Inclusion Criteria:  Any patient with a solid tumor diagnosis and any number of brain metastasis clinically indicated for cranial irradiation with whole brain radiation therapy Age ≥ 18 Karnofsky Performance Status ≥ 70 Prior stereotactic radiosurgery (SRS) permissible per physician discretion Prior craniotomy permissible per physician discretion. Protocol radiation therapy should be initiated ≥2 weeks after craniotomy. Prior partial brain radiation therapy permissible if target volume < 50% brain and per physician discretion Expectant > 6 months survival Ability to understand and the willingness to sign a written informed consent document. Fluency in English, able to complete questionnaires and neurocognitive testing Ability to undergo MRI with gadolinium examination Ability to return for follow-up examinations throughout the course of this study for a maximum of 2 years after radiation treatment completion Any prior, concomitant, or post-radiotherapy systemic therapy is permitted at discretion of treating physicians Negative pregnancy test for premenopausal women  Exclusion Criteria:  Leptomeningeal disease (by any one or more of clinical assessment, radiographic assessment, or cerebrospinal fluid study) Prior whole brain radiation therapy Pre-existing or current use of memantine or other NMDA antagonists Known allergy to contrast used in imaging studies and/or inability to have MRI imaging Uncontrolled intercurrent illness that could significantly affect baseline cognitive function as determined by the enrolling clinician, such as symptomatic congestive heart failure, unstable angina pectoris, prior CVA, significant uncontrolled epilepsy or psychiatric illness/social situations that would limit compliance with study requirements Pregnant or unwilling to use appropriate contraception to prevent pregnancy during the time of radiation therapy Concurrent participation in an investigational systemic therapy protocol.