Signet ring cell basal cell carcinoma: a basal cell carcinoma with myoepithelial differentiation.

Basal cell carcinoma (BCC) can show a variety of routes of differentiation, but myoepithelial differentiation has rarely been described. We describe a case of BCC showing histologic and immunohistochemical features of myoepithelial differentiation. Histologically, the lesion showed well-demarcated tumor nodules composed of two different components. One component was typical of BCC, and the other component was composed of tumor cells containing abundant cytoplasm, eccentric nuclei, and no peripheral palisading, with scattered signet ring-shaped cells. Immunohistochemically, the tumor cells in the typical BCC component stained with CKAE1/AE3 and smooth muscle actin (SMA), but not with S-100 protein. They stained weakly with CAM5.2, epithelial membrane antigen, and glial fibrillary acidic protein (GFAP). The tumor cells in the other component stained strongly with CKAE1/AE3 and SMA, moderately with epithelial membrane antigen and GFAP, and weakly with CAM5.2. In a small area, the tumor cells stained with S-100 protein.