Pleomorphic adenoma with extensive squamous metaplasia and keratin cyst formations in minor salivary gland: a case report

Abstract

Pleomorphic adenoma (PA), the most common salivary gland tumor, accounts for 54 to 65% of all salivary gland neoplasias and 80% of the benign salivary gland tumors. It most frequently affects the parotid gland, followed by the submandibular and the minor salivary glands. Microscopically, mucous, sebaceous, oncocytic and squamous metaplasia, sometimes with the formation of keratin pearls, may be present, but the latter rarely results in the formation of extensive keratin-filled cysts lined by squamous epithelium. Extensive squamous metaplasia can be mistaken for malignancy, including mucoepidermoid carcinoma and squamous cell carcinoma. Here, we present an unusual case of PA with extensive squamous metaplasia and keratin cyst formations in a minor salivary gland, and discuss its microscopic features, including the immunohistochemical characteristics, and differential diagnosis of this uncommon presentation.

Figure 1

Intraoral photograph at initial examination showing well circumscribed and sessile nodule in the right upper vestibule, measuring 2.0 cm in diameter and with a normal-appearing oral mucosa surface

Figure 2

Encapsulated lesion composed of multiple keratin cysts of variable size (A). Cysts lined by stratified squamous epithelium (B), which is intensely positive for high-molecular-weight cytokeratins (CKs; C). Numerous duct-like formations (D), lined by a double layer of cells (E). Intense expression of CK7 in luminal cells of the duct-like structures (F). Hematoxylin and eosin stain (A,B,D,E). Immunoperoxidase stain (C,F)

Figure 4

Immunoprofile of duct-like structures. Luminal cells intensely positive for cytokeratin (CK) 7 (A) and CK19 (B) and moderately positive for CK8 (C). Abluminal cells moderately positive for high-molecular-weight (CKs) (D), p63 (E) and vimentin (F). Immunoperoxidase stain