Persistent Fluorodeoxyglucose Uptake on Post-Definitive Radiotherapy PET/CT Evaluation Predicts Poor Survival in Pediatric Metastatic Ewing Sarcoma: A Single-Center Cohort Study.

BACKGROUND

Though 18F-FDG-PET CT scan is established as a standard of care imaging technique for staging and restaging in Ewing Sarcoma (ES), its utility in the prognostic assessment of residual masses post definitive radiotherapy to the primary is still unknown. This study audits the above.

PROCEDURE: Children ≤ 15 years of age with ES treated from January 2012 to December 2023 who received definitive radiotherapy to the primary and had an 18F-FDG-PET CT 3 months post completion of RT (PET-3) were analyzed retrospectively. Disease status of the soft tissue component based on PET-3 was classified as having either no residual, anatomic residual with no FDG-avidity, or FDG-avid residual.

RESULTS

One hundred and forty-nine patients formed the study cohort. Metastatic disease was present in 46.3% (n = 69). PET3 showed FDG-avid residual in 17.4% (n = 26), anatomic residual in 30.2% (n = 45), and no residual in 52.3% (n = 78) of patients. Five-year EFS of those with no residual, anatomic residual, and FDG-avid residual in the whole cohort were 52.8% (95% CI: 40.4%-65.2%), 45.9% (95% CI: 28.4%-63.4%), 42.6% (95% CI: 20.7%-64.6%) (p = 0.038). Among patients with FDG-avid residual disease, 5-year EFS was 22.9% (95% CI: 5.0%-45.2%) for metastatic cases compared to 75.0% (95% CI: 44.0%-100%) for localized disease (p = 0.003). A PET3 SUVmax > 3.9 was prognostic for EFS (HR-9.59, 95% CI: 3.94-23.3, p ≤ 0.001) in the metastatic cohort.

CONCLUSIONS

In patients with metastatic Ewing Sarcoma treated with definitive radiotherapy of the primary, the persistence of FDG-avid residual on post-treatment PET/CT signifies a markedly poor prognosis. In particular, a PET3 SUVmax exceeding 3.9 delineates an ultra-high-risk cohort.