A case of metastatic breast cancer with outgrowth of HER2-negative cells after eradication of HER2-positive cells by humanized anti-HER2 monoclonal antibody (trastuzumab) combined with docetaxel.

A 38-year-old woman with cancer of the left breast underwent a modified radical mastectomy with lymph node dissection. Twenty-one months later, massive liver metastases and pleural carcinomatosis occurred. The liver metastases responded completely to chemotherapy with trastuzumab combined with docetaxel, but the pleural carcinomatosis was refractory to the therapy.