The CompassHER2 Trials (Comprehensive Use of Pathologic Response Assessment to Optimize Therapy in HER2-Positive Breast Cancer) CompassHER2 Residual Disease (RD), a Double-Blinded, Phase III Randomized Trial of T-DM1 Compared With T-DM1 and Tucatinib

This study is being done to answer the following question: Is the combination of T-DM1 and a newer drug tucatinib, better than usual treatment with T-DM1 alone at preventing your cancer from returning? We are doing this study because we want to find out if this approach is better or worse than the usual approach for your breast cancer. The usual approach is defined as care most people get for breast cancer.