[A Case of Primary Breast Cancer and Gastric Cancer Metastasis to the Skin-Usefulness of Immunohistochemistry in Differentiating Primary Breast Cancer from Metastatic Breast Cancer].

We herein report the case of a 76-year-old female patient who had undergone gastrectomy for advanced gastric cancer (histologically tubular adenocarcinoma)before 5 months, presenting with abdominal skin tumor. A skin biopsy revealed tubular adenocarcinoma. Positron emission tomography-computed tomographic scanning detected right breast tumor. A partial mastectomy of the right breast and local resection of abdominal skin tumor were performed and both tumors depicted similar histology of tubular adenocarcinoma in routine pathological examination. Immunohistochemically, positive for CDX2 and MUC5AC in previously resected gastric cancer and skin tumor tissues, whereas negative for both antigens in breast cancer. Thus, the final pathological diagnosis demonstrated skin metastasis originating from gastric cancer and primary breast cancer(invasive ductal carcinoma)histologically mimicking gastric cancer. We emphasize difficulties in diagnosis of this situation and that immunohistochemistry is helpful to distinguish primary breast cancer from gastric cancer metastasizing to the breast.