Inclusion criteria:

1. Histopathologically proven diagnosis of glioblastoma. Since gliosarcoma is a variant of glioblastoma, gliosarcoma is also an eligible diagnosis.
2. Patients must have at least 1 block of tissue available for analysis of MGMT status; fresh frozen tumor tissue acquisition is encouraged.
3. Diagnosis must be established by open biopsy or tumor resection. Patients who have only had a stereotactic biopsy are not eligible.
4. The tumor must have a supratentorial component.
5. Patients must have recovered from the effects of surgery, postoperative infection, and other complications before study registration.
6. A diagnostic contrast-enhanced magnetic resonance imaging (MRI) or computerized tomography (CT) scan (if MRI is not available) of the brain must be performed preoperatively and postoperatively. The postoperative scan must be done within 28 days of registration and prior to the initiation of radiotherapy. Preoperative and postoperative scans must be the same type. If CT scans were performed perioperatively, a CT and an MRI should be performed before randomization.

6.1. Patients unable to undergo MRI imaging because of non-compatible devices can be enrolled, provided pre- and post-operative contrast-enhanced CT scans are obtained and are of sufficient quality.
7. Therapy must begin ≤ 5 weeks after the most recent brain tumor surgery.
8. History/physical examination within 14 days prior to study registration.
9. Neurologic examination within 14 days prior to study registration.
10. Documentation of steroid doses within 14 days prior to study registration and stable or decreasing steroid dose within 5 days prior to registration.
11. Karnofsky performance status of ≥ 60.
12. Age ≥ 18 years.
13. Complete blood count (CBC)/differential obtained within 14 days prior to study registration, with adequate bone marrow function as defined below: 13.1 Absolute neutrophil count (ANC) ≥ 1500 cells/mm3. 13.2 Platelets ≥ 100,000 cells/mm3. 13.3 Hemoglobin ≥ 10 g/dl. (Note: The use of transfusion or other intervention to achieve Hgb ≥ 10 g/dl is acceptable.)
14. Adequate renal function, as defined below:

14.1 Blood urea nitrogen (BUN) ≤ 25 mg/dl within 14 days prior to study registration 14.2 Creatinine ≤ 1.7 mg/dl within 14 days prior to study registration
15. Adequate hepatic function, as defined below:

15.1 Bilirubin ≤ 2.0 mg/dl within 14 days prior to study registration 15.2 Alanine aminotransferase (ALT) ≤ 3 x normal range within 14 days prior to study registration 15.3 Aspartate aminotransferase (AST) ≤ 3 x normal range within 14 days prior to study registration
16. Patients must sign a study-specific informed consent prior to study registration.

If the patient's mental status precludes his/her giving informed consent, written informed consent may be given by the responsible family member.
17. For females of child-bearing potential, negative serum pregnancy test within 72 hours prior to starting temozolomide.
18. Women of childbearing potential and male participants must practice adequate

Exclusion criteria:

1. Prior invasive malignancy (except for non-melanomatous skin cancer) unless disease free for ≥ 3 years. (For example, carcinoma in situ of the breast, oral cavity, and cervix are all permissible).
2. Recurrent or multifocal malignant gliomas
3. Metastases detected below the tentorium or beyond the cranial vault.
4. Prior chemotherapy or radiosensitizers for cancers of the head and neck region; note that prior chemotherapy for a different cancer is allowable. Prior use of Gliadel wafers or any other intratumoral or intracavitary treatment are not permitted. See Section 1.
5. Prior radiotherapy to the head or neck (except for T1 glottic cancer), resulting in overlap of radiation fields.
6. Severe, active co-morbidity, defined as follows:

* 6.1. Unstable angina and/or congestive heart failure requiring hospitalization.
* 6.2. Transmural myocardial infarction within the last 6 months.
* 6.3. Acute bacterial or fungal infection requiring intravenous antibiotics at the time of registration.
* 6.4. Chronic Obstructive Pulmonary Disease exacerbation or other respiratory illness requiring hospitalization or precluding study therapy at the time of registration.
* 6.5. Hepatic insufficiency resulting in clinical jaundice and/or coagulation defects; note, however, that laboratory tests for liver function and coagulation parameters are not required for entry into this protocol.
* 6.6. Acquired Immune Deficiency Syndrome (AIDS) based upon current Centers for Disease Control and Prevention (CDC) definition; note, however, that HIV testing is not required for entry into this protocol. The need to exclude patients with AIDS from this protocol is necessary because the treatments involved in this protocol may be significantly immunosuppressive.
* 6.7. Major medical illnesses or psychiatric impairments that in the investigator's opinion will prevent administration or completion of protocol therapy.
* 6.8. Active connective tissue disorders, such as lupus or scleroderma, that in the opinion of the treating physician may put the patient at high risk for radiation toxicity.
7. Pregnancy or women of childbearing potential and men who are sexually active and not willing/able to use medically acceptable forms of contraception; this exclusion is necessary because the treatment involved in this study may be significantly teratogenic.
8. Pregnant or lactating women, due to possible adverse effects on the developing fetus or infant due to study drug;
9. Prior allergic reaction to temozolomide.
10. Patients treated on any other therapeutic clinical protocols within 30 days prior to study entry or during participation in the study.
11. No tissue provided for histopathologic central review and MGMT status.
12. Tissue provided by stereotactic biopsy method.