Inclusion Criteria:
* Histologic diagnosis of malignant glioma, World Health Organization (WHO) grade 4, including glioblastoma and astrocytoma, based on recent biopsy or resection.
* Age 18 and old
* Karnofsky performance status (KPS) 60-100
* Acceptable labs, including platelets of greater than or equal to 100,000, glomerular filtration rate (GFR) greater than or equal to 25.
* Ability to provide informed consent.
* Planning for treatment with radiation and chemotherapy.
Exclusion Criteria:
* Evidence of deep venous thrombosis (DVT), pulmonary embolism (PE), or cerebral vein thrombosis (CVT) within 6 months prior to enrollment on the study.
* Pregnancy.
* Significant bleeding risk including: history of symptomatic intracranial hemorrhage, active bleeding at time of study registration, clinically significant bleeding within 14 days of study registration, intracranial trauma in the past 6 months, including stroke or traumatic brain injury, major surgery or major procedure within 48 hours.
* Allergy to apixaban or contraindication to prophylactic anticoagulation. Contraindications include patients who are currently on unfractionated heparin, low molecular weight heparin, heparin derivatives (examples: fondaparinux), or other direct oral anticoagulants (examples: dabigatran, edoxaban, rivaroxaban), who are either unable or unable to discontinue these agents in favor of the investigational drug. Apixaban is also contraindicated in patients who are on strong CYP3A4 inhibitor and P-glycoprotein inhibitors.
* Indication for full anticoagulation (i.e. atrial fibrillation, mechanical valve, etc.).
* Estimated life expectancy of \<3 months.
