Answer: Mucoepidermoid carcinoma, high grade

Histology: The tumor forms a solid mass that replaces most of the parotid parenchyma. Cystic growth is not apparent. The tumor cells form cohesive cords and nests in a desmoplastic stroma. Their nuclei are large and irregular with prominent nucleoli. Mitotic activity is brisk. Although the tumor cells have an epidermoid appearance, keratin pearl formation and individual cell keratinization is only focally developed. Occasional cells have a foamy cytoplasm, but goblet-type mucous cells and gland formations are not identified. A mucicarmine stain demonstrates the presence of mucin vacuoles in some of the cells with foamy cytoplasm.

Discussion: Mucoepidermoid carcinomas (MEC) is by far the most common malignant salivary gland tumor, in both children and adults. Histologically, MEC may demonstate any combination of solid and cystic growth. Its cellular composition is often a mixture of mucous cells, intermediate cells, clear cells, and epidermoid cells. The proportion of each cell type varies from tumor to tumor.

Although there is a consensus that histopathologic grade is strong predictor of tumor behavior, the morphologic distinction between low-, intermediate- and high-grade tumors has been a contentious issue. Solid growth, perineural invasion, anaplasia, tumor necrosis, and high mitotic activity are some of the more reliable histopathologic features that, when present, predict an aggressive tumor behavior. Of note, grading is less useful in predicting the behavior of MEC when this tumor arises in the submandibular gland.

Recognizing MEC and distinguishing it from primary and metastatic squamous cell carcinoma can be problematic when its light microscopic appearance is dominated by anaplastic epidermoid cells. As in the presence case, the diagnosis of high grade mucoepidermoid carcinoma relies on the histochemical demonstration of mucin within tumor cells.