An Exceptional Primary Breast Tumor: A Report of Two Mucinous Cystadenocarcinoma Cases.

Mucinous cystadenocarcinoma of the breast (BMCA) is an exceptionally rare malignant tumor. Histologically, BMCA features cystic structures lined with tall columnar cells rich in intracytoplasmic and extracellular mucin, resembling mucinous cystadenocarcinomas of the ovary, pancreas, and appendix. Typically triple-negative for hormone receptors and human epidermal growth factor receptor 2 (HER2), BMCA paradoxically demonstrates a better prognosis compared to other triple-negative breast cancers. Diagnosis requires exclusion of metastatic mucinous carcinomas from other primary sites and differentiation from other mucin-producing breast tumors through a combination of clinical, radiological, histopathological, and immunohistochemical evaluations. We present two cases illustrating the variable histological features and hormone receptor status of BMCA, emphasizing differential diagnosis challenges. Given its rarity, further studies are necessary to better clarify the molecular pathogenesis of BMCA, guiding optimal management strategies.