Oncocytic mucoepidermoid carcinoma: clinicopathologic description in a series of 12 cases.

Mucoepidermoid carcinoma (MEC) is the most common salivary gland malignancy. Oncocytic MEC (OMEC) has been rarely reported with previous cases suggesting they are largely cystic low-grade neoplasms with a favorable prognosis. The differential diagnosis of OMEC includes numerous oncocytic/"oncocytoid" neoplasms. Some are benign while others are aggressive. Recent evidence suggests that p63 is a reliable marker in the diagnosis of conventional MEC but has not been explored in OMEC. We searched the archives of various institutions for examples of OMEC to re-appraise the grade, and to evaluate p63 immunohistochemistry as a tool to separate OMEC from its potential mimics. A total of 12 cases were identified. There were 6 males and 6 females with an age range of 30 to 79 years. Most occurred in the parotid (9) with 1 each in the sublingual gland, hard palate and neck. They showed minimal cystic content and were infiltrative and solid tumors spanning all grades. All tumors had focal mucin production and were composed almost exclusively of oncocytic cells with 2 cases demonstrating conventional MEC areas. All 6 cases tested showed the majority of oncocytic cells staining with p63 in a diffuse pattern, demonstrating its utility in the diagnosis of OMEC. Of the 6 cases with follow-up information, 1 case had local recurrence 8 years after the initial surgery. Three cases showed skin or bone invasion. None had lymph node/distant metastases. In summary, OMEC behaves as a low-grade tumor, and is diffusely positive for p63, which may aid in its differential diagnosis.