(P 319) THE SURVIVAL DISPARITY BETWEEN CHILDREN AND ADOLESCENTS & YOUNG ADULTS WITH EWING SARCOMA IN THE NETHERLANDS DID NOT CHANGE SINCE THE 1990S DESPITE IMPROVED SURVIVAL: A POPULATION-BASED STUDY

Objective: Ewing sarcoma is a highly aggressive type of cancer originating in the bone or soft tissues that typically affects children and adolescents & young adults (AYAs). AYAs with Ewing sarcoma have a worse prognosis than children. Population-based survival evaluations stratifying findings by important clinical factors are, however, limited. This Dutch population-based study comprehensively compared survival of children and AYAs with Ewing sarcoma over three decades considering diagnostic period, tissue of origin, tumor site, and disease stage. Findings for the Netherlands can likely be extrapolated to other developed countries with compulsory health insurance and complete coverage of costs of anti-cancer therapies.

Methods: Data on all children (0-17 years, N=463) and AYAs (18-39 years, N=379) diagnosed with Ewing sarcoma in the Netherlands between 1990-2018 were collected from the nationwide Netherlands Cancer Registry. Follow-up was complete until February 2023. Five-year relative (disease-specific) survival was calculated using the traditional cohort method. Multivariable analyses were conducted through Poisson regression.

Results: Children with Ewing sarcoma had a significantly higher 5-year relative survival than AYAs (65% vs. 44%). Survival increased over time to a similar extent in both age groups, reaching 70% in children and 53% in AYAs in 2010-2018. Results were similar for Ewing bone sarcoma and extraosseous Ewing sarcoma. AYAs had a poorer prognosis than children for the majority of tumor sites and regardless of disease stage. Stage-specific survival differences were -18 percent-points (60% vs. 78%) for localized disease and -13 percent-points (20% vs. 33%) for metastatic disease. Detailed analysis showed a superior survival of 78% for the youngest children aged 0-4 years. In subsequent age groups, survival gradually decreased, plateauing at 40-50% in AYAs aged 18 years or older. Multivariable-regression analysis, adjusted for follow-up time, diagnostic period, sex, disease stage, and tumor site, confirmed increased excess mortality within 5 years of diagnosis among AYAs compared with children [excess hazard ratio (HR), 95% confidence interval: 1.7, 1.3-2.1]. Using 0-4 year-olds as reference, statistically significantly increased excess HR were obtained for older children aged 15-17 years (excess HR: 2.2) and all AYAs (excess HR: 2.7-3.4).

Conclusion: Despite survival improvements since the 1990s, AYAs with Ewing sarcoma in the Netherlands continue to fare considerably worse than children. This survival disparity did not change over time and was present irrespective of tissue of origin, tumor site, and disease stage. Future research should focus on determining the contribution of patient characteristics, tumor biology, treatment and therapy-related factors, and social aspects to the survival disparity to provide guidance for improving survival in both children and AYAs.