Histopathological Varieties of Intraductal Papilloma of the Oral Cavity

Objectives: Intraductal papilloma (IDP) has been considered to be a rare salivary gland tumor characterized by its papillary growth within a unicystic ductal space. However, it is rare to encounter such a typical histology when IDP arises in minor salivary gland within the oral cavity. To prove our hypothesis that IDP does not limit its growth to the luminal space only but involve the outside of the primary duct wall, that “extra-luminal IDP” involving the peri-luminal space is often misdiagnosed as pleomorphic adenoma (PA), and that there must be a malignant counterpart of IDP, we examined our archives of intraoral PA cases retrospectively.
Methods: Reviewing oral pleomorphic adenoma cases filed at Niigata University and Sichuan University in the last thirty-three year period, we found 85 typical cases of extra-luminal IDP, including three cases of carcinoma ex IDP. They were examined for common clinicopathological characteristics as IDP. In addition, immunohistochemical profiles for the constituent tumor cells were investigated.
Results: Histopathologically, IDP had dilated cystic spaces, which were basically lined by two-cell layered duct epithelium-like cells, in the periphery of the tumor but had no complete intraluminal growth. There was definite papillary proliferation of two-cell layered cells with hyaline stroma containing blood vessels. The luminal cells were immunohistochemically keratins 7/19 positive and MUC-1, while basal cells were positive for vimentin, calponin, P63, α-SMA and occasionally S-100 protein. There were malignant foci within IDP.
Conclusions: The present result indicates that the extra-luminal IDP is not so rare but accounts for about 10% of PA, which may be nearly the same as the frequency of myoepithelioma. There is no apparent difference in prognoses between IDP and PA, and secondarily malignant changes also occur in IDP as in PA.