Oncocytic carcinoma is an extremely rare malignancy in salivary glands, accounting only for 11% of all oncocytic salivary gland neoplasms, 0.5% of all epithelial salivary gland malignancies and 0.18% of all epithelial salivary gland tumours . This neoplasm is characterized by epithelial cells with abundant eosinophilic and granular cytoplasm, filled with numerous mitochondria.
Malignant oncocytoma, malignant oxyphilic adenoma and oncocytic adenocarcinoma have been used synonymously for oncocytic carcinoma. The malignant nature of the neoplasm can be recognized by its morphologic features and infiltrative growth. Morphologic criteria for the diagnosis of a malignant nature are cellular pleomorphism, necrosis and frequent mitoses. Infiltrative growth of the neoplasm is represented by perineural, vascular or lymphatic invasion, destruction of adjacent structures and local lymph node metastasis.
Immunohistochemical study and ultrastructural examination are essential ancillary studies necessary for a correct diagnosis of oncocytic carcinoma to be made; these procedures show the presence of abundant mitochondria in cytoplasm.
In our case, the malignant nature of the neoplasm was evidenced by the presence of perineural invasion and by infiltration of subcutaneous tissue. No regional or distant lymph node metastases clinically or radiologically were observed.
Oncocytic differentiation of neoplastic cells was demonstrated by immunohistochemical positivity for mithochondrial antigen , keratin, alpha-1-antichymotrypsin . On ultrastructural analysis numerous mitochondria seemed to fill the cytoplasm. These organuli were not clear because of fixation of tissue for light microscopy, which is similar to the case reported by Mizutary et al . Other neoplasms that arise from the salivary gland with a granular cytoplasm are oncocytoma, acinic cell adenocarcinoma and salivary duct carcinoma .
Oncocytic carcinoma can be differentiated from benign oncocytoma by the presence of a connective tissue capsule in the latter. Moreover, compared to oncocytoma, oncocytic carcinoma usually shows a greater mitotic activity and more nuclear pleomorphism.
Acinic cell adenocarcinoma can be differentiated from oncocytic carcinoma since its cytoplasmic granules are amphophilic or basophilic. Moreover, the patterns of growth in acinic cell adenocarcinoma can be microcystic or papillary and the neoplastic elements are negative for mithochondrial antigen when examined immunohistochemically. Salivary duct carcinoma, in contrast to oncocytic carcinoma, forms duct-like spaces with papillary and cribriform growth and also shows comedonecrosis .
The non-neoplastic proliferation of a salivary gland, which can mimic oncocytic carcinoma is oncocytosis. This lesion is a condition that predominantly affects adults over the age of 60 years, and can be differentiated from malignant oncocytoma by the presence of variably sized foci of oncocytic cells within glandular lobules without altering the normal architecture of the gland .
Primary oncocytic carcinoma of the salivary glands should also be differentiated from metastatic oncocytic carcinomas to the salivary glands from a precise clinical history, revealing the previous primary neoplasm and by specific immunohistochemical studies.
Metastatic oncocytic carcinoma of the thyroid (Hürthle cell carcinoma) can be diagnosed because of the immunohistochemical expression of thyroglobulin .
The diagnosis of a rare metastatic oncocytic adenocarcinoma of the stomach to the salivary gland is facilitated by the presence of a tubular pattern of growth and by the presence of microvilli on the luminal surfaces of neoplastic cells, which are absent in salivary oncocytic carcinoma of the salivary gland .
Metastatic renal cell carcinoma, the granular type, must be considered in the differential diagnosis of primary oncocytic carcinoma in a salivary gland. This variant of renal carcinoma is characteristically composed of cells organized in sheets, cords or as papillary fronds . On immunohistochemical examination, oncocytic carcinoma in a salivary gland is negative for carcinoembryonal antigen (CEA) and S-100 protein  in contrast with renal carcinoma [11, 12] which is positive to either markers. In our case, the diagnosis of primary oncocytic carcinoma of parotid gland was made by immunohistochemical analysis revealing negativity for S100 and CEA according to other studies reported in the literature  and by a negative clinical history for a renal tumour.
The prognosis of oncocytic carcinoma in salivary gland is not well known, because of its rarity. Goode and Corio have reported that tumours smaller than 2 cm in diameter appeared to have a better prognosis than those that were larger .
In our case, the neoplasm was 2.5 cm in diameter and was not associated with local or distant metastases. A good prognosis is expected for our patient, because there was not involvement of the lymph nodes.