RecruitingPhase 2Radiation therapyStereotactic Radiation for Growing/Changing Brain Metastases With Same-Day Radiation Planning and Treatment With Margin ReductionThe goal of this study is to evaluate the feasibility and effectiveness of same-day radiation planning and treatment. The study will shorten the time interval between radiation planning (radiation mapping) and radiation treatment. The intent of this shorter time interval is to increase the likelihood that the brain metastases being treated remain fully within the high-dose radiation fields. Participants will be randomized to receive brain-directed stereotactic radiation with a 1mm margin or 0mm margin, have their simulation/radiation planning imaging on the same day that brain-directed stereotactic radiation is delivered, and have repeat simulation/radiation planning scans during the course of treatment if more than 2-3 days have elapsed since the most recent scans.