A 30-year-old woman presented with a 2 cm solid mass in the superficial lobe of her right parotid gland. A superficial parotidectomy was performed and yielded the specimen seen below.
What is your diagnosis?
The specimen consisted of multinodular nests of distinctly oncocytic cells in a fibroblastic stroma. The lesion was not encapsulated and oncocytosis was not noted in the surrounding parotid gland. Necrosis was not present and mitotic figures were extremely rare.
A careful search of the lesion yielded occasional goblet cells seen below.
An immunohistochemical stain for p16 was performed and the results are shown below.
This is an example of an oncocytic mucoepidermoid carcinoma. These tumors may be virtually entirely oncocytic, leading to obvious confusion with an oncocytoma. The latter diagnosis should be approached with considerable caution in younger patients as oncocytomas primarily occur in older adults. Salivary oncocytomas are often associated with scattered nests of oncocytic cells (oncocytosis) in the surrounding gland. These were absent in the current case. When present they may help support the diagnosis of a primary oncocytoma.
Immunoreactivity for p63 as in this case is also of value, as this is not present in oncocytomas but is a common feature of mucoepidermoid carcinomas.
Recent studies have demonstrated that low- and intermediate-grade mucoepidermoid carcinomas, including the oncocytic vaiants, harbor a characteristic t(11;19) translocation that results in a MECT1-MAML2 fusion gene. This translocation is present in approximately 70% of low-and intermediate-grade tumors, but is notably absent from high-grade mucoepidermoid carcinomas. A commercially available breakapart FISH probe is available and can be used for for this distinction as well.