[A Case of Peritoneal Metastasis of Breast Cancer Diagnosed by Laparoscopic-Assisted Right Hemicolectomy].

The patient was an 86-year-old woman. She underwent right breast-conserving surgery and sentinel lymph node biopsy for breast cancer in August 2006. The pathological diagnosis was invasive ductal carcinoma, T1N0M0, Stage Ⅰ, ER (+), PgR (-), HER2 (-). She was treated with tamoxifen for 5 years as adjuvant therapy and showed no signs of recurrence. In November 2014, CA15-3 was elevated and an accumulation of FDG in the right paracolic sulcus was observed on PET-CT. Peritoneal metastasis of breast cancer was suspected, and an operation was performed for a definitive diagnosis. During the operation, the tumor was seen on the paracolic sulcus, and laparoscopic-assisted right hemicolectomy was performed. A poorly differentiated adenocarcinoma was diagnosed by pathological examination, and immunostaining results were as follows: CK7(+), CK20(-), mammaglobin (-), GCDFP-15 (-), ER (-), PgR (-), and HER2 (-). Because there was no original lesion other than the breast cancer, the tumor was diagnosed as a metastasis of breast cancer. The frequency of peritoneal metastasis of breast cancer is low. In this case, pathological diagnosis was necessary for a definitive diagnosis. A change of subtype was also confirmed, and the treatment strategy was decided appropriately. Surgical resection should be considered for peritoneal metastasis of breast cancer when the operation can be performed safely.