Mucinous metaplasia of breast carcinoma with macrocystic transformation resembling ovarian mucinous cystadenocarcinoma in a case of synchronous bilateral infiltrating ductal carcinoma

Mammary mucinous cystadenocarcinoma (MCA) is a rare, invasive ductal carcinoma (IDC) of the breast that is virtually identical morphologically to MCA of the ovary, pancreas or appendix. Synchronous bilateral breast tumors, not uncommonly encountered in fibroadenoma and lobular carcinoma, are unusual in IDC. Reported herein is a primary MCA of the right breast coexisting with a bilateral ordinary IDC in a 55-year-old Taiwanese woman who underwent modified radical mastectomy of both breasts with bilateral axillary level I and II lymph node dissection. In the right breast a 2.5 cm unilocular mucus-filled cyst was found. It had complex papillae, some of which were supported by delicate fibrovascular stroma, lined by simple to slightly stratified columnar neoplastic epithelial cells with intracellular mucin and an abundance of intracystic extracellular mucin, coexisting with a low-grade ordinary IDC. In the left breast a high-grade ordinary IDC was discovered. The patient had undergone simple abdominal total hysterectomy for myoma uteri along with bilateral salpingo-oophorectomy 10 years previously. Based on pathological studies and a literature review, it is suggested that mammary MCA arises from mucinous metaplasia and macrocystic transformation of ordinary breast carcinoma. A brief discussion of bilateral breast cancers is also given.