Objective: To investigate the clinicopathological characteristics and diagnostic-therapeutic strategies of oncocytic mucoepidermoid carcinoma (OMEC) of the parotid gland, and to enhance awareness of this rare variant among clinicians and pathologists. Methods: The clinical data, imaging findings, histopathological features, immunophenotype, and molecular characteristics of two patients with parotid OMEC were retrospectively analyzed, and the relevant literature was reviewed. Results: Case 1 was a 50-year-old man who presented with a painless mass behind the right earlobe for more than 2 years. The patient underwent extended parotidectomy with preservation of the facial nerve. Histopathological examination revealed that the tumor was predominantly composed of oncocytic cells with a small proportion of mucous cells. Immunohistochemically, the tumor cells were partially positive for cytokeratin 5/6, cytokeratin 7, and P63. Special staining with alcian blue, periodic acid-Schiff, and phosphotungstic acid hematoxylin yielded positive results. The diagnosis of right parotid OMEC was established. No recurrence or metastasis was observed during a 1 year follow-up. Case 2 was a 61-year-old man with a 3-month history of a mass beneath the left ear. After partial parotidectomy at an outside institution, pathological consultation at the Stomatological Hospital of Jilin University demonstrated that the tumor consisted almost entirely of oncocytic cells, exhibited infiltrative growth, and lacked typical mucous, epidermoid, and intermediate cells. Fluorescence in situ hybridization confirmed positive mastermind-like transcriptional activator 2 (MAML2) gene rearrangement, establishing the diagnosis of left parotid OMEC. The patient subsequently underwent total parotidectomy with preservation of the facial nerve, and no recurrence was detected during a short-term 3 months follow-up. A review of the literature indicated that OMEC most commonly arises in the parotid gland and is generally a low-grade malignancy with favorable prognosis. When tumors are composed exclusively of oncocytic cells, exhibit minimal cytological atypia, and lack the classical cellular components of mucoepidermoid carcinoma, they are highly prone to misdiagnosis as oncocytoma, nodular oncocytic hyperplasia, or other benign oncocytic lesions. Accurate differential diagnosis relies on recognition of infiltrative growth patterns, supportive immunophenotypic markers (e.g., P63 positivity), and detection of characteristic MAML2 gene rearrangement. Complete surgical excision remains the treatment of choice. Conclusion OMEC dominated by oncocytic cells carries a high risk of clinical misdiagnosis. Integrating the assessment Conclusion OMEC dominated by oncocytic cells carries a high risk of clinical misdiagnosis. Integrating the assessment of infiltrative histopathological features with immunohistochemistry and molecular detection of MAML2 rearrangement is crucial for accurate diagnosis, appropriate assessment of tumor behavior, and optimal surgical decision making.

This paper systematically elaborates on the key points of diagnosis and differential diagnosis of salivary gland tumors characterized by a substantial amount of extracellular mucus as a main or prominent feature, and clarifies the core differential features. The term "mucus-rich" specifically denotes that mucus is a major component of the tumor, rather than a focal or minor one. This phenomenon is associated with distinct histogenetic mechanisms: it may result from specific genetic mutations (e.g., AKT1 E17K in mucinous adenocarcinoma) that drive ductal epithelial differentiation into mucus-secreting cells, or from myoepithelial cells secreting glycosaminoglycans that form a myxoid stroma. Salivary gland tumors with abundant extracellular mucus include mucinous cystadenoma, sialadenoma papilliferum-like intraductal papillary tumors, mucinous myoepithelioma, pleomorphic adenoma with mucin-rich stroma, mucinous adenocarcinoma, low-grade mucoepidermoid carcinoma, mucin-rich salivary duct carcinoma and intestinal-type adenocarcinoma. The diagnosis of these tumors is complicated by the dual nature of extracellular mucus: while it is a defining feature of some entities, it can also obscure key diagnostic architectural features in others, leading to histological overlap and inconspicuous diagnostic areas. Given the frequent histological morphological overlap among these tumors, immunohistochemical findings and molecular characteristics have emerged as crucial differential diagnostic criteria. Core differential diagnostic points include the following: histologically, there must be meticulous identification of typical structures obscured by mucin (such as squamoid cells in mucoepidermoid carcinoma and apocrine features in salivary duct carcinoma); in immunohistochemical staining, CK20 is useful for distinguishing intestinal-type adenocarcinoma (positive) from mucinous adenocarcinoma (negative), while androgen receptor aids in differentiating salivary duct carcinoma (positive) from mucoepidermoid carcinoma (negative); and molecular testing plays a critical role in definitive diagnosis (e.g., the AKT1 E17K mutation for mucinous adenocarcinoma, MAML2 rearrangement for mucoepidermoid carcinoma, and MEF2C::SS18 fusion for microsecretory adenocarcinoma). This paper systematically summarizes the core pathological features and differential diagnostic points of mucin-rich salivary gland tumors, aiming to provide a practical reference for clinical pathological diagnosis.