Epidemiological landscape and clinical heterogeneity of ultra-rare sarcomas: A population-based study from the SEER database.
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INTRODUCTION: Ultra-rare sarcomas (URS), defined by the Connective Tissue Oncology Society (CTOS) as subtypes with an incidence of ≤ 1 per 1000,000 individuals per year, present significant diagnostic and therapeutic challenges. This study aimed to elucidate the epidemiological landscape and clinical characteristics of URS using a large-scale population-based registry. METHODS: Patients diagnosed with bone sarcomas (BS) and soft tissue sarcomas (STS) between 2000 and 2019 were identified from the Surveillance, Epidemiology, and End Results (SEER) database. Cases were classified into URS and non-URS groups based on the CTOS definition. Patient demographics, tumor characteristics, and survival outcomes were analyzed. RESULTS: The collective incidence rates of bone (B-URS) and soft tissue URS (ST-URS) were 2 and 14 per 1000,000 respectively. Among 14,695 BS and 94,455 STS cases included, URS accounted for 3099 (21.1%) and 15,155 (16.0%). B-URS were more frequently diagnosed in older patients (≥40 years) and presented with lower pathological grades. Conversely, ST-URS were more common in younger patients and exhibited higher rates of metastases. A substantial proportion had unknown pathological grades (53.4% in B-URS; 35.9% in ST-URS), reflecting diagnostic complexity. Although survival analysis indicated outcomes for URS were comparable to or better than non-URS, this likely reflects the prevalence of indolent subtypes rather than superior treatment efficacy. CONCLUSION: This study provides a comprehensive epidemiological description of URS based on the CTOS definition. Our findings highlight the significant clinical heterogeneity of URS, characterized by indeterminate grading and diverse biological behaviors.