Primary adenoid cystic carcinoma of the skin. Histologic and ultrastructural study of two cases localized in the external auditory canal

: Primary cutaneous adenoid cystic carcinoma is a rare tumor, even if its frequency has often be underestimated, its localization in the external auditory meatus being generally considered separately. The results of the histological and ultrastructural study of two cases of adenoid cystic carcinoma, arising in the external auditory meatus, are presented, confirming the essential morphological similarity of adenoid cystic carcinoma of the skin and salivary glands. In these two cases, the ultrastructural study revealed two types of unique intra luminal inclusions, composed respectively of parallel straight tubular arrays and paracrystalline quadrangular structures, probably corresponding to a peculiar organization of secretion products or cellular debris. The review of 126 published cases of adenoid cystic carcinoma of the skin, including 89 cases developed in the auditory canal, shows that 78% of the cases were located in areas containing numerous apocrine glands, supporting an apocrine gland origin for this tumor. Contrasting with the low (8%) metastatic potential of cutaneous adenoid cystic carcinomas of other locations, adenoid cystic carcinoma of the auditory meatus present a 30% metastatic risk, similar to those or adenoid cystic carcinoma of salivary glands.