Basal Cell Carcinoma or Trichoblastoma? Dermoscopic Examination of Black Macules Developing in the Same Nevus Sebaceus

Nevus sebaceus (NS) is a common congenital birthmark, and various tumors have been reported to develop in NS. Basal cell carcinoma (BCC) seldom occurs in NS, and it is very important to be able to clinicopathologically distinguish BCC from trichoblastoma. Herein, we describe a case of BCC and trichoblastoma occurring simultaneously in the same NS, including the differential dermoscopic features. BCC is clinically difficult to distinguish from trichoblastoma because the clinical manifestations are similar. In a dermoscopic examination of BCC, arborizing vessels are one of the diagnostically significant features. In our case, the BCC showed ‘multiple’ black structures, and the trichoblastoma showed a ‘single’ black structure without arborizing vessels. To the best of our knowledge, there have been no reports on the dermoscopic findings of secondary tumors on NS.