The classic findings of cutaneous adenocystic carcinoma (ACC) occur in the deep dermis and are characterized by basaloid cells in islands that form cribriform patterns and tubular structures (Fig. 15.2). Multiple cystic spaces can be seen containing mucin that stains positively with hyaluronic acid (Figs. 15.3 and 15.4). Perineural invasion is observed in most cases. The lumina of the tubular structures have prominent basement membrane material that is PAS positive and diastase-resistant [1–3].

Unlike the similarly appearing adenoid basal cell carcinoma, ACC typically does not have a connection to the overlying epidermis or adnexal structures. This separation is a helpful clue for pathologists when making the diagnosis, as oftentimes these two can be difficult to differentiate. Immunohistochemical studies demonstrate that cutaneous ACC stains positively for S-100, epithelial membrane antigen (EMA), and is occasionally positive for carcinoembryonic antigen (CEA) [1–3].