Answer: Polymorphous low grade adenocarcinoma

Histology: The tumor is composed of bland ductal cells. The tumor cells have uniform, oval nuclei featuring fine chromatin. Mitoses are rare. The tumor has a varied architecture, ranging from tubules to cribriform structures and cords. The tumor infiltrates adjacent normal salivary glands and fat. An S100 immunostain is diffusely positive.

Discussion: The name polymorphous low grade adenocarcinoma (PLGA) is a very accurate description. PLGA is a low grade malignancy that arises almost exclusively from the minor salivary glands of the oral cavity (palate is the classic location). Also as its name indicates, PLGA is polymorphous in the sense that it can be very diverse architecturally, growing as cords, nests, ducts, tubules, sheets of spindled cells, and cribriform structures.

PLGA is most likely to be confused with pleomorphic adenoma and adenoid cystic carcinoma. Like pleomorphic adenoma, PLGA can have myxoid stromal material, but it does not have a well developed mesenchymal component. Most important, unlike the benign pleomorphic adenoma, PLGA is very infiltrative at its border, commonly invading adjacent seromucinous acini and frequently exhibiting perineural invasion. (In a small biopsy, however, tumor invasion can be difficult or impossible to assess.) Adenoid cystic carcinoma is also very infiltrative, but is characteristically composed of hyperchromatic, angulated tumor cells. Finally, both pleomorphic adenoma and adenoid cystic carcinoma are biphasic tumors composed of ductal cells and myoepithelial cells. PLGA, on the other hand, has only one cell type. S-100 is very helpful in illustrating this: in adenoid cystic carcinoma and pleomorphic adenoma, S100 staining will be patchy, only staining myoepithelial tumor cells, while in PLGA virtually every cell will be positive.