Clinical and Histopathologic Study of Apocrine-Type Mixed Tumor of the Skin

Mixed tumor of the skin (MTS) is a tumor characterized by folliculosebaceous-apocrine differentiation. Because of the wide range of histological variations, understanding the unique features of MTS can help improve diagnosis. This study describes the histopathological characteristics of MTS, mainly apocrine-type MTS (AMT), using 166 cases of AMT. We found that nodular aggregates of myoepithelial cells, mucinous changes in the stroma, and follicular differentiation were standard characteristic features of MTS. Among the cases studied, 67% showed prominent follicular germinative cells and 40% showed prominent lipomatous metaplasia in the stroma. These cases often pose difficulties for the diagnosis of AMT because of insufficient evidence of sweat glands or myoepithelial cell differentiation. This is the first study to examine how the histological features of AMT change as the tumor extends deeper into the dermis. We found that the proportion of AMT with folliculosebaceous differentiation and large lumina increased as it got deeper into the dermis. Histopathological diagnosis of MTS is vital because the clinical symptoms lack specificity. This study enhances our understanding of the histopathological characteristics of MTS.