Photon Craniospinal Irradiation for the Treatment of Leptomeningeal Disease Secondary to Breast Cancer or Non-small Cell Lung Cancer

This study is testing a type of radiation therapy called photon craniospinal irradiation — radiation delivered to the brain and spine — for people with breast cancer or non-small cell lung cancer that has spread to the lining of the brain and spinal cord (called leptomeningeal disease). **You may be eligible if...** - You are 18 or older - You have been diagnosed with breast cancer or non-small cell lung cancer - Cancer has spread to the lining around your brain or spinal cord (confirmed by imaging or spinal fluid testing) - You are physically well enough to handle treatment (a doctor will assess this) - Your blood counts and kidney function meet certain minimum levels - You can read English or Spanish, or complete questionnaires with an interpreter **You may NOT be eligible if...** - You received chemotherapy, immunotherapy, or targeted therapy within the last 7 days - You have had prior radiation that would prevent safe delivery of spinal radiation - You have severe neurological symptoms such as severe confusion - Your cancer outside the brain/spine is extensive and uncontrolled - You are pregnant or breastfeeding - You are unable to have an MRI with contrast dye Talk to your doctor to see if this trial is right for you.