Primary Cutaneous Spindle Cell Squamous Carcinoma Expressing Smooth Muscle Actin: Diagnostic Pitfalls.

ABSTRACT

Alpha smooth muscle actin (SMA) belongs to the actin proteins. It is a known immunohistochemical marker for tumors of mesenchymal origin. There have been reports of expression of SMA in certain epithelial malignancies in the head and neck and genital regions. In this study, the authors report a primary cutaneous spindle cell squamous carcinoma expressing SMA. Both high- and low-molecular-weight keratins and p63 were positive, and S100 protein, SOX10, MART-1/Melan-A, and muscle-specific actin stains were negative. This case highlights that an epithelial tumor could express a mesenchymal marker, thereby making the diagnosis problematic.