NgFUS NIVO: NeuroNavigation-Guided Focused Ultrasound With Nivolumab in Relapsed and Progressive DMG and Other High Grade Brain Tumors

This is an open-label phase 1 safety and feasibility study evaluating a novel combination therapy for progressive and relapsed diffuse midline glioma (DMG) and other progressive and relapsed high-grade brain tumors. This study combines intravenous nivolumab therapy infused following transient blood-brain barrier opening (BBBO) using low-intensity focused ultrasound with microbubble (LIFU-MB) treatment using NeuroNavigation-Guided Focused Ultrasound (NgFUS). There are two groups in this study: * Group A: Patients with relapsed or progressive diffuse midline glioma in the brainstem * Group B: Patients with relapsed or progressive high grade brain tumor that clinically require surgical resection The primary outcome is to evaluate the safety and feasibility of 3 cycles of nivolumab with BBB disruption using NgFUS with microbubbles in pediatric patients with progressive or relapsed brainstem DMG or with high grade brain tumors after surgery. Secondary outcomes include preliminary efficacy and immunological effects.