Basal cell carcinoma with myoepithelial differentiation: a distinct plasmacytoid cell variant with hyaline inclusions.

We describe a 74-year-old man presenting with a 10-year history of an asymptomatic, skin-colored hard plaque on his left cheek. Microscopy revealed well-demarcated tumor with peripheral palisading and peritumoral clefting in the dermis. The tumor included many plasmacytoid cells with cytoplasmic hyaline inclusions and eccentric nuclei. Electron microscopic examination showed that hyaline inclusions consisted of aggregates of filaments. Immunoperoxidase staining showed diffuse cytoplasmic expression of 34bE12, smooth muscle actin, calponin and p63. We diagnosed the patient with basal cell carcinoma (BCC) with myoepithelial differentiation and treated him with excision. Eight months postoperatively, the patient remained well with no recurrence. This distinct variant of BCC with myoepithelial differentiation, showing cytoplasmic hyaline inclusions and eccentric nuclei, has previously been confusingly termed 'signet-ring cell BCC'. However, these plasmacytoid cells differ from signet-ring cells with vacuolated cytoplasm. To avoid confusion, we suggest the designation 'BCC with myoepithelial differentiation'.