6 Months Adjuvant Temozolomide (TMZ) vs No Adjuvant TMZ in Newly Diagnosed MGMT Methylated Glioblastoma (GBM)

Inclusion Criteria: 1. Patients must be 18 years of age or older. 2. Patients must have a Karnofsky Performance Status ≥ 60% (i.e. the patient must be able to care for himself/herself with occasional help from others). 3. Patients must have had a pathologically confirmed recently diagnosed primary glioblastoma before they began concurrent radiation and temozolomide. 4. Patients must have tumor MGMT methylation status of methylated using a Clinical Laboratory Improvement Amendments (CLIA)-approved diagnostic test. Results of routinely-used methods for MGMT methylation testing (e.g. Mutagenically separated- polymerase chain reaction (MS-PCR) or quantitative PCR) are acceptable. 5. Documented mutated isocitrate dehydrogenase 1 gene (IDH1) isocitrate dehydrogenase 1 gene (IDH2)status: patients with either wild-type or(IDH) status are eligible. Patients with no documented status are also eligible and the status should be listed as unknown. 6. Patients must be completing or have completed 6 weeks of standard concurrent RT/TMZ. 7. Patients' post-operative treatment must have included at least 80% of standard radiation and concomitant temozolomide. Patients may not have received any other prior chemotherapy, immunotherapy or therapy with biologic agent (including immunotoxins, immunoconjugates, antisense, peptide receptor antagonists, interferons, interleukins, tumor-infiltrating lymphocytes (TIL), lymphokine activated killer cells (LAK) or gene therapy), or hormonal therapy for their brain tumor. Prior Gliadel Wafers are allowed. Glucocorticoid therapy is allowed. 8. Patients must be clinically eligible for the six months of adjuvant temozolomide. 9. Patients must be able to provide written informed consent. 10. Patients must have baseline MRI performed within the 21 days prior to starting entering the "adjuvant" treatment or observation period. 11. Patients must have the following organ and marrow functions: Absolute neutrophil count ≥1,500/µL Platelets ≥100,000/µL Hemoglobin ≥ 9 g/dL Total bilirubin ≤1.5 × institutional upper limit of normal (ULN), (except for patients with known Gilbert's syndrome who must have normal direct bilirubin) aspartate aminotransferase (AST) serum glutamic-oxaloacetic transaminase (SGOT)/ alanine transaminase (ALT) serum glutamic-pyruvic transaminase (SGPT) ≤ 2.5 × ULN Creatinine ≤1.5 × ULN OR Creatinine clearance ≥ 60 mL/min/1.73m2 Exclusion Criteria: 1. Patients receiving any other standard or investigational agents or who plan to use the OPTUNE device or other therapies for the glioblastoma are ineligible. 2. Patients with uncontrolled intercurrent illness including, but not limited to, ongoing or active infection or psychiatric illness/social situations that would limit compliance with study requirements, are ineligible. 3. Patients must not have received prior RT, chemotherapy (except for their concurrent radiation and temozolomide for their recently diagnosed glioblastoma), immunotherapy or therapy with a biologic agent (including immunotoxins, immunoconjugates, antisense, peptide receptor antagonists, interferons, interleukins, tumor-infiltrating lymphocytes, lymphokine-activated killer cells, or gene therapy), or hormonal therapy for their brain tumor. Corticosteroid therapy is allowed. 4. Patients must not have had a prior diagnosis of a lower grade glioma. 5. No active treatment for other cancers in the past 3 years.