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Renal oncocytoma and chromophobe renal cell carcinoma have been long recognized as distinct tumors; however, it remains unknown if uniform diagnostic criteria are used to distinguish these tumor types in practice. A survey was distributed to urologic pathologists regarding oncocytic tumors. Responses were received from 17 of 26 invitees. Histologically, more than 1 mitotic figure was regarded as most worrisome (n=10) or incompatible (n=6) with oncocytoma diagnosis. Interpretation of focal nuclear wrinkling, focal perinuclear clearing, and multinucleation depended on extent and did not necessarily exclude oncocytoma if minor. Staining techniques most commonly used included the following: cytokeratin 7 (94%), KIT (71%), vimentin (65%), colloidal iron (59%), CD10 (53%), and AMACR (41%). Rare cytokeratin 7-positive cells (≤5%) were regarded as most supportive of oncocytoma, although an extent excluding oncocytoma was not universal. Multiple chromosomal losses were most strongly supportive for chromophobe renal cell carcinoma diagnosis (65%). Less certainty was reported for chromosomal gain or a single loss. For tumors with mixed or inconclusive features, many participants use an intermediate diagnostic category (82%) that does not label the tumor as unequivocally benign or malignant, typically "oncocytic neoplasm" or "tumor" with comment. The term "hybrid tumor" was used variably in several scenarios. A slight majority (65%) report outright diagnosis of oncocytoma in needle biopsies. The morphologic, immunohistochemical, and genetic characteristics that define oncocytic renal tumors remain incompletely understood. Further studies correlating genetics, behavior, and histology are needed to define which tumors truly warrant classification as carcinomas for patient counseling and follow-up strategies.

Succinate dehydrogenase B (SDHB) is a mitochondrial enzyme complex subunit. Loss of SDHB protein expression has been found to correlate with SDHx gene mutations. Little is known about its expression in subtypes of renal cell carcinoma (RCC) and whether it is a prognostic indicator. Four hundred fifty renal epithelial neoplasms were analyzed for SDHB, comprising clear cell RCC (CCRCC) (n = 240), papillary RCC (n = 84), chromophobe RCC (n = 49), renal oncocytoma (n = 47), clear cell papillary RCC (CCPRCC) (n = 19), and von Hippel-Lindau (VHL)-associated CCPRCC-like tumors (n = 11). Succinate dehydrogenase B expression was graded based upon staining intensity using a 4-tiered system (0-3+), in which 3+ was strongest and complete absence was 0. Neoplasms were further categorized based upon staining extent into SDHB weak (1+-2+) and strong (3+). Succinate dehydrogenase B was strongly preserved in 131 (55%) of 240 CCRCCs, 84 (100%) of 84 papillary RCCs, 49 (100%) of 49 chromophobe RCCs, 1 (5%) of 19 CCPRCC, 5 (45%) of 11 VHL-associated CCPRCC-like tumors, and 47 (100%) of 47 renal oncocytomas. The remaining 109 CCRCCs, 18 CCPRCCs, and 6 VHL-associated CCPRCC-like tumors had weak but preserved SDHB. Succinate dehydrogenase B expression in CCRCCs with high International Society of Urological Pathology nucleolar grade (G3-G4) correlated significantly with survival (log-rank, P = .0004). Succinate dehydrogenase B is variably expressed in RCCs with clear cell morphology and strongly preserved in most other neoplasms. Therefore, weak staining, particularly in clear neoplasms, should not be misinterpreted as negative. Finally, SDHB expression in CCRCCs with high nucleolar grade (G3-G4) is significantly associated with survival, indicating it may be both a diagnostic and prognostic marker in RCC.

Renal oncocytomas are benign neoplasms that are often excised, as clinically they cannot be distinguished with certainty from renal cell carcinoma. One of the least common findings in oncocytomas is vascular invasion, and their behavior is not well characterized with only reports of isolated examples and smaller case series. Whether vascular invasion is acceptable for the diagnosis of oncocytoma still remains controversial, even amongst genitourinary pathologists with expertise in renal tumor pathology. Of 1474 cases of renal oncocytoma identified at 3 large medical centers, 22 (1.5%) had vascular invasion. Patients included 12 men and 10 women with an average age at diagnosis of 67.5 years (range, 48-91 years). Thirteen cases showed large vessel invasion, and the remainder involved medium or small vessels. Tumor was grossly visible in the renal vein in 2 cases. Clinical data were available on 16 of the 22 cases with an average follow-up time of 29.9 months (range, 7.5-94.5 months). Of the cases with clinical follow-up, all but one individual was alive. All living individuals were free of recurrence or metastatic disease at the time of last follow-up. Our cohort showed no metastasis or recurrence and overall survival of 94.7% at 2.5 years following diagnosis, supporting the finding that vascular invasion does not alter the favorable prognosis of oncocytoma. The presence of vascular invasion should not lead to any uncertainty about the diagnosis in an otherwise typical oncocytoma.

A cohort of a heretofore not described rare subtype of renal oncocytoma, small cell oncocytoma with pseudorosettes is presented. Patients were 6 women and 4 men with ages ranging from 51 to 76 years. The tumors displayed areas composed of small cells ("oncoblasts") featuring scant cytoplasm and small, round monomorphic nuclei. The small cell areas constituted 15% to 60% of the total tumor volume (mean, 28.5%; median, 22.5%). No necrosis or mitotic activity was discerned. All tumors also contained areas composed of characteristic oncocytes comprising 40% to 85% of the total tumor volume. In all cases, a varying number of pseudorosettes were identified. The pseudorosettes were composed of small globules of (periodic acid-Schiff-positive) hyaline basal membrane-like material surrounded by small "oncoblastic" cells. The immunohistochemical profile was variable, including at least focal positivity for AE1-3 (10/10), cytokeratin 7 (7/10), epithelial membrane antigen (10/10), c-kit (6/10), antimitochondrial antigen (MIA;10/10), PAX-2 (9/10), AMACR (racemase;6/10), CD10 (5/10), parvalbumin (8/10), vimentin (6/10), claudin 7 (10/10), and claudin 8 (3/10). No immunoreactivity for carbonic anhydrase 9, HMB-45, S-100A1, and TFE3 was documented. We found no differences in the immunophenotype in the small cell oncocytes/oncoblasts that formed pseudorosettes and those that did not. However, there were differences in the immunohistochemical profile of classic oncocytes and small cell oncocytes/oncoblasts. Using array comparative genomic hybridization, no chromosomal changes were identified in any of the cases examined (n = 3). No numerical changes of chromosomes 7 and 17 were revealed on fluorescence in situ hybridization analysis (n = 3). In conclusion, we herein present the first study on small cell renal oncocytomas with formation of pseudorosettes. This is a rare subtype of oncocytoma, which may, especially on a core biopsy, present differential diagnostic difficulties. The immunohistochemical profile of these tumors is variable and differs in significant respects from that of conventional renal oncocytoma. Awareness of this entity and its immunohistochemical variability should help in distinguishing this rare tumor from malignant tumors with similar (small cell) histomorphologic features. All tumors behaved in a benign fashion during follow-up (mean, 3.1 years; median, 1 year).

Summary Renal oncocytoma is a benign tumor occurring singly or as multiple synchronous lesions. The histologic features of renal oncocytoma may overlap with those of chromophobe renal cell carcinoma. Chromosomal translocations involving the CCND1 locus at 11q13 and overexpression of cyclin D1 occur in a subset of renal oncocytomas. We evaluated a series of 63 renal oncocytomas and 36 chromophobe renal cell carcinomas and assessed the clinical features, cyclin D1 overexpression by immunohistochemistry, and alterations of the CCND1 gene by fluorescence in situ hybridization. All 36 chromophobe renal cell carcinomas were negative for cyclin D1 overexpression and alterations of CCND1 . Of the 63 renal oncocytomas, 21 (33%) showed cyclin D1 overexpression. Of 21 renal oncocytomas with cyclin D1 overexpression, a CCND1 rearrangement was detected in 12 (57%). A CCND1 rearrangement was also identified in 1 (2%) of the 42 renal oncocytomas without cyclin D1 overexpression. Of 42 renal oncocytomas without cyclin D1 overexpression, 16 (38%) were from patients with multiple renal oncocytomas at nephrectomy. Of 21 renal oncocytomas with cyclin D1 overexpression, only 1 (5%) patient had multiple renal oncocytomas ( P = .006). Of the 25 patients whose original tumor showed no cyclin D1 overexpression, 8 (32%) developed a subsequent renal oncocytoma. None of 15 patients whose original tumor showed cyclin D1 overexpression had a subsequent renal oncocytoma ( P = .016). The findings of this study suggest that renal oncocytomas lacking cyclin D1 overexpression may be associated with the development of multiple renal oncocytomas and that these patients are more likely to develop subsequent renal oncocytomas suggesting the need for more frequent clinical for these patients and little need for follow-up in patients with renal oncocytomas overexpressing cyclin D1. The data also show that cyclin D1 overexpression and CCND1 rearrangements by fluorescence in situ hybridization are absent in chromophobe renal cell carcinoma, suggesting that these are useful when differentiating between renal oncocytoma and chromophobe renal cell carcinoma.

Renal oncocytoma can present with calcification seen on the plain roentgenogram. From a review of the literature (both before and after 1976, when the concept of oncocytoma was popularized in the English literature), 10 cases of calcification in association with renal oncocytoma were found. In three of these 10 cases, the calcifications could be grossly demonstrated on the plain roentgenogram; in the other seven cases, the calcifications were microscopic in nature. The presence of calcification in a renal tumor does not rule out the diagnosis of oncocytoma. Grossly, a renal oncocytoma has a brownish mahogany color. A case is presented in which simple excision of an oncocytoma resulted in a 33-year survival. In the future, excision instead of nephrectomy could become the standard treatment for these tumors.

Claudin-7 and claudin-8 code for tight junction proteins expressed in distal nephron epithelium. In a recent oligonucleotide microarray study, we identified claudin-7 and claudin-8 as candidate markers to distinguish chromophobe renal cell carcinoma from other renal tumors, including oncocytoma. Distinction of these lesions can be difficult by light microscopy but is clinically important because chromophobe renal cell carcinoma has malignant biological potential, whereas renal oncocytoma is benign. Claudin-7 and claudin-8 expression was studied by immunohistochemistry in 11 chromophobe renal cell carcinomas and 17 oncocytomas using formalin-fixed paraffin-embedded tissue sections of tumor with adjacent nonneoplastic kidney. Steam antigen retrieval was performed before immunohistochemistry. Specificity was verified by negative control reactions without primary antibody and appropriate membranous staining patterns in positive control tissues (colon carcinoma and adjacent nonneoplastic kidney). Claudin-7 protein was expressed in a membranous pattern in 10 of 11 chromophobe renal cell carcinomas and 4 of 17 oncocytomas (P < .01). Claudin-8 was expressed in multiple patterns: In oncocytoma, 11 of 17 cases showed cytoplasmic, 4 of 17 membranous, and 2 of 17 negative reactions. In chromophobe renal cell carcinoma, 0 of 11 cases showed cytoplasmic, 3 of 11 membranous, and 8 of 11 negative reactions (P < .01). The immunohistochemical pattern of membranous claudin-7 and negative claudin-8 was seen in 7 of 11 chromophobe renal cell carcinomas and 1 of 17 oncocytomas (63% sensitivity, 84% specificity, 88% positive predictive value for chromophobe renal cell carcinoma). Negative claudin-7 and cytoplasmic claudin-8 were observed in 10 of 17 oncocytomas and 0 of 11 chromophobe renal cell carcinomas (59% sensitivity, 100% specificity and positive predictive value for oncocytoma). The distal nephron proteins claudin-7 and claudin-8 have potential use as immunohistochemical biomarkers in the differential diagnosis of chromophobe renal cell carcinoma and oncocytoma. Expression of claudin-7 and claudin-8 may reflect the relationship of chromophobe renal cell carcinoma and oncocytoma to intercalated cells of the cortical collecting duct. It may be necessary to identify additional biomarkers to include with claudin-7 and claudin-8 in a larger immunohistochemical panel to improve diagnostic sensitivity and specificity.

Distinguishing renal oncocytoma from chromophobe and other renal carcinomas is essential, considering their differing biological potentials. Although renal oncocytoma is considered a benign tumor, chromophobe renal cell carcinoma has potentially malignant biological behavior. The numerous reported studies on distinguishing these 2 entities have been based on morphological, histochemical, immunohistochemical, ultrastructural, and cytogenetic features. But none of these features has proven to be reliably specific, especially in tumors with overlapping phenotypes. We report a novel immunohistochemical approach based on the expression of a recently described kidney-specific cadherin (Ksp-cadherin) for the differential diagnosis of these 2 tumors. We compared Ksp-cadherin expression in 212 renal tumors, including 102 clear cell renal carcinomas, 46 papillary renal cell carcinomas, 30 chromophobe carcinomas, 3 collecting duct carcinomas, and 31 oncocytomas. In addition, we examined the expression of epithelial membrane antigen, vimentin, CK7, and Hale's colloidal iron staining. We found that chromophobe renal cell carcinomas consistently (96.7% of cases) demonstrated a distinctive membrane pattern of Ksp-cadherin expression, whereas renal oncocytomas (3.2%), clear cell renal cell carcinomas (0%), papillary renal cell carcinomas (2.2%), and collecting duct carcinomas (0%) usually did not express Ksp-cadherin. CK7 expression was found in 90.0%, 6.5%, 7.8%, 76.1%, and 33.3% of these tumor cases, respectively. Whereas CK7 was detected in different types of renal cell carcinomas, Ksp-cadherin was expressed almost exclusively in chromophobe renal cell carcinomas. Immunohistochemical analysis of Ksp-cadherin offers a fast, reliable approach for the distinguishing between renal oncocytoma and chromophobe renal cell carcinoma that is applicable for routine pathology laboratory studies without the need for time-consuming and costly ancillary studies.

Several unsettled histogenetic, nosologic and diagnostic considerations for renal epithelial tumors may have ultrastructural ramifications. Yet, a comprehensive electron microscopic study of renal epithelial neoplasms, in light of the recent classification, is not available. The ultrastructural findings from fifty-five renal epithelial neoplasms [31 clear cell renal cell carcinomas (RCC), 11 papillary RCC, 5 chromophobe RCC, 3 sarcomatoid RCC and 5 oncocytomas] were correlated with their light microscopic appearance. Clear cell RCC showed long microvilli similar to the brush border of the normal proximal tubules, with abundant cytoplasmic lipid and glycogen. Papillary RCC showed variably sized microvilli, and small amounts of cytoplasmic lipid, but no glycogen. Chromophobe RCC showed many cytoplasmic vesicles and abnormal mitochondria, with rare short and stubby microvilli. Renal oncocytoma showed many mitochondria with a few vesicles in the apical portion of the cytoplasm and rare short and stubby microvilli. The eosinophilic cell variants of clear cell RCC, papillary RCC and chromophobe RCC showed ultrastructural features similar to those of their respective prototypes, except for an increased numbers of mitochondria in the cytoplasm. One sarcomatoid clear cell RCC showed skeletal muscle differentiation. Two types of cytoplasmic inclusions, i.e. hyaline globules and granules similar to those in the Paneth cells (PC-like granules) were identified only in clear cell RCC, which displayed distinctive ultrastructural features. The current EM study demonstrates distinctive ultrastructural features of renal epithelial neoplasms. The findings lend additional support to the current classification of the pertinent tumor types, facilitate the differential diagnoses, and provide insights into the possible histogenesis of renal epithelial neoplasms.

Oxyphil cells are characterized by cytoplasm packed with large numbers of mitochondria. Study of these unusual cells may provide information about the regulation of mitochondrial biogenesis. Although it has been suggested that this is a compensatory proliferation due to a mitochondrial dysfunction, no such dysfunction has been well documented. In this study we considered the possibility of dysfunction in the mitochondrial enzyme Fl/Fo-adenosine triphosphatase(ATPase) as a stimulating factor involved in the mitochondrial proliferation of oxyphil cells. Mitochondria isolated from frozen tissue of a renal oncocytoma showing structural integrity and purity by electron microscopy were studied. Submitochondrial particles formed by sonic disruption showed the presence of the Fl component of mitochondrial ATPase with electron microscopy which was functionally active. The oligomycinsensitive ATPase activity from the renal oncocytoma was 0.133 μmol/min.mg submitochondrial particle protein which was higher than the readings obtained from normal kidney tissue (0.091 μmol/min.mg SMP protein) obtained from hamsters. Normal human renal tissue obtained at autopsy contained only nonfunctional mitochondria and therefore could not be used as control tissue. Mitochondrial ATPase dysfunction does not appear to be the inciting factor in the proliferation of mitochondria seen in oxyphil cell metaplasia and future studies should consider other possibilities. Preliminary functional studies of this nature can be performed with properly prepared frozen surgical tissue.

Five renal oncocytomas were studied by light and electron microscopy. Five glutaraldehyde-fixed blocks from each tumor were examined. Scarce short, blunt microvilli, plasmalemmal interdigitations, basement membrane protuberances, and round or oval mitochondrial profiles were found, suggesting that these neoplasms arise from the epithelium of the distal tubule. Morphologic criteria for the diagnosis of renal oncocytoma, including ultrastructural demonstration of cytoplasmic filling by mitochondria, are presented.