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Definition    Benign tumour of salivary gland origin composed exclusively of large epithelial cells with characteristic bright eosinophilic granular cytoplasm (oncocytic cells).  

 ICD-O code 8290/0  

 Synonym    Oncocytic adenoma, oxyphilic adenoma  

 Epidemiology    Oncocytoma accounts for about 1% of all salivary gland neoplasms and occurs most commonly in the 6-8th decades{257}. The mean age of the patients is 58 years. There is no sex predilection.  

 Etiology    Approximately 20% of all the patients will have a history of radiation therapy to the face or upper torso or long-term occupational radiation exposure five or more years prior to tumour discovery {257}. Patients with previous radiation exposure are on the average 20 years younger at tumour discovery than those without a documented history of irradiation.                 

Histopathology  

 Histologically, the oncocytic cells are arranged in a solid or trabecular pattern. Microcyst formation can rarely be observed. The oncocytes display ample granular acidophilic cytoplasm. Typically the predominant cells have abundant oncocytic cytoplasm and an oval, vesicular nucleus (light cells). In addition,  there are cells with very brightly eosinophilic cytoplasm and pyknotic nuclei (dark cells). The cells are arranged in uniform sheets and they may aggregate into clusters, and sometimes they form duct-like structures. Rarely,  oncocytomas present with large polyhedral clear cells in an organoid distribution.  A thin fibrovascular stroma is also present. An intimate mixture of typical eosinophilic and clear cell oncocytes may be encountered within the same tumour. Tumours with a predominantly    clear cell component are referred to as    clear cell oncocytoma {665}. The optically    clear cell appearance is due to fixation    artefact and/or intracytoplasmic glycogen    deposition {551,2291}.    The tumour cells typically stain with        phosphotungstic acid haematoxylin    (PTAH). Electron microscopy shows elongated    cristae and a partial lamellar internal    structure {1227}. The nuclei of the    oncocytes are irregular and contain    inclusions and glycogen granules.

   Differential diagnosis

   The most important differential diagnosis    of oncocytoma includes acinic cell carcinoma    and clear cell carcinoma.    Mucoepidermoid carcinoma with prominent    clear cell alteration and metastatic    renal cell carcinoma may also be practical    considerations. Also, stroma-poor    Warthin tumour, oncocytic carcinoma,    and metastatic thyroid carcinoma should    be included.    The clear-cut separation of an oncocytic    adenomatous (nodular) hyperplasia of    the parotid gland from a multinodular    oncocytoma (a true neoplasm) is not    always possible since the two entities    overlap histologically {223,882,2427}.