Testing the Addition of an Anti-Cancer Drug, Mycophenolate Mofetil, to the Usual Treatment (Radiation Therapy and Temozolomide) for Advanced Brain Cancer

Inclusion Criteria: * Newly diagnosed primary glioblastoma by World Health Organization (WHO) 2021 criteria * No spinal cord glioblastoma * No leptomeningeal disease * No extracranial metastatic disease * Patient is a candidate for first-line standard of care chemoradiation per treating physician(s) * No prior treatment for glioblastoma other than resection (i.e. prior chemotherapy, radiation therapy, or other therapies such as laser ablation are not allowed) * Prior biopsy and/or resection of glioblastoma must be completed at least 14 days prior to registration with adequate wound healing * Age ≥ 18 years * Karnofsky performance status (KPS) ≥ 60 * Absolute neutrophil count (ANC) ≥ 1,500/mm\^3 * Platelet count ≥ 100,000 cells/mm\^3 * Hemoglobin (Hg) ≥ 9.0 g/dL * Calculated (Calc.) creatinine clearance (CrCl) ≥ 25 mL/min \* Calculated using the Cockcroft-Gault equation * Bilirubin ≤ 1.5 x upper limit of normal (ULN) * Aspartate aminotransferase (AST)/alanine aminotransferase (ALT) ≤ 3.0 x ULN * Not pregnant and not nursing, because this study involves an agent that has known genotoxic, mutagenic and teratogenic effects. Therefore, for women of childbearing potential only, a negative pregnancy test done ≤ 14 days prior to registration is required. Women and men of reproductive potential must agree to use a highly effective method of birth control throughout their participation in this study and for at least 6 weeks (women) or 90 days (men) after last dose of treatment. R…