Breast metaplastic squamous cell carcinoma associated with a squamous-lined cyst: A potential distinct clinicopathologic subset

Pure metaplastic squamous cell carcinoma (SCC) of the breast is a rare subtype of breast cancer that is generally considered aggressive and is usually triple-negative (estrogen receptor-negative, progesterone receptor-negative, and human epidermal growth factor receptor-negative). The aim of this study was to report a small series of breast pure SCCs with distinct morphologic and prognostic features. Seven cases of pure breast SCC were identified and were divided into two categories according to the presence (five cases, group 1) or absence (two cases, group 2) of an associated benign squamous-lined cyst. In group 1, two patients did not receive systemic chemotherapy, and one patient only received limited cycles of neoadjuvant chemotherapy. The remaining two patients in group 1 and both patients in group 2 received neoadjuvant and/or adjuvant chemotherapy. All five patients in group 1 including three without systemic therapy remained disease free (follow up 20 to 99 months, median 27 months). One patient from group 2 developed local recurrence at 17 months and died of disease at 36 months. The presence of an associated benign squamous-lined cyst may identify a potential distinct clinicopathologic subset of pure breast SCC with less aggressive biological behavior. Additional studies are needed to further confirm this observation.