Testing the Addition of an Anti-Cancer Drug, AZD1390, During Radiation Therapy for Newly Diagnosed High Grade Glioma, Diffuse Midline Glioma, or Diffuse Intrinsic Pontine Glioma

Inclusion Criteria: * COHORT A and COHORT B: For the dose escalation phase, patients must be ≥ 12 months and \< 18 years of age at the time of study enrollment * COHORT C and COHORT D: For the disease expansion phase, patients must be ≥ 12 months and \< 22 years of age at the time of study enrollment * Patients with newly diagnosed primary high-grade glioma (HGG), diffuse midline glioma (DMG) (excluding primary spinal tumors), or diffuse intrinsic pontine glioma (DIPG) who are eligible to receive 54-59.4 grey (Gy) fractionated radiation at 1.8 Gy/day. Patients must have had histologic verification of malignancy at original diagnosis except in patients with DIPG as defined below. * COHORTS A AND C (SUPRATENTORIAL TUMORS): * HGG and non-pontine DMG: * Patients with newly diagnosed HGG (including diffuse hemispheric glioma, H3 G34-mutant; diffuse pediatric-type high-grade glioma, H3-wildtype and IDH-wildtype; astrocytoma; IDH-mutant; or glioblastoma, IDH-wildtype): or non-pontine DMG (including diffuse midline glioma, H3 K27-altered; diffuse pediatric-type high-grade glioma, H3-wildtype and IDH-wildtype; astrocytoma; IDH-mutant; or glioblastoma, IDH-wildtype) require histologic diagnosis. * COHORT B AND D (INFRATENTORIAL TUMORS): * DIPG/pontine DMG or infratentorial HGG or DMG: * Patients with newly diagnosed typical DIPG, defined as tumors with a pontine epicenter and diffuse involvement of at least 2/3 of the pons on at least 1 axial T2-weighted i…