The purpose of this study is to compare the effects, good and/or bad, of memantine or placebo on memory and thinking in participants receiving whole brain radiotherapy. Doctors hope that memantine (the study agent being tested in this study) will be effective in preventing thinking problems after whole brain radiotherapy, although there is no proof of this yet.   In addition, we want to find out what effects, good and/or bad, memantine has on participants and their brain tumors.