Are all adenomatoid tumors adenomatoid mesotheliomas?

The ultrastructure of three histologically typical adenomatoid tumors of the epididymis is presented. Two of the three tumors showed morphologic features of mesothelium similar to those reported in the literature. The third tumor demonstrated a multilayered basal lamina and Weibel-Palade bodies, both of which have been found consistently in sclerosing hemangiomas. Although the ultrastructure of the first two tumors confirms the theory of a mesothelial origin of adenomatoid tumors, the findings of the third tumor indicate that a histologically typical adenomatoid tumor may not necessarily be a mesothelioma and that electron microscopy is indispensable in establishing their nature. These findings also lead us to suggest that the term adenomatoid tumor should remain in use for light microscopic diagnosis, and that the term adenomatoid mesothelioma should be applied only when the mesothelial nature of an adenomatoid tumor is proven by electron microscopy. For those that show ultrastructural evidence of endothelial origin, the term adenomatoid angioma seems to be appropriate and accurate. It is possible that the adenomatoid angioma represents a variant of the histiocytoid hemangioma.