint origin and 250 patients with ESOS. The 1-, 3-, and 5-year tumor-specific survival rates for ESOS were lower than those for skeletal osteosarcoma. Multivariate Cox analysis showed that older age at diagnosis, distant staging, and presence of bone metastases were independent risk factors affecting patient prognosis, and surgery of the primary site was independently associated with improved survival. A nomogram was created based on these factors to predict overall survival at 1, 3, and 5 year in patients with ESOS. An internally validated nomogram consistency index showed satisfactory results between predictions. Conclusions: Primary focus surgery is an important factor in improving survival outcomes in patients with ESOS. The nomogram for predicting the prognosis of patients with ESOS was shown to have favorable accuracy and reliability. Such a prognostic nomogram may assist clinicians in optimizing.