Additional Study Information: This study is being done to answer the following question: Is treatment with HER2-targeted therapy without radiation therapy as good as the usual treatment of radiation and HER2-targeted therapy in patients with HER2-positive breast cancer who have had surgery, chemotherapy, and HER2-targeted therapy? This study is being done to find out if this approach is better or worse than the usual approach for your breast cancer. The usual approach for patients who have undergone surgery, chemotherapy, and HER2-targeted therapy and are not in a study is treatment with radiation therapy to the breast and continuing HER2-targeted therapy. Radiation therapy can be given to part or all of the breast delivered over 1-6 weeks. For patients who get the usual approach for this cancer, about 95 out of 100 are cancer free after5 years.