[Pleomorphic adenoma of salivary glands: diagnostic pitfalls and mimickers of malignancy]

Pleomorphic adenoma is the most common salivary gland tumor, characterized by a complex biphasic proliferation of epithelial and myoepithelial cells intermingled with a mezenchymal component with frequent metaplastic changes and protean histomorphology of the cells. This review describes several unusual histological findings in pleomorphic adenoma that may mimic malignancy, and therefore they represent a diagnostic pitfall. Intravascular invasion of tumor cells is generally suspicious of malignancy; however, intravascular tumor deposits may be rarely found within the capsule of clinically benign salivary pleomorphic adenomas. It is important not to render a malignant diagnosis in such neoplasms, in the absence of other evidence of malignancy. Pleomorphic adenomas, particularly of minor glands of palate, may contain large areas of squamous and mucinous metaplasia suspicious of mucoepidermoid carcinoma (MEC). In contrast to MEC, metaplastic pleomorphic adenomas do not harbour the distinctive translocations t(11;19) and t(11;15), they are not invasive, in contrast they reveal at least focally myxochondroid stroma. Cribriform structures in pleomorphic adenoma may mimic adenoid cystic carcinoma. Oncocytic metaplasia in cellular rich pleomorphic adenoma/myoepithelioma may be associated with significant nuclear polymorphism and hyperchromasia suspicious of malignancy. The most common pitfall in diagnosis of pleomorphic adenoma is so called "atypical PA" that must be distinguished from early malignant transformation to in situ-carcinoma ex pleomorphic adenoma.