Cutaneous adnexal differentiation and stromal metaplasia in palate pleomorphic adenomas: a potential diagnostic pitfall that may be mistaken for malignancy.

Cutaneous adnexal differentiation is well-recognized in benign mixed tumors occurring in cutaneous sites. The incidence of this histologic finding in salivary gland sites is not known. We sought to describe the incidence of cutaneous adnexal differentiation in benign mixed tumors of the palate, lip, and parotid gland. Benign mixed tumors of the palate (n=30), lip (n=13), and parotid gland (n=37) resected between 1980 and 2009 at a single academic medical institution were reviewed. All hematoxylin and eosin-stained sections containing neoplasm were reviewed by all authors including one dermatopathologist (S.H.O.). After confirming the diagnosis of benign mixed tumor, we evaluated for morphologic evidence of cutaneous adnexal differentiation and metaplastic epithelial and stromal changes. Chart review was conducted to obtain pertinent clinical information. Cutaneous adnexal differentiation was seen in 20% of palate and 39% of lip benign mixed tumors but in no parotid tumors. The most frequent features of cutaneous adnexal differentiation were tricholemmal differentiation (20% of palate and 39% of lip tumors), infundibulocystic structures (17% and 31%), and trichohyalin granules (13% and 31%). Sebaceous differentiation was seen in only one palate tumor. Varying amounts of stromal adipose were seen in 62, 37, and 22% of lip, palate, and parotid tumors. Osseous metaplasia was seen in one tumor from each site. When cutaneous adnexal differentiation occurs in salivary gland pleomorphic adenomas, it can present a diagnostic pitfall that must not be misinterpreted as carcinoma at biopsy, fine needle aspiration, or frozen section.