Radiation Therapy Followed by Intrathecal Trastuzumab/Pertuzumab in HER2+ Breast Leptomeningeal Disease

This study tests a combination of radiation therapy plus targeted antibody drugs (trastuzumab and pertuzumab) injected directly into the spinal fluid for people with HER2-positive breast cancer that has spread to the membranes surrounding the brain and spinal cord — a condition called leptomeningeal disease. **You may be eligible if...** - You have HER2-positive cancer (a specific protein found on some breast cancer cells) that has spread to the lining of the brain or spinal cord - You have acceptable kidney, liver, and blood counts - Your heart function is adequate (ejection fraction over 50%) - Your overall health and function level meets minimum requirements - You are willing to have a small device (Ommaya reservoir) placed under your scalp to allow drug delivery into the spinal fluid **You may NOT be eligible if...** - You were in another research study within the past 2 weeks - You are currently on certain chemotherapy drugs that can enter the brain (unless your disease progressed while on them) - You have significant lung disease causing shortness of breath at rest - You are pregnant or breastfeeding - You have had a serious reaction to any of the study drugs Talk to your doctor to see if this trial is right for you.