[A Case of Metaplastic Squamous Cell Carcinoma of the Breast Successfully Treated with S-1].

Metaplastic squamous cell carcinoma(MSCC)of the breast is very unusual and is histologically characterized by rapid progression. Conventionalchemotherapy for ductalcarcinoma of the breast is ineffective against MSCC. Here, we report a case of MSCC of the breast successfully treated with S-1. A 57-year-old woman was admitted to our hospital because of a left breast tumor. A tumor approximately 10 cm in diameter was palpable in the lower-outer quadrant(D region)of the left breast. Core needle biopsy indicated estrogen receptor(ER)-negative, progesterone receptor(PR)-negative, and human epidermalgrowth factor receptor 2(HER2)-negative MSCC of the breast. Computed tomography(CT)showed left axillary lymph node metastases but did not indicate distant metastasis. A diagnosis of T4N3cM0, Stage C, MSCC of the left breast was made. Each treatment course consisted of the administration of S-1(120mg/body/day)for 4weeks, followed by 2 drugfree weeks. After the second course, significant tumor and lymph node reduction was observed. We concluded that S-1 chemotherapy seems to be effective for patients with MSCC of the breast.