Participants will either get treatment with SRS plus HA-WBRT and memantine, or treatment with SRS alone.
Participants will receive the usual treatment, Stereotactic radiosurgery, which delivers a high dose of radiation only to the small areas of cancer in the brain and avoids the surrounding normal brain tissue. SRS is a single treatment over one day (or in some cases a few days) as an outpatient procedure.
If participants receive HA-WBRT, they will receive it 5 days per week Monday through Friday for about 2 weeks as an outpatient procedure. Whole brain radiation therapy delivers a low dose of radiation to the entire brain including the normal brain tissue. Hippocampus avoidance during whole-brain radiation therapy (HA-WBRT) decreases the amount of radiation that is delivered to the hippocampus which is a brain structure that is important for memory. The medicine memantine is also often given with whole brain radiation therapy because it may decrease the risk of side effects of radiation on thinking and memory. Participants will receive memantine as a tablet by mouth for 6 months.
After treatment, the doctor and study team will watch participants for side effects and follow their condition. They will check every 2 to 3 months for at least 1 year after finishing SRS. If the participants are receiving memantine, the doctor will continue to see them in the clinic as needed.