Additional Study Information: The purpose of this study is to compare any good and bad effects of avoiding the hippocampus during whole-brain radiation plus memantine to using the usual whole-brain radiation plus memantine. The hippocampus is a brain structure that is important for memory. The addition of the hippocampal avoidance technique to the usual whole-brain radiation plus memantine will decrease the dose of radiation to your hippocampus. It is hoped hippocampal avoidance technique will decrease the chance of cognitive side effects, however it is possible hippocampal avoidance could have no impact on cognitive side effects and could even cause side effects. This study will allow the researchers to know whether this different approach is better, the same, or worse than the usual approach. Memantine is already FDA-approved for use in patients with dementia and is commonly used off-label (that is, for a purpose for which it is not FDA approved) for patients receiving whole-brain radiation therapy for cancer that has spread to the brain.