Fibromatosis-like metaplastic carcinoma of breast: a challenge for clinicopathologic diagnosis.

Fibromatosis-like metaplastic carcinoma of breast (FLMC) is a newly described metaplastic tumor in the (4th) edition of the WHO classification of breast tumors in 2012. It is a low-grade tumor formed by spindle cells with mild or absent nuclear atypia, embedded in collagenized stroma, and it has only <5% tumor cells showing epithelial traits. Because its biological behavior is better than that of general spindle cell metaplastic cancer, this cancer type is added to the new WHO classification. Due to their mild morphology, breast FLMCs are often misdiagnosed as benign interstitial proliferative lesions or benign mesenchymal tumors. Accurate diagnosis is a challenging task particularly in needle core biopsies. We systematically reviewed 23 cases of metaplastic breast carcinoma (MBC), 8 cases of metaplastic carcinoma with squamous cell component, 12 cases of spindle cell/sarcomatoid metaplastic carcinoma, and 3 cases of FLMC. Also, we performed CK, CK high molecular weight (34βE12), CK7, CK5/6, P63, CD10, ER, PR, HER-2, SMA, Desmin, CD34, CD117, and S-100 immunohistochemistry, using Envision staining. This study was focused on clinical and pathological features of 3 FLMC cases and assessed the immunoprofile of MBC subtypes in a large series to improve the understanding of these diseases.