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

## TL;DR

This paper reports two rare cases of breast mucinous cystadenocarcinoma, a tumor with unique features and better prognosis despite being triple-negative.

## Contribution

The novelty lies in presenting two distinct BMCA cases and highlighting diagnostic challenges and the need for further molecular studies.

## Key findings

- BMCA is a rare breast tumor with cystic structures and mucin-rich cells.
- BMCA is typically triple-negative but has a better prognosis than other triple-negative breast cancers.
- Accurate diagnosis requires ruling out metastatic tumors and using clinical, radiological, and histopathological methods.

## Abstract

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.