Epithelioid Myoepithelioma of the Accessory Parotid Gland: Pathological and Magnetic Resonance Imaging Findings

Tumors of the accessory parotid gland (APG) are rare, and pleomorphic adenoma (PA) is the most common benign APG tumor subtype. Myoepithelioma of the APG is much rarer than PA, and to date, only 5 cases have been sporadically reported in the English literature. We describe the clinicopathological and MRI findings of an epithelioid myoepithelioma of the APG that was treated in our hospital. The patient's only clinical symptom was a slow-growing and painless mid-cheek mass. The tumor was suspected to be PA before surgery based on the following MRI findings: (1) a well-circumscribed and lobulated contour, (2) isointensity and hyperintensity relative to the muscle on T1- and T2-weighted images (WIs), respectively, (3) good enhancement on contrast-enhanced T1-WIs, (4) peripheral hypointensity on T2-WIs, and (5) a gradual time-signal intensity curve enhancement pattern on gadolinium-enhanced dynamic MRI. The tumor was completely resected via a standard parotidectomy approach, and the postoperative pathological examination of the tumor, including immunohistochemistry, confirmed the diagnosis of epithelioid myoepithelioma. As it is hardly possible to distinguish myoepithelioma from PA and low-grade malignant tumors preoperatively, a pathological examination using frozen sections is helpful for surgical strategy-related decisions.