Answer: Adenoid cystic carcinoma (primary of the external ear canal)

Histology: The tumor demonstrates mixed architectural patterns including a tubular pattern and a cribriform pattern. In the tubular pattern, a hyalinized stroma compresses the tumor nests into thin cords and tubules. In the cribriform pattern, the tumor cells are arranged around sharply punched-out holes. These holes, or cysts, are not true glandular spaces. Instead, they are lined by myoepithelial cells and filled with matrix ground substance. Duct epithelial cells are also present, and they form true glandular spaces. The cellular features of the tumor are deceptively non-threatening. The cells have uniform round-to-angulated basophilic nuclei, and they do not demonstrate appreciable pleomorphism or mitotic activity. Nerves are not identified in this limited sampling, such that the presence or absence of invasion cannot be addressed.

Discussion: Adenoid cystic carcinoma is generally thought of as a tumor arising from the major and minor salivary glands. Less well recognized is that adenoid cystic carcinoma can also arise from the ceruminal glands of the external ear (and, for that matter, apocrine glands elsewhere in the skin). Given the anatomic approximation of the external ear to the parotid gland, the diagnosis of adenoid cystic carcinoma of ceruminal gland origin can only be confidently made after direct extension from a primary parotid gland tumor has been excluded on clinical grounds. Despite differences in origin, ceruminal gland adenoid cystic carcinomas are morphologically identical to their salivary gland counterparts. In contrast to ceruminal gland adenocarcinoma, true glandular formations are not conspicuous in adenoid cystic carcinomas.