High-grade surface Osteosarcoma – Data from the Cooperative Osteosarcoma Study Group (COSS)

High-grade surface osteosarcoma (HGSOS) is a rare subtype of osteosarcoma, accounting for approximately 1% of all cases. Due to its rarity, data on clinical characteristics, treatment, and outcomes remain limited. Patients with a histologically confirmed diagnosis of HGSOS registered in the Cooperative Osteosarcoma Study Group (COSS) between 1978 and 2021 were retrospectively analyzed. Event-free survival (EFS) and overall survival rate (OAS) were estimated using Kaplan–Meier analysis, and potential prognostic factors were explored. Thirty-one patients met the inclusion criteria. Median age at diagnosis was 16.8 years, and tumors were predominantly located in the extremities (83.9%). Primary metastatic disease at diagnosis was rare (n = 1). At a median follow-up of 5.33 years, 3- and 5-year EFS were 79.5% and 75.3%, while 3- and 5-year OAS were 82.7% and 78.1%, respectively. Larger tumor volume, poor histological response to neoadjuvant chemotherapy, and early recurrence were associated with inferior outcomes in univariate analyses. In multivariate analysis, tumor localization in an extremity was identified as the only independent predictor of overall survival. Patients with HGSOS treated within multimodal osteosarcoma protocols demonstrated favorable survival outcomes. Prognostic interpretation is limited by the small cohort size; however, tumor localization, tumor volume, histological response, and timing of recurrence appear to influence outcome. Larger collaborative studies are needed to further define prognostic factors in this rare tumor entity. • This study includes 31 HGSOS patients and represents one of the largest cohorts reported to date. • Two patients were diagnosed with Li-Fraumeni syndrome, emphasizing genetic predisposition in OS. • Histological response remains an important prognostic marker in HGSOS.