Answer: Canalicular adenoma

Histology: The tumor is architecturally characterized by double columns of cells forming interconnecting channels that course through a loose myxoid stroma. The cells forming the channels have columnar, uniformly aligned nuclei without atypia or mitotic activity.

Discussion: Canalicular adenoma is a benign tumor of minor salivary gland origin. It is notable for its distinct topography in that it has a striking predilection for the upper lip. Involvement of the maxillary sinus is unusual. Histologically, they are characterized by columns of elongated cells forming interconnecting channels in a loose stroma. They should not cause confusion with a malignant process. Canalicular adenomas are not infiltrative, and their cells are entirely bland and uniform.

In contrast to benign mixed tumors, canalicular adenomas do not demonstrate a prominence of myoepithelial cells or a chondromyxoid stroma, and their epithelial component is less diverse. Some canalicular adenomas harbor foci of basaloid cells, and this finding has led some to believe that canalicular adenomas represent one variant of basal cell adenoma. On the other hand, canalicular adenomas lack the solid cellular trabeculae and the collagenized stroma of basal cell adenomas. The distinction between canalicular adenoma and basal cell adenoma is more academic than practical. Both tumors have a benign clinical behavior and are treated in the same manner. Some prefer to group these tumors under the umbrella of “monomorphic adenoma”. As the term encompasses all benign salivary gland tumors (e.g. Warthins tumor, oncocytoma) other than pleomorphic adenomas, the designation is too broad to be of much use regarding precise tumor classification. A canalicular adenoma involving the maxillary sinus is unusual, and at this site may cause some confusion with ameloblastomas – particularly a plexiform type of ameloblastoma. However on close inspection, the columns of tumor cells lack a central stellate reticulum and do not demonstrate subnuclear vacuoles.