Inclusion Criteria:
* Participants must have confirmed newly diagnosed glioblastoma multiforme (GBM), World Health Organization (WHO) grade 4, IDH wildtype, either by histological or molecular criteria.
* Molecular analysis needs to confirm a positive MGMT promoter methylation status using standard institutional testing methods.
* Treatment needs to involve a planned 6-week course of standard of care radiation therapy with concurrent and adjuvant 6 monthly chemotherapy with temozolomide. Patients scheduled to receive an abbreviated radiation course (e.g., 3 weeks in elderly patients) are eligible.
* Age ≥18 years. GBM is considered a biologically distinct disease in children. Children are excluded from this study but will be eligible for future pediatric clinical trials.
* Karnofsky Performance Status (KPS) \> 60, see Appendix A
* No prior cranial irradiation.
* No existing diagnosis of clinical dementia or high clinical suspicion for presence of any neurodegenerative disease (e.g., Alzheimer's Disease, Fronto-temporal Dementia (FTD), Parkinson's Disease, Motor Neuron Disease, etc.) prior to diagnosis of GBM.
* Life expectancy of greater than 6 months.
* Must be able to undergo repeated brain Magnetic resonance imaging (MRI) studies with administration of gadolinium (contrast enhanced brain MRI).
* Participants must have adequate organ and bone marrow function (as defined below) to be able to receive standard chemoradiation therapy:
* leukocytes ≥2,500/mcL
* absolute neutrophil count≥1,500/mcL
* platelets ≥100,000/mcL
* total bilirubin≤ institutional upper limit of normal (ULN)
* AST(SGOT)/ALT(SGPT)≤3 × institutional ULN creatinine≤ institutional ULN OR
* glomerular filtration rate (GFR) ≥60 mL/min/1.73 m2 unless data exists supporting safe use at lower kidney function values, no lower than 30 mL/min/1.73 m2.
For patients with Gilbert's syndrome, total bilirubin can be ≤ 3xULN.
* For participants with evidence of chronic hepatitis B virus (HBV) infection by history, the HBV viral load must be undetectable on suppressive therapy, if indicated.
* Participants with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen (use of granulocyte colony stimulating factor (G-CSF)) are eligible for this trial.
* Ability to understand and the willingness to sign a written informed consent document.
Exclusion Criteria:
* Participants who are receiving any other investigational agents.
* History of allergic reactions attributed to compounds of similar chemical or biologic composition to G-CSF (Filgrastim associated allergic reactions).
* Participants with uncontrolled intercurrent illness that could influence leukocyte counts, such as severe infection requiring intravenous antibiotics, or known HIV (human immunodeficiency virus), since HIV/AIDS is an immunocompromising disease affecting lymphocyte counts (one of the correlative biomarkers in this study)
* Pregnant women are excluded from this study because of the use of cytotoxic chemotherapy (temozolomide) and radiation, given as part of standard of care in this trial, is of teratogenic potential or has abortifacient effects. Because there is a risk for adverse events in nursing infants secondary to treatment of the mother with cytotoxic chemotherapy, breastfeeding should be discontinued if the mother is treated with cytotoxic chemotherapy.
* Participants must be able to undergo repeated neurocognitive testing in English (or Spanish). As cognitive outcome is one of the main secondary endpoints of this study, the lack of normative and comparison data for non-English or non-Spanish-speaking patients would confound this outcome in our small sample size (see Statistical Analysis Plan for more details). Presence of significant aphasia or any other language impairment at time of diagnosis with GBM is considered an exclusion criterion. Any concerns or questions about a subject's ability to participate in neurocognitive testing can be directed to the study investigators for further discussion and clarification.
* Participants with active thromboembolic event (pulmonary embolism or deep venous thrombosis) or prior thromboembolic event within 6 months prior to diagnosis of GBM may need to be excluded because of possible risks of thromboembolism with the use of G-CSF and will require further discussion with the PI prior to enrollment on a case-by-case basis.
* Participants with the following medical conditions are excluded and not eligible based on elevated risk of G-CSF associated toxicity: Sickle cell disease or sickle cell trait, congenital neutropenia, hematological malignancy (leukemia or myelodysplastic syndrome).
* Patients who are dependent on high doses of corticosteroids equivalent to 8mg of daily dexamethasone or more, or who are expected to be unable to taper steroids post-operatively to a dose of 4mg of dexamethasone or less prior to start of chemo-RT.
