Testing Addition of an Anti-cancer Drug, Vorasidenib to Temozolomide, After Radiation for Advanced Brain Cancer

Inclusion Criteria: * STEP 0: Histologic diagnosis of astrocytoma, IDH-mutant (central nervous system \[CNS\] WHO grade 3) * STEP 0: Available diagnostic slides (hematoxylin and eosin staining method \[H\&E\] and immunohistochemical stains for central review) * STEP 0: Tissue available for central biomarker testing (CDKN2A/B and1p/19q co-deletion \[all patients\], and IDH1/IDH2 \[if needed\]) * STEP 1: Centrally-confirmed diagnosis of astrocytoma, IDH-mutant (CNS WHO grade 3) * STEP 1: Presence of IDH1 p.R132 or IDH2 p.172 mutation, confirmed by central review of immunohistochemical stain or molecular testing results, with central confirmation of equivocal results * STEP 1: Absence of CDKN2A/B homozygous deletion by central testing * STEP 1: Absence of whole arm 1p/19q co-deletion (i.e. intact 1p/19q) by central testing * STEP 1: No evidence of spinal or leptomeningeal disease * STEP 1: No prior chemotherapy, cranial irradiation, IDH-inhibitor therapy, radiotherapy, vaccine therapy, small-molecule therapy, or laser ablation * STEP 1: Prior diagnostic surgery/resection/biopsy ≤ 6 months of registration * STEP 1: Planned radiotherapy and adjuvant chemotherapy * STEP 1: Age ≥ 12 years * STEP 1: Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2 (or Karnofsky performance status \[KPS\] ≥ 60%) * STEP 1: Absolute neutrophil count (ANC) ≥ 1,500/mm\^3 * STEP 1: Hemoglobin ≥ 9 g/dL * STEP 1: Platelet count ≥ 100,000/mm\^3 * STEP 1: Total bilirubin ≤ 1.5 x upper limit of…