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 t umor, composed entirely of clear cells of putative ductal origin. It bears striking morphologic similarities to renal cell carcinoma (RCC) of clear ce ll type on hematoxylin and eosin stains. Differentiation between CCASC and metastatic RCC to the salivary glands has been considered problematic or ev en impossible on morphologic grounds. We examined three cases of CCASC and 12 cases of RCC (6 primary and 6 metastatic) by hematoxylin and eosin stain ing, immunohistochemistry, and Electron microscopy. Two distinctive immunoh istochemical and ultrastructural patterns emerged from this analysis, CCASG showed positivity for high molecular weight cytokeratin and carcinoembryon ic antigen and ultrastructurally showed prominent squamoid differentiation, glycogen pools, and absence of lipid. In contrast, RCC was characterized b y positivity for vimentin and complete absence of staining for high molecul ar weight cytokeratin and carcinoembryonic antigen. On ultrastructural stud ies, RCC lacked any squamoid differentiation, and the tumor cells contained abundant cytoplasmic lipid in addition to glycogen. Thus, based on the con sistent differences on the immunohistochemical staining patterns and their characteristic subcellular morphology, CCASG and RCC can be distinguished o n pathologic evaluation. The different direction of differentiation of the cells in CCASG and RCC (i.e., ductal in the former and renal tubular and me sodermal in the latter) results in their distinctive immunophenotypical and ultrastructural features.