T-DM1 therapy has shown a survival benefit in previously trastuzumab-treated HER2-positive locally-advanced unresectable or metastatic breast cancer patients from clinical trials. However, the real-world efficacy and safety of T-DM1 in KOREA were not evaluated outside the controlled clinical trials. Therefore, this large multicenter retrospective analysis was designed to evaluate the real-world efficacy and safety of T-DM1 under the Korea National Health Insurance System. The medical records in approximately 1,000 patients with relapsed or De Novo metastatic breast cancer, who have received T-DM1 between Aug 03, 2017 and Dec 31, 2018 will be collected. Eligibility criteria included age ≥ 19 years, histologically confirmed HER2-positive, relapsed after primary surgery or initially metastatic breast cancer, and previous trastuzumab treated. Efficacy was evaluated by overall survival, progression free survival, time to progression, objective response rate, disease control rate, duration of response and time to next treatment. Safety was evaluated by hematologic or non-non hematologic toxicities and adverse events of special interest with T-DM1 therapy.
