Owing to its relatively recent pathological distinction, multiple classification systems have been adopted for MCB among different organizations and research groups. The World Health Organization classifies MCB under two categories: Epithelial-type and mixed-type. The epithelial-type is further classified as squamous cell carcinoma, adenocarcinoma with spindle cell differentiation, and adenosquamous carcinoma. The mixed-type is further classified as carcinoma with chondroid metaplasia, carcinoma with osseous metaplasia, and carcinosarcoma. Wargotz et al. proposed a classification system for MCB according to its cytopathological features. These classifications are spindle cell, squamous cells, matrix-producing, carcinosarcoma, and MCB with osteoclastic giant cell. Another research group, Oberman et al., proposed a classification system consisting of spindle cell carcinoma, invasive ductal carcinoma with extensive squamous metaplasia, and invasive ductal carcinoma with pseudosarcomatous metaplasia. The lack of consensus between these various classification systems for MCB has led to complications in both clinical practice and research studies.
Metaplastic carcinoma
Anaplastic carcinoma
Verrucous carcinoma
Adenosquamous carcinoma
Spindle cell carcinoma
Mucinous carcinoma
Adenosarcoma
Mucoepidermoid carcinoma