Polymorphous adenocarcinoma of the salivary glands: An overview of immunohistochemical features and insights on molecular pathology.

Polymorphous adenocarcinoma (PAC), represents a common minor salivary gland tumor (SGT) characterized by local growth, low metastatic potential and non-aggressive biologic behavior. Due to the clinical aggressiveness noted in a subset of such tumors, the former term polymorphous low-grade adenocarcinoma (PLGA) was recently revised. PAC's clinical features and histological diversity result in clinical overlap of this entity with several other SGTs including mainly adenoid cystic carcinoma (AdCC). Differential diagnosis among the entities is crucial, in terms of tumor management and patients' prognosis. The aim of the present review is to summarize the histological, cytological, immunohistochemical and molecular features of PAC. Histopathological examination is usually adequate for PAC differential diagnosis from other SGTs, except of AdCC. Several immunohistochemical markers including c-Kit, S-100/ MG, Mcm-2 and Integrin β-1, -3, -4, are reported to be useful diagnostic aids in borderline cases. Limitations in sample numbers and study methodology issues of the immunohistochemical PAC studies complicate the identification and selection of appropriate markers useful in the differential diagnosis. Additionally, molecular analyses of PAC specimens indicate that the PAC spectrum phenotypes result from different genotypes (protein kinase D positive; PRKD(+) and PRKD(-) tumors). PAC pathogenesis remains to be determined in each particular genotype while the convergence issue should be addressed in future studies.