This is a retrospective, multicenter, non-interventional study for the evaluation of real-world efficacy and safety of T-DM1 in metastatic/relapsed HER2-positive breast cancer as part of the establishment of hospital medical record collection system to evaluate drug effectiveness by Health Insurance Review & Assessment Service (HIRA). The medical records in approximately 1,000 patients of HER2-positive locally-advanced unresectable or metastatic breast cancer, who have received Kadcyla(Trastuzumab Emtansine, T-DM1) previously, will be collected.

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.

Inclusion Criteria:

1. Age ≥19 years at the time of study registration
2. Participants must have histologically confirmed HER2-positive breast cancer
3. Locally advanced unresectable or metastatic patients
4. Patients who have received T-DM1 therapy between Aug 2017 and December 2018 under the Korea National Health Insurance System

Exclusion Criteria:

• Patients who have received T-DM1 therapy outside of the Korea National Health Insurance System

Patients with locally-advanced unresectable or metastatic HER2-positive breast cancer, who have received T-DM1 therapy between Aug 2017 and December 2018 under the Korea National Health Insurance System