Differential diagnosis between monomorphic clear cell adenocarcinoma of salivary glands and renal (clear) cell carcinoma

Clear cell adenocarcinoma of salivary glands (CCASG) is a relatively rare tumor, composed entirely of clear cells of putative ductal origin. It bears striking morphologic similarities to renal cell carcinoma (RCC) of clear cell type on hematoxylin and eosin stains. Differentiation between CCASG and metastatic RCC to the salivary glands has been considered problematic or even impossible on morphologic grounds. We examined three cases of CCASG and 12 cases of RCC (6 primary and 6 metastatic) by hematoxylin and eosin staining, immunohistochemistry, and electron microscopy. Two distinctive immunohistochemical and ultrastructural patterns emerged from this analysis. CCASG showed positivity for high molecular weight cytokeratin and carcinoembryonic antigen and ultrastructurally showed prominent squamoid differentiation, glycogen pools, and absence of lipid. In contrast, RCC was characterized by positivity for vimentin and complete absence of staining for high molecular weight cytokeratin and carcinoembryonic antigen. On ultrastructural studies, RCC lacked any squamoid differentiation, and the tumor cells contained abundant cytoplasmic lipid in addition to glycogen. Thus, based on the consistent differences on the immunohistochemical staining patterns and their characteristic subcellular morphology, CCASG and RCC can be distinguished on pathologic evaluation. The different direction of differentiation of the cells in CCASG and RCC (i.e., ductal in the former and renal tubular and mesodermal in the latter) results in their distinctive immunophenotypical and ultrastructural features.