| Summary: | Background: Ewing sarcoma (ES) of bone is accounting for the second most common type of primary bone cancer in children and adolescents. However, the patterns of distant metastasis (DM) and the effect of the sites of DM on survival outcomes were not investigated. Aims: This study aimed to investigate the patterns of DM and the prognostic factors related to outcomes in primary metastatic ES of the bone. Methods: Patients who were diagnosed with primary metastatic ES between 2010 and 2018 were identified from the Surveillance, Epidemiology, and End Results database. Kaplan–Meier analysis, log-rank tests, and Cox proportional-hazards regression models were used for statistical analyses. Results: We identified 277 patients in this study and 95.3% of them (n = 264) receiving chemotherapy. A total of 371 sites of DM were observed. Lung was the most common distant metastatic site (n = 182, 49.1%), followed by bone (n = 139, 37.5%), distant lymph node (n = 26, 7.0%), liver (n = 14, 3.8%), and brain (n = 10, 2.7%). Three-year cause-specific survival (CSS) was 56.1% in the entire cohort. Older age (hazard ratio [HR] 2.210, P < 0.001) and bone metastasis (HR 1.903, P = 0.002) were the independent prognostic factors associated with inferior CSS. Similar results were found in those with bone-only metastasis (n = 80) or lung-only metastasis (n = 117), which showed that patients with bone-only metastasis had an inferior CSS compared to those with metastases only to the lung (HR 1.926, P = 0.005). Conclusions: Lung and bone are the most frequently distant metastatic sites in patients with primary metastatic ES of bone. Bone metastasis is an independent risk factor for inferior survival.
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