(A518) Challenges in Differentiating Parotid Oncocytoma from Mucoepidermoid Carcinoma - A Case Report

Introduction: Mucoepidermoid carcinoma (MEC) is the most common salivary gland malignancy but the oncocytic variant of MEC is rare and may be misdiagnosed as benign lesions, including oncocytomas and Warthin’s tumors. In this case report, we present a case of a parotid tumor initially resected and diagnosed as oncocytoma recurring 5 years later as oncocytic mucoepidermoid carcinoma.

Methods: A 71-year-old male was referred to otolaryngology clinic by his primary care physician for a new-onset right jaw lesion. Physical examination as well as CT and MRI imaging showed a well-circumscribed 1.5cm right parotid gland lesion. Ultrasound-guided fine needle aspiration demonstrated a cystic salivary gland lesion with marked oncocytic change.

Results: After a period of observation, the patient opted for surgical resection and underwent right superficial parotidectomy. Pathologic diagnosis was challenging and the specimen was sent to a local tertiary care center for confirmation; pathologists at both sites independently determined the most likely diagnosis to be benign oncocytoma. Five years later, the patient’s tumor recurred. Revision right parotidectomy was performed with removal of the mass as well as a cuff of adherent masseter muscle. Final pathology showed oncocytic MEC and molecular testing was positive for MAML2 rearrangment.

Conclusions: The oncocytic variant of MEC is rare and can be difficult to differentiate histologically from other salivary gland lesions with oncocytic change. Intraoperative findings such as adherence to facial nerve branches or underlying musculature should alert the surgeon to the possibility of a malignant process in such cases. Molecular testing for MAML2 translocations can also aid in supporting the diagnosis of MEC.