ultrasonography showed an irregularly shaped hypoechoic mass in the cranial pole of the right kidney .
cytology revealed moderate cellularity smears composed of epithelial cell clusters , which consisted of an exclusive population of oncocytic cells seen in sheets and papillary clusters along with abundant single cells .
a moderate - to - abundant amount of densely stained granular cytoplasm with round nuclei and indistinct nucleoli was seen .
a final diagnosis of renal oncocytoma was made on the basis of histology , immunohistochemical staining profile ( positive for cytokeratin , and negative for chromogranin a , neuron - specific enolase and vimentin ) of neoplastic cells , together with the electronic microscopy results .
we believe that this is the first report of the cytological features of feline renal oncocytoma .
primary renal tumours are uncommon and make up about 1.52.5% of all feline neoplasms . in 1999 , henry et al reported 19 primary feline renal tumours that consisted of tubulopapillary or tubular carcinoma ( n = 13 ) , transitional cell carcinoma ( n = 3 ) , adenoma ( n = 1 ) , haemangiosarcoma ( n = 1 ) and nephroblastoma ( n = 1 ) .
feline renal leiomyosarcoma , squamous cell carcinoma and sarcomatoid renal cell carcinoma have also been sporadically reported .
oncocytes are large cylindrical or polyhedral cells characterised by abundant eosinophilic , granular cytoplasm , which is caused by the presence of large numbers of mitochondria .
so far , feline oncocytomas have only been reported in the nasal cavity , salivary gland and nasopharyngeal lesion .
however , renal oncocytomas were first reported in humans in 1942 and in dogs in 2000 .
human renal oncocytomas are relatively common : 37% of all renal tumours vs two cases of canine renal oncocytomas .
a 9-year - old male neutered domestic shorthair cat was brought to the irion animal hospital in korea appearing anorexic .
the cat was formerly diagnosed with chronic renal failure ( iris stage 3 ) and had been treated with isotonic polyionic replacement fluid for several years .
biochemical analysis of the blood with vettest8008 ( idexx laboratories ) revealed highly elevated concentrations of blood urea nitrogen ( > 130 mg / dl ; reference interval [ ri ] 1636 mg / dl ) , creatinine ( 7.5 mg / dl ; ri 0.82.4 mg / dl ) and phosphate ( 8.8 mg / dl ; ri 3.17.5 mg / dl ) .
urine specific gravity was 1.010 and urine protein creatinine ratio ( urc ) was 0.1 ( vettest8008 ; idexx laboratories ) .
ultrasonography revealed an irregularly shaped mass that was hypoechoic to renal cortex and medulla at the cranial pole of the right kidney and a minimally hyperplastic lymph node in the cranial abdomen region .
ultrasound - guided fine - needle aspiration ( fna ) of the renal mass was performed using a 23 g needle attached to a 10 ml syringe .
blood mixed particulate material was obtained and air dried , and 95% ethanol - fixed smears were then made and stained with diff - quik .
the cranial abdominal lymph node was insufficiently large and surrounded with vasculatures for the performance of fna .
cytological examination revealed moderate cellularity smears composed of epithelial cell clusters consisting of an exclusive population of oncocytic cells seen in sheets and papillary clusters , along with abundant single cells .
a moderate - to - abundant amount of densely stained granular cytoplasm with round nuclei and indistinct nucleoli was seen .
the differential diagnoses were oncocytoma , chromophobe carcinoma , clear cell renal cell carcinoma and neuroblastoma - associated renal cell carcinoma . under general anaesthesia ,
( a ) cytological examination revealed moderate cellularity smears composed of epithelial cell clusters consisting of an exclusive population of oncocytic cells seen in sheets and papillary clusters , along with abundant single cells ( magnification  40 ) .
( b ) moderate - to - abundant densely stained granular cytoplasm with round nuclei and indistinct nucleoli ( magnification  100 ) in gross appearance , the tumours were mahogany or dark reddish brown , well circumscribed and contained a central scar ( approximate size 2.83 cm  2.2 cm  3.5 cm ) ( figure 2 ) .
the tumours were mahogany / dark reddish brown , well circumscribed and contained a central scar ( approximate size 2.83 cm  2.2 cm  3.5 cm ) .
a rim of compressed normal renal parenchyma was found the mass was sent to the comparative pathology department of asan medical center ( korea ) for histopathological examination .
the tissues were fixed with 10% buffered formalin and embedded in paraffin ; 5 m sections were stained with haematoxylin and eosin .
histologically , the tissue was composed of solid nests , anastomosing cords and closely packed glands separated by a delicate fibrovascular stroma .
individual neoplastic cells were cylindrical to polyhedral in shape , had distinct cell borders and contained moderate amounts of finely granular eosinophilic cytoplasm and round - to - oval nuclei ( figure 3 )
commercially available antibodies against chromogranin ( clone dak - a3 , 1:1600 ; dako ) , cytokeratin ( clone ae1/ae3 , 1:400 ; novo ) , neuron - specific enolase ( clone bbs / nc / vi - h14 , 1:400 ; dako ) and vimentin ( clone v9 , 1:500 ; zymed ) were used , with various secondary polyclonal antibodies .
immunohistochemical staining was performed using a benchmark xt autoimmunostainer with optiview dab detection kit ( ventana medical systems ) according to the manufacturer s instructions and using the reagent supplied with the kit .
the neoplastic cells were negative for chromogranin a , neuron - specific enolase and vimentin , indicating that the origin was not mesenchymal or neuronal / neuroendocrine ( figure 3b ) .
( a ) the tissue was composed of solid nests , anastomosing cords and closely packed glands separated by a delicate fibrovascular stroma ( haematoxylin and eosin ,  10 ) .
( b ) individual neoplastic cells were cylindrical to polyhedral in shape , had distinct cell borders and contained moderate amounts of finely granular eosinophilic cytoplasm and round - to - oval nuclei ( haematoxylin and eosin ,  40 ) .
( f ) vimentin negative ( magnification  40 ) ancillary testing using an electron microscope ( jem-1400 ; jeol ) was performed on slices prepared from fixed formalin mass .
ultrastructurally , neoplastic cells were interconnected via junctional complexes and contained numerous round - to - oval , electron - dense mitochondria ( figure 4 ) .
on the basis of the immunohistochemical staining profile of the neoplastic cells , together with electron microscopy results , a diagnosis of renal oncocytoma was made .
the neoplastic cells were interconnected via junctional complexes and contained numerous round - to - oval , electron dense mitochondria (  2500 electron microscope )
cytologically , smears from well - sampled human renal oncocytoma reveal numerous isolated cells or loose clusters with abundant , eosinophilic , granular cytoplasm , well - demarcated cell borders , and round nuclei with small or medium nucleoli . in the current case ,
the smear consisted of dyshesive cells and eosinophilic - to - basophilic granular cytoplasm , and most features were equivalent to human renal oncocytoma .
however , some cells were also aggregated in tubular or papillary forms along with eosinophilic intracellular materials .
benign renal neoplasms according to the 2004 world health organization classification of renal tumours in human medicine , mai et al recently reported two cases of human oncocytic renal cell carcinoma .
those tumours not only revealed cytological atypia and lymph node metastasis , but also showed similar immunological features to renal oncocytoma .
buergelt and adjiri - awere reported a canine bilateral renal oncocytoma that invaded the lumbar muscle . with this in mind ,
renal oncocytomas should be considered as having very low rather than no malignant potential , although the typical classification is benign . in the present case
a slightly enlarged lymph node in the cranial abdomen could not be evaluated by ct scan , but the cranial abdominal lymph node is being evaluated ultrasonographically every 12 months . to date , the cat only manifests clincal signs associated with chronic renal failure .
although oncocytoma has characteristic features and fna is helpful for preoperative diagnosis , jing and christina reported a great degree of overlap in the cytological spectrum between renal oncocytoma and renal cell carcinomas with eosinophilic granular cytoplasm .
this may impede accurate diagnosis in routine examination of fna specimens , even although each tumour has classic cytological features .
in human medicine , clinical diagnostic dilemmas and the difficulty of cytological or histopathological differentiation of renal oncocytoma from chromophobe renal cell carcinoma still persist . in feline medicine , as renal cell carcinoma with eosinophilic granular cytoplasm has not been reported , a comparison was not possible .
however , helpful information will be provided by ancillary immunohistochemical staining with electron microscopy examination in difficult cases , as in human medicine .
cytological features of feline renal oncocytoma were oncocytic cells in sheets or clusters with abundant single cells .
however , some cells were also aggregated in tubular or papillary forms along with eosinophilic intracellular materials . a moderate to abundant amount of densely stained granular cytoplasm with round nuclei and indistinct nucleoli was also noted .
most cytological features of feline renal neoplasia are equivalent to human renal oncocytoma . to the best of our knowledge ,
further analyses of feline renal oncocytoma and feline renal cell carcinoma with eosinophilic granular cytoplasm are needed .
 case summarya 9-year - old male neutered domestic shorthair cat presented with anorexia . 
 ultrasonography showed an irregularly shaped hypoechoic mass in the cranial pole of the right kidney . 
 ultrasound - guided fine - needle aspiration of the renal mass was performed . 
 cytology revealed moderate cellularity smears composed of epithelial cell clusters , which consisted of an exclusive population of oncocytic cells seen in sheets and papillary clusters along with abundant single cells . 
 a moderate - to - abundant amount of densely stained granular cytoplasm with round nuclei and indistinct nucleoli was seen . 
 the cytological diagnosis was renal oncocytic neoplasm . 
 ct and surgical resection revealed a firm tan mass in the right kidney . 
 a final diagnosis of renal oncocytoma was made on the basis of histology , immunohistochemical staining profile ( positive for cytokeratin , and negative for chromogranin a , neuron - specific enolase and vimentin ) of neoplastic cells , together with the electronic microscopy results.relevance and novel informationwe believe that this is the first report of the cytological features of feline renal oncocytoma . 
Case summary
Relevance and novel information
Introduction
Case description
Discussion
Conclusions