This study is testing a drug called selinexor combined with radiation therapy in children and young adults with specific types of brain cancer: diffuse intrinsic pontine glioma (DIPG) and high-grade glioma (HGG). Glioma is a kind of cancer found in the brain or spine. DIPG is a type of glioma that affects the brainstem, which helps control breathing and other important functions. The study will check if selinexor is safe, its side effects, and the best dose to use with radiation therapy. Selinexor works by stopping a protein that helps cancer cells grow. The study has two parts: in Part 1, researchers will find the safest dose, and in Part 2, they'll see how well it works. The treatment involves selinexor taken as a pill and radiation therapy over several weeks. After initial treatment, patients will continue taking selinexor every month for up to two years if it’s safe and helping. Patients will have regular check-ups with MRI scans to monitor progress.

• Treatment lasts several months, with follow-ups for up to 5 years.
• Regular visits for radiation therapy and taking selinexor pills are required.
• Participants must be ≤ 25 years old and have certain types of brain tumors.