[A Case of Stage IV Metaplastic Breast Cancer in Which Primary Tumor Was Removed after Treatment with Anti-HER2 Systemic Therapy].

The patient was a 58-year-oldpostmenopausal woman. Vacuum assistedbiopsy of the left breast tumor revealedinvasive ductal carcinoma. Immunohistochemical examination was negative for estrogen receptor(ER), negative for progesterone receptor(PgR), andshowedan HER2 score of 3+. FDG-PET/CT revealedmultiple metastases to the left supracravicular and axillary lymph nodes and lungs. She was diagnosed with HER2-positive T3N3M1, Stage IV breast cancer. A 2-year regimen of chemotherapy with trastuzumab andvinorelbine achieveda complete response with regardto the metastatic sites; however, the size of the primary tumor increasedd espite the chemotherapy, andsurgical resection of the left breast with axillary lymph node dissection was performed for local control. Pathological examination of the surgical specimen revealed metaplastic carcinoma with sarcoma component surrounded by non-invasive ductal carcinoma. No component of invasive ductal carcinoma was found. Immunohistochemically, metaplastic carcinoma was negative for ER, negative for PgR, andrevealedan HER2 score of 0. There was discordance of HER2 status between pre- andpost -chemotherapy. The patient receivedno further chemotherapy following surgery andhas been without disease progression for 6 years. We suggest there is heterogeneity, that is, the metastatic sites andthe partial primary tumor were HER2-positive invasive ductal carcinoma and the remainder of the primary tumor was triple negative metaplastic carcinoma. As a result, the patient was able to discontinue chemotherapy with higher quality of life.