Characterization of undifferentiated carcinoma of the salivary gland: clinicopathological and immunohistochemical analyses in comparison with lymphoepithelial carcinoma.

Background: This study aimed to reclassify a subset of poorly differentiated salivary gland carcinoma that do not conform to any entities of the current World Health Organization (WHO) classification into the category of undifferentiated carcinoma (UDC) because they lack specific histologic differentiation or immunophenotype.

Methods: Cases of salivary gland carcinomas from Asan Medical Center (2002-2020) that did not fit any existing WHO classification criteria and were diagnosed as poorly differentiated carcinoma, high-grade carcinoma, or UDC, were retrospectively reviewed. Immunohistochemical (IHC) staining for p40, neuroendocrine markers, androgen receptor (AR), and gross cystic disease fluid protein 15 (GCDFP-15) and Epstein-Barr virus (EBV) in situ hybridization (ISH) were performed. Clinical data were collected from the electronic medical records.

Results: Six salivary gland carcinomas did not align with any specific entities and lacked distinct differentiation. Two of six cases displayed lymphoepithelial carcinoma (LEC)-like morphology but were negative or showed negligible immunoreactivity for p40 and EBV ISH, distinguishing them from LEC of the salivary gland. Two cases showed strong AR positivity, suggesting a potential overlap with salivary duct carcinoma (SDC) but lacked classic SDC morphologies and GCDFP-15 expression. No cases expressed neuroendocrine markers.

Conclusions: This study proposes reclassifying these poorly differentiated or high-grade salivary gland carcinomas as UDC based on their indeterminate differentiation and IHC profiles. This may lead to a clearer diagnostic category and enhance our understanding of these high-grade tumors.