Stroma-poor Warthin's tumor with significant oncocytic hyperplasia: case presentation and considerations regarding its histogenesis.

Although Warthin's tumor is one of the common tumors of the salivary glands, Warthin's tumors with a prominent component of nodular oncocytic hyperplasia reminiscent of oncocytoma are rare. Here we report such a tumor, measuring 3 cm in diameter, found in the parotid gland of an 81-year-old man. Histologically, approximately 70% of the mass was a component of nodular oncocytic proliferation, and the remaining portion was a component of conventional Warthin's tumor. We performed immunohistochemical analysis to explore what factors determined the morphogenesis of the two components in the single mass. Cytokeratin (CK) 5÷6-positive tumor cells, which represent basal cells, were aligned in a layer in the conventional Warthin's tumor component, whereas they were localized around blood vessels in the nodular oncocytic hyperplasia component. Immunostaining for CD34 showed that capillaries were sparsely present beneath the bilayered epithelia in the former component, while blood vessels resembling sinusoids separated the trabeculae of the tumor cells in the latter component. Ki-67 labeling index was slightly higher in the latter component. Double immunostaining for CK5÷6 and Ki-67 revealed that most of Ki-67-positive proliferating tumor cells were CK5÷6-positive, suggesting that CK5÷6-positive population contained proliferative progenitor cells of the tumor. These findings imply that the regional difference in the distribution pattern and proliferative activity of CK5÷6-positive putative progenitor cells along with the difference in the pattern of vascular network occurred during the tumorigenic process of the tumor and determined one region to become conventional Warthin's tumor morphology and the other to become nodular oncocytic hyperplasia.