[A long survival of a patient with poor performance status who suffered from advanced right breast cancer with multiple lung metastases controlled by trastuzumab as a key drug].

We report a case of a 60-year-old woman with poor performance status (PS). She suffered from advanced right breast cancer with multiple lung metastases, which was controlled by chemotherapy with trastuzumab as the key drug. The patient presented with a 4 cm-sized large right breast mass. Her PS was poor due to progressive spinocerebellar degeneration. The biopsy specimen of the breast mass showed scirrhous type of the invasive ductal carcinoma (ER+, HER2 2+). Multiple lung metastases were also detected by computed tomography. Considering her poor PS, the patient was treated with mild systemic therapy using trastuzumab as the key drug. A different drug response was achieved between the breast mass and lung metastatic lesions, and the tumors were maintained as stable disease (SD) during first 18 months. However, she finally passed away due to respiratory failure resulting from lung metastasis, 33 months after starting treatment. The autopsy findings showed a difference of HER2 expression between the breast tumor and lung metastatic lesions. It must be recognized that differences of HER2 expression between the primary tumor and metastatic lesions are sometimes demonstrated in patients with breast cancer, and that trastuzumab can be used as a key drug in some patients as in the current case.