[A case of bilateral breast cancer and metastatic gastric cancer with peritonitis carcinomatosa successfully treated with a combination therapy of S-1 and paclitaxel].

A 46-year-old woman was diagnosed with complaints of bilateral breast tumor with massive ascites retention. The patient was examined as scirrhous carcinoma by lacteal gland inspection and dysplastic cell by ascites cytotechnology. We diagnosed her case to be bilateral breast cancer with peritonitis carcinomatosa, lymph node metastases and bony metastases. In addition to that, gastric metastasis was diagnosed by the result of widespread irregular gastric mucosa, which was inspected through upper gastrointestinal endoscope. The patient was treated with S-1 and paclitaxel and has achieved a remarkable response. The patient's tumor, gastric metastasis, and ascites were disappeared almost completely.