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 myoepit helial differentiation. Histologically, the lesion showed well-demarcated t umor nodules composed of two different components. One component was typica l of BCC, and the other component was composed of tumor cells containing ab undant cytoplasm, eccentric nuclei, and no peripheral palisading, with scat tered signet ring-shaped cells. Immunohistochemically. the tumor cells in t he typical BCC component stained with CKAE1/AE3 and smooth muscle actin (SM A), but not with S-100 protein. They stained weakly with CAM5.2, epithelial membrane antigen, and glial fibrillary acidic protein (GFAP). The tumor ce lls in the other component stained strongly with CKAE1/AE3 and SMA. moderat ely with epithelial membrane antigen and GFAP, and weakly with CAM5.2. In a small area, the tumor cells stained with S-100 protein.