We present a case of breast metastases of advanced primary serous ovarian cancer, in which differential diagnosis from invasive ductal carcinoma (IDC) was required.

A 43-year-old woman presented with a mass of the right breast. during chemotherapy for postoperative recurrence of ovarian cancer. Ultrasonography showed a low-echoic mass measuring 15 mm in diameter in the lower-lateral quadrant of the right breast and swelling of the right axillary lymph nodes. The pathological diagnosis by a biopsy of the right breast tumor was IDC and that of the right axillary lymph node was ovarian cancer metastasis. She received chemotherapy with a poly ADP-ribose polymerase inhibitor which was only effective for the axillary lymph nodes, but not for the right breast mass. Additionally, another left breast tumor appeared and was diagnosed with IDC. Then we performed bilateral mastectomy. The bilateral breast masses and axillary lymph nodes were positive for CA125 on immunohistochemistry (IHC), suggesting metastases from the ovarian cancer (high-grade serous carcinoma). Ovarian cancer metastases to the breast and/or axillary lymph nodes are very rare, and the accurate diagnosis is difficult. However, the history of advanced ovarian cancer and IHC of CA125 are expected to be helpful for the differential diagnosis.