Primary cutaneous sarcomatoid squamous cell carcinoma: a comprehensive clinicopathological and immunohistochemical study focusing on diagnostic pitfalls

Aims Primary cutaneous sarcomatoid squamous cell carcinoma (PCSSCC) is an uncommon variant of squamous cell carcinoma. Histologically, these tumours are composed of deeply infiltrative spindled and pleomorphic cells arranged in fascicles or cohesive nests and usually lacking keratinisation. Its distinction from other malignant spindle cell lesions is a difficult task that requires ancillary immunohistochemical studies to make a definitive diagnosis. A retrospective analysis of 74 cases of PCSSCC, obtained from large academic institutions, was performed, and is summarised below. Methods and results Immunohistochemical studies were performed with antibodies against pan‐cytokeratin monoclonal antibody (AE1/AE3), anti‐cytokeratin (CAM 5.2) mouse monoclonal primary antibody, MNF‐116, p63, p40, high‐molecular‐weight kininogen (HMWK), cytokeratin (CK)7, CK5/6, smooth muscle actin (SMA), desmin and H‐caldesmon. All cases were poorly differentiated; however, there were some histopathological clues to the diagnosis of PCSSCC; namely, the presence of a well‐differentiated component in 13 cases. Using immunohistochemistry, all cases were positive for at least one epithelial marker (AE1/AE3, MNF116, CAM5.2, p63, p40, HMWK and CK5/6). Eighteen cases had aberrant expression of SMA. Conclusions PCSSCC can be a difficult neoplasm to diagnose, as it can appear histologically similar to other malignant spindle cell neoplasms. We describe 74 cases of PCSSCC, highlighting their salient clinical, histopathological and immunohistochemical features, and discuss the potential diagnostic pitfalls in distinguishing this entity from other malignant neoplasms. Our results indicate that a combination of thorough histological inspection coupled with an adequate battery of immunohistochemical stains is necessary to support the diagnosis of PCSSCC.