[One Case of Accessory Breast Cancer Complicated by Contralateral Breast Cancer].

We experienced a case of right sided accessory breast cancer complicated by contralateral breast cancer. A 50-year-old woman came to us for an examination because a tumor in her left breast was pointed out at breast cancer screening. A breast MRI confirmed a tumor in her left breast and a tumor continuing from the skin to the subcutis of the right axilla. A skin biopsy for the tumor in the right axilla and a core needle biopsy(CNB)for the tumor in the left breast were performed. The pathological result of the CNB for the left breast indicated an invasive ductal carcinoma of the tubular formative scirrhous type. Although the tumor of the right axilla was poorly differentiated adenocarcinoma demonstrating cord-like arrays, it was examined by skin biopsy and therefore no deep part of the tissue was included. We conducted immunostaining, in consideration of the possibility of metastasis from the left sided breast cancer. ER, PgR, mammaglobin, GATA 3 were positive, strongly suggesting that the tumor in the right axilla was also derived from a mammary gland. We also performed a wide local excision of the right axilla plus axillary dissection(level Ⅰ)in addition to conducting a left mastectomy plus sentinel lymph node biopsy, in consideration of the possibility of primary right sided accessory breast cancer. The pathological result following surgery confirmed a difference in the histologic features between both sides, residual normal accessory mammary glands around the tumor on the right side, and the presence of rich DCIS and a lobular replacement image, leading to a definitive diagnosis of primary invasive ductal carcinoma of the accessory breast on the right side.