S397
Clinical - CNS
ESTRO 2026
Material/Methods: Seven children (median age 10 years) with molecularly characterized WNT-MB and rigorously defined low-risk disease were accrued and treated with post-operative focal conformal RT to the tumor-bed alone (54Gy/30 fractions/6-weeks) followed by 6-cycles of adjuvant systemic chemotherapy after written informed consent/assent. All time-to-event outcomes were calculated using Kaplan-Meier method and expressed as 6-year survival with 95% confidence interval (CI). The study was terminated prematurely due to unacceptably high incidence of neuraxial failure (3 of 7 patients). Results: Three of 7 children were detected with leptomeningeal relapse on surveillance neuro-imaging within 2-years of index diagnosis, while one child succumbed to chemotherapy-related toxicities during treatment. All three children with neuraxial failure were treated with aggressive multi-modality salvage therapy including full-dose CSI plus focal boost irradiation to nodular metastatic deposits and systemic chemotherapy. Two of them remain alive and disease free while one child succumbed to further disease local and neuraxial progression despite 3rd line salvage chemotherapy. The remaining 3 children treated with only focal radiotherapy plus chemotherapy who did not experience a relapse remain alive, event free, and disease free well beyond 5-years from index diagnosis. At a median follow up of 74 months, the 6-year Kaplan-Meier estimates of event-free survival (EFS), relapse-free survival (RFS), and overall survival (OS) were 42.9% (95%CI: 18.2-100%), 50.0% (95%CI: 22.5- 100%), and 71.4% (44.7-100%) respectively. Conclusion: Omission of upfront CSI in low-risk WNT-MB is associated with unacceptably high risk of neuraxial failure. Although full-dose salvage CSI achieves excellent clinic-radiological response, remissions may not be durable with resultant sub-optimal long-term survival. Judicious de-escalation with upfront low-dose CSI (15-18Gy) is presently being tested in low-risk WNT-MB globally. Keywords: WNT, de-intensification, CSI
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