Inclusion Criteria:  Age ≥18 years at diagnosis of brain metastases. Eastern Cooperative Oncology Group (ECOG) Performance Score of <2. Participants must have a biopsy-proven solid malignancy (histologic proof or unequivocal cytologic proof solid tumor malignancy from either the primary or any metastatic site) with intracranial lesions radiographically consistent with or pathologically proven to be brain metastases. Patients who have undergone prior systemic therapy are eligible. Life expectancy from extracranial disease greater than six months. Patients with measurable brain metastasis who have not been or will not be treated with stereotactic radiosurgery (SRS) or surgery (Note: The treatment options are only permitted at relapse) If an open biopsy is performed, the patient must be at least one-week post-biopsy. This requirement is not necessary for stereotactic biopsies. Creatinine clearance is ≥ 30 mL/min. Start of PRDR WBRT within two weeks following registration. Ability to complete the Neurocognitive Function (NCF) test battery (including people whose primary language is English). Patients with previous or other malignancies whose disease is controlled and not impacting ECOG performance or life expectancy. Willing and able to give consent and to comply with treatment and follow-up schedule.  Exclusion Criteria:  Metastases from hematological malignancy, or central nervous system malignancy. Patients whose malignancy is being treated with curative intent. Leptomeningeal metastases. Contraindication to MRI imaging with contrast. Contraindication to memantine including concurrent use of N-methyl-D-aspartate (NMDA) antagonists. Stage IV-V chronic kidney disease or end-stage renal disease. Participants with a maximum tumor diameter exceeding 5 cm (if not resected). Prior cranial radiotherapy. Past medical history of dementia which is thought to be unrelated to the brain metastases. Women of childbearing potential who are known to be pregnant or are unwilling to use an acceptable method of contraception from the time of informed consent until completion of the course of radiotherapy. Patients must not have a serious medical or psychiatric illness that would, in the opinion of the treating physician, prevent informed consent or completion of protocol treatment, and/or follow-up visits. Non-native English speakers will be excluded since patients often lose their faculty with the language they acquired second before their native language is affected in the context of cognitive decline. This could adversely affect performance on verbal cognitive tasks.