A malignant mass in the breast is not always breast cancer.

A 37-year-old woman presented to the Internal Medicine Clinic with complaints of abdominal pain and constipation which had begun 3 months earlier. A colonoscopy was performed, and wall thickening of the sigmoid colon was detected. A biopsy of the sigmoid colon revealed a poorly differentiated, mucin-producing adenocarcinoma with a signet-ring pattern. No distant metastasis was detected. The patient was treated with chemotherapy consisting of 5-fluorouracil, leucovorin, and oxaliplatin. One and a half years later, a painless mass, which was not fixed to the skin, measuring 1 cm in diameter, was found in the lower outer quadrant of the left breast. A core biopsy of the mass was performed, and a histopathological report confirmed metastasis to the breast from mucinous adenocarcinoma of an intestinal primary.