MDB-53. Salvage Craniospinal Irradiation for Relapsed WNT-Pathway Medulloblastoma Patients Treated With Upfront Focal Conformal Radiotherapy and Chemotherapy at Initial Diagnosis: Long-Term and Mature Outcomes From a Prospective De-Intensification Study

Abstract BACKGROUND Excellent long-term outcomes (5-year overall survival 90%) in WNT-pathway medulloblastoma (MB) patients have prompted systematic attempts at de-intensification of therapy to mitigate treatment-related toxicities. Mature outcomes of a prospective study (FOR-WNT) treating low-risk WNT-MB children with focal conformal radiotherapy (RT) and adjuvant systemic chemotherapy omitting upfront craniospinal irradiation (CSI) are being reported herein. METHODS Seven children with rigorously-defined low-risk WNT-MB were treated with post-operative focal conformal RT to the index tumor-bed (54Gy/30 fractions) followed by 6-cycles of adjuvant systemic chemotherapy after written informed consent/assent. Survival outcomes were computed using Kaplan-Meier method and reported as point estimate with 95% confidence intervals (CI). The trial was terminated prematurely due to unacceptably high risk of neuraxial failures. RESULTS One child succumbed to chemotherapy-related toxicity while 3 of 7 children were detected with leptomeningeal relapse on surveillance neuro-imaging within 2-years of index diagnosis. Full-dose salvage CSI (35Gy/21 fractions) at relapse followed by further chemotherapy led to excellent clinico-radiological response in all 3 children; however, one child developed further local recurrence and leptomeningeal metastases and succumbed to recurrent/progressive disease despite further aggressive salvage therapy. Five of 7 children including the other two treated with salvage CSI and systemic chemotherapy are alive and continue to remain in remission on long-term follow-up. At a median follow up of 61 months, the 5-year Kaplan-Meier estimates of event-free survival, relapse-free survival, and overall survival were 42.9% (95%CI: 18.2-100%), 50.0% (95%CI: 22.5-100%), and 71.4% (44.7-100%) respectively. CONCLUSIONS 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 response, remissions are not durable resulting in sub-optimal long-term overall survival. Judicious de-escalation with upfront low-dose CSI (18Gy) is currently being tested in a successor study (FOR-WNT 2).