[A Case in Which Multiple Biopsies from Breast Cancer Skin Metastases Were Performed and Trastuzumab Deruxtecan Was Used after Low HER2 Expression Was Confirmed].

In March 2023, Trastuzumab deruxtecan(T-DXd)was added for the treatment of inoperable or recurrent breast cancer with low HER2 expression who has a history of chemotherapy. The patient was a 61-year-old woman who had undergone surgery for bilateral breast cancer, and was diagnosed with Stage ⅡA triple negative(HER2-IHC score 0)on the right and non-invasive ductal carcinoma of the breast on the left. Biopsy from metastases was triple-negative(HER2-IHC score 0, PD-L1 negative). Exacerbation of skin metastases was observed despite repeated regimen changes 3rd skin biopsy was diagnosed with low expression of HER2 for the first time, and T-DXd was started. The therapeutic effect of T-DXd was temporarily observed, such as a decrease in tumor markers. It is useful to repeatedly collect tissues from primary and metastatic lesions and re-evaluate biomarkers in order to develop an appropriate treatment plan.