EPT early phase trial IQR interquartile range PES postevent survival R/R relapsed/refractory 1 Introduction Extracranial solid tumors represent approximately 40% of childhood cancer diagnoses, with upfront prognosis varying substantially according to specific diagnosis and respective risk stratification criteria [1]. In the event of on-therapy tumor progression, treatment refractoriness, or off-therapy recurrence, tumors are deemed relapsed/refractory (R/R).