A recently described and rare variant of breast carcinoma, mucinous cystadenocarcinoma (MCA), is reported in a 65-year-old post-menopausal woman. She presented with a gradually enlarged breast tumor. A well-circumscribed tumor measuring about 3 cm in diameter was noted in the mammographic and ultrasonographic examinations. The mammographic and ultrasonographic findings were indistinguishable from more common mucinous carcinoma (colloid carcinoma) of the breast. The gross appearance of the tumor was well-defined and cystic, consisting of abundant transparent to bloody mucin, as well as whitish solid parts. Microscopically, the tumor was characterized by abundant extracellular and intracellular mucin. It looked like a mucinous cystic neoplasm of the ovary and pancreas. Particularly, few microscopic foci of ordinary intermediate-grade infiltrating ductal carcinoma (IDC) and ductal carcinoma in situ (DCIS) were observed around the main lesion in this case. A transition from ordinary DCIS to MCA in situ was found. It might indicate MCA derives from a metaplasia process of ordinary DCIS. MCA can be easily differentiated from mucinous carcinoma by quite different histologic and immunohistochemical findings. According to the previously reported and present cases, MCA of the breast more commonly affects elderly women and has a relatively favorable prognosis.

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