RecruitingPhase 1Drug trialDetermine Maximum Tolerated Dose, Safety, and Tolerability of Rhenium (186Re) in Pediatric Recurrent, Refractory or Progressive Ependymoma and High-Grade GliomaPediatric patients 6-21 years of age with supratentorial recurrent, refractory, or progressive pediatric ependymoma and high-grade glioma (HGG) will be included in this study of treatment with Rhenium-186 Nanoliposome (186RNL). Phase 1 of the study will look to determine the maximum tolerated dose (MTD) of 186RNL in this patient population. Phase 2 of the study will use the recommended dose determined in Phase 1 to continue to look at overall response rate and progression-free survival following 186RNL treatment.