2.1. The SEER database and related codes

The SEER database contains information on cancer epidemiology among the United States population.^[19] All patients with osteosarcoma listed in the SEER database from 1973 to 2014 were identified. We used code number “011” under the North American Association of Central Cancer Registries item “CS Schema v0204+” as the first filter, which helped find all bone malignancies. Subtypes of osteosarcoma were coded as 9180 to 9187 and 9192 to 9194, and COS was coded as 9181 according to the International Classification of Diseases for Oncology (ICD-O-3 edition). Clinical factors, including age, gender, race, marital status, year of diagnosis, primary site, histology grade, metastasis at diagnosis, tumor size (largest dimension), treatment strategies, follow-up time and survival status were retrieved for this research. All data were extracted using Perl language. We signed the Data-Use Agreement for the SEER 1973 to 2014 Research Data File and had the permission to use the database for research.