nevus sebaceus ( ns ) is a common congenital birthmark that occurs most frequently on the head and face . removal is preferable because various tumors have been reported to develop in ns [ 1 , 2 ] . 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 . a 77-year - old woman had been diagnosed at the age of 15 years with ns which had started from the left side of her ear and extended to her chest . she was referred to our hospital with a 3-year history of black macules on the left ear . a physical examination showed yellowish plaques around the left side of the neck and small , numerous yellow papules in the ns . a black macule of 2 mm in diameter was observed on the left auricle ( fig . a dermoscopic examination revealed multiple blue - gray ovoid nests and whitish veils , but arborizing vessels were not found ( fig . a surgical specimen showed that tumor cells were proliferating from the epidermis to the upper dermis and were arranged in a palisading pattern at the periphery with basal melanosis . deposits of mucin and clefts were observed in the surrounding area with dilated vessels in the papillary dermis ( fig . three months after the surgery , a small black macule appeared on the left earlobe which slowly enlarged . a physical examination indicated the presence of a small black macule with a diameter of 1 mm on the left earlobe ( fig . a dermoscopic examination revealed a round , homogeneous , brownish structure without arborizing vessels ( fig histopathologically , the surgical specimen showed that the tumor cells were proliferating in the dermis , presented as multiple different shapes , were either clear or basophilic , and were arranged in a partial palisading pattern . the tumor cells also resembled follicular germinative cells without keratinizing cysts , and islands of tumor cells were present as hair bulbs surrounding fibrous stroma ( fig . a study on tumors occurring in ns found that trichoblastoma was observed in 4.7% of cases ( 28 of 596 cases ) and bcc in 0.8% of cases ( 5 of 596 cases ) . in a dermoscopic examination of bcc , arborizing vessels were observed in 84.1% of cases and multiple blue - gray ovoid nests in 36% of cases . multiple black structures , and the trichoblastoma showed a  single black structure without arborizing vessels . we theorize that this difference is a specific clue when distinguishing between bcc and trichoblastoma in ns . in a dermoscopic examination of ns , it is difficult to precisely evaluate a macule with vessels directly under the epidermis due to histopathological modifications of the background , particularly under the influence of epidermal proliferation . this is why our bcc did not reveal arborizing vessels . in conclusion , most ns are removed to avoid secondary tumors , including bcc ; thus , there have been no reports on their dermoscopic findings .
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 .