This study looks at how to give the amino acid arginine safely with whole brain radiation therapy (WBRT) for people whose cancer has spread to the brain, called brain metastases. Arginine is important because it helps proteins form in our body and may make radiation therapy work better. The study tests two ways to give arginine: by mouth (oral) or through a vein (intravenous or IV). Researchers want to find the best way to give arginine and see what side effects might happen. Patients are split into two groups: one group gets arginine through an IV, and the other group takes it by mouth. Both groups also receive WBRT for about 10 days over two weeks. Participants will have tests like CT and MRI scans and blood samples taken.

• Study length: 2 weeks of treatment, with follow-ups for one year.
• Must be 18 or older and able to consent.
• Must not be pregnant or taking certain medications during the study.