Answer: Mucoepidermoid carcinoma, low grade

Histology: The tumor demonstrates a combination of solid and cystic growth. Its cellular composition includes a mixture of mucous cells, intermediate cells and clear cells. There is no evidence of cellular anaplasia, high mitotic activity, tumor necrosis or neural invasion.

Discussion: Mucoepidermoid carcinoma (MEC) is by far the most common malignant salivary gland tumor, both in children and in adults. Most patients with MECs are entirely asymptomatic. They tend to present as slow growing, painless, cystic masses.

As is demonstrated in the present case, MECs may demonstrate any combination of solid and cystic growth. Their 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 an overwhelming consensus that tumor grade is a strong predictor of clinical behavior, the morphologic distinction between low and high grade tumors has been a contentious issue. In some studies, the histologic features which were most useful in predicting an aggressive behavior were:
1) a minor cystic component (less than 20%),
2) tumor necrosis,
3) neural invasion,
4) cellular anaplasia, and
5) brisk mitotic activity.

Based on the presence or absence of these features, tumors could be categorized as low, intermediate, or high grade. Notably, grading is not very reliable in predicting the behavior of submandibular tumors.

Recognition of MEC can be difficult when the histologic picture is dominated by any one cell type. Those with a large number of clear cells must be distinguished from other clear cell salivary gland tumors including acinic cell carcinoma, epimyoepithelial carcinoma, clear cell adenocarcinoma and others. In these instances, the use of mucin stains to discern the presence of mucinous cells can be helpful in establishing the diagnosis of MEC.