oncocytes are observed in lesions of several organs such as the thyroid , kidney , pancreas , ovary , liver , and salivary gland . in the salivary glands , oncocytes are known to arise in warthin tumor , oncocytoma , oncocytic carcinoma , and oncocytosis . oncocytosis is a rare non - neoplastic lesion that is classified as diffuse oncocytosis and multifocal adenomatous oncocytic hyperplasia ( maoh ) ; it comprises approximately 0.1% of salivary gland lesions [ 3 , 4 ] . the correct cytological diagnosis of oncocytosis can be difficult because oncocytes are seen in a variety of other salivary gland lesions , and it is usually diagnosed by histological examination [ 5 , 6 , 7 , 8 ] . here she consulted an otolaryngology clinic because of discomfort at the left side of her neck . at the clinic , a mass was noted in the left side of her neck and she was referred to our hospital . a 1.5-cm mass with no lymph node swelling was identified using magnetic resonance imaging . from the above clinical findings , warthin tumor or a malignant parotid tumor was suspected . five slides were prepared for cytological evaluation , including 3 fixed smears for papanicolaou staining , 1 air - dried smear for may - grnwald giemsa ( mgg ) staining , and 1 smear examined by liquid - based cytology ( lbc ) based on the lbc technique . papanicolaou staining of direct smears revealed a loose sheet - like cluster formed of round to polygonal cells with granular cytoplasm against a hemorrhagic background . the cells had round to oval , centrally located nuclei with granular chromatin and without distinct nucleoli . in the lbc preparation , the cytoplasm showed microvacuoles in contrast to the direct smear ( fig . there were only a few lymphocytes , and basophils were not detected in any of the smears . the cut surface of the resected lesion revealed a milky - white lobular mass with an unclear border ( fig . microscopically , the lesion was formed of many variable - sized nodules that comprised oncocyte - like cells with small round nuclei and eosinophilic granular cytoplasm . a definite capsule was not seen around the nodules and the surrounding adipose tissue and acinus . the oncocyte - like cells were diffusely positive for cytokeratin antibodies and strongly positive for mitochondrial antibodies ( fig . basophils were not detected in the lesion , using immunohistological staining based on anti - cd117 ( c - kit ) antibodies . typical findings for low - grade cancer of the salivary glands were not observed . from the above findings it has been reported that maoh develops in women in their 60s and is localized unilaterally in the parotid glands . histologically , maoh is thought to arise from the ductal epithelium and the remnants of the original salivary gland . the lesion was observed unilaterally in the left parotid gland in a 71-year - old woman . the remnants of the existing salivary ducts were observed by p63 and ck34e12 staining , and a strong response to mitochondrial antibodies was found in the cytoplasm . from the above findings described the cytological findings of maoh in a case report : ( 1 ) a low n / c ratio ; ( 2 ) central round nuclei ; ( 3 ) anisonucleosis ; ( 4 ) prominent nucleoli ( in part of the cells ) , and ( 5 ) abundant eosinophilic cytoplasm . in the present case , however , we could not make a diagnosis of maoh cytologically because of the high n / c ratio and because the nucleoli were unclear . cytologically , the major lesions in which oncocytes are observed include warthin tumor , oncocytoma , and oncocytic carcinoma . typical cytological findings of warthin tumor reveal oncocytes and lymphocytes with an inflammatory or necrotic background . have reported that mast cells are present with oncocytes in fine - needle aspiration preparations . kobayashi et al .   have investigated the frequency of mast cells in cytological samples , as compared with immunocytochemical identification using human mast cell tryptase antibodies in warthin tumor , and have indicated that mast cells are frequent in the epithelial cell component . in contrast , there are no reports to suggest any association with mast cells of other lesions . we also searched for mast cells by cytological mgg staining and immunohistochemical staining using the cd117 ( c - kit ) antibody . . the identification of basophils seems to be a diagnostic checkpoint when differentiation from warthin tumor is necessary . however , the typical cytological findings such as a lymphocyte and/or basophil appearance were not observed in any of the cases , which may thus be mistakenly diagnosed as oncocytoma   or maoh . cytological features of oncocytoma comprise large , round to polygonal cells with abundant granular cytoplasm , centrally located nuclei , prominent nucleoli , binucleation , sheet - like clustering , and an often necrotic background . in oncocytic carcinoma , although the neoplastic cells are pleomorphic , with nuclear atypia and hyperchromatism , other findings overlap with oncocytoma [ 7 , 8 , 13 ] . thus , it can be difficult to differentiate between maoh , warthin tumor , and oncocytoma . in addition , a case of oncocytoma arising from maoh has been reported . however , the distinction between maoh and oncocytoma is possible by capsular interpretation . therefore , it should be noted that neoplastic identification can not be achieved from partial samples microscopically . furthermore , oncocytic metaplasia has recently been shown in other salivary gland lesions ( e.g. , pleomorphic adenoma , myoepithelioma , and mucoepithelial carcinoma ) , except for maoh , warthin tumor , and oncocytic tumor . therefore , it is necessary to carefully carry out the assessment of oncocytes . when oncocytes are observed , it is necessary to perform a surgical resection . the authors have no potential conflicts of interest with respect to the authorship and/or publication of this article .