Chemotherapy is often difficult to treat human epidermal growth factor receptor 2 (HER2)-positive metastatic recurrent breast cancer in the elderly, and no standard treatment has been established at this point. We experienced a case in which trastuzumab (Tmab) + anastrozole (ANA) was ineffective (progressive disease; PD) in elderly HER2-positive breast cancer with postoperative multiple liver metastases, but T-DM1 was significantly effective (complete response; CR), and treatment could be continued safely. An 82-year-old woman was referred to our department with a right breast mass. A close examination revealed right breast cancer cT1bN0M0 cStage I, and total mastectomy and sentinel lymph node biopsy were performed. The postoperative pathological result was pT1bN0M0 pStage I (luminal HER2 type). The patient was elderly and had no adjuvant treatment after the operation. Approximately 2 years after the operation, multiple liver metastases were observed, and treatment with ANA and Tmab was started. Four months later, MRI showed that the number of multiple liver metastases increased. The patient was diagnosed with PD, and the anti-HER2 drug was changed from trastuzumab to trastuzumab emtansine (T-DM1). The dose was reduced due to vomiting (grade 3). Two months later, MRI showed that the multiple liver metastases shrank and became obscure after 5 months. After that, T-DM1 was continued, and the disease did not worsen. In elderly people with difficulty in administering chemotherapy, T-DM1 may have a safe and sufficient therapeutic effect by adjusting the dose and managing side effects appropriately.

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