Ultrastructural distinction of basaloid-squamous carcinoma and adenoid cystic carcinoma

Basaloid-squamous carcinoma is a unique and infrequent variant of squamous cell carcinoma with a predilection for occurring in the upper aerodigestive tract. The cardinal diagnostic criteria are the presence of basaloid cells arranged in lobules or cords and a squamous component. Another feature that distinguishes basaloid-squamous carcinoma from conventional squamous cell carcinoma is the development of multiple foci of amorphous, eosinophilic, or mucinous materials among the basaloid tumor cells. It is this feature in basaloid-squamous carcinoma that causes confusion with adenoid cystic carcinoma, especially the solid variant of adenoid cystic carcinoma. To clarify the distinctive features of these two types of tumors, the ultrastructural findings in three basaloid-squamous carcinomas from the head and neck region were compared with three adenoid cystic carcinomas, one from the head and neck region and one each from trachea and bronchus. Widened intercellular spaces, tonofilaments, and replicated basal lamina were present in both types of tumor. However, the ultrastructural features of glandular differentiation were present only in adenoid cystic carcinoma, whereas the ultrastructural features of squamous cell differentiation were present only in basaloid-squamous carcinoma. The distinction between basaloid-squamous carcinoma and adenoid cystic carcinoma is important since basaloid-squamous carcinoma is a clinically aggressive, high-grade form of squamous cell carcinoma, generally with a worse prognosis than adenoid cystic carcinoma.