The International Society of Urological Pathology (ISUP) Vancouver Classification of Renal Neoplasia

Abstract

The classification working group of the International Society of Urological Pathology consensus conference on renal neoplasia was in charge of making recommendations regarding additions and changes to the current World Health Organization Classification of Renal Tumors (2004). Members of the group performed an exhaustive literature review, assessed the results of the preconference survey and participated in the consensus conference discussion and polling activities. On the basis of the above inputs, there was consensus that 5 entities should be recognized as new distinct epithelial tumors within the classification system: tubulocystic renal cell carcinoma (RCC), acquired cystic disease–associated RCC, clear cell (tubulo) papillary RCC, the MiT family translocation RCCs (in particular t(6;11) RCC), and hereditary leiomyomatosis RCC syndrome–associated RCC. In addition, there are 3 rare carcinomas that were considered as emerging or provisional new entities: thyroid-like follicular RCC; succinate dehydrogenase B deficiency–associated RCC; and ALK translocation RCC. Further reports of these entities are required to better understand the nature and behavior of these highly unusual tumors. There were a number of new concepts and suggested modifications to the existing World Health Organization 2004 categories. Within the clear cell RCC group, it was agreed upon that multicystic clear cell RCC is best considered as a neoplasm of low malignant potential. There was agreement that subtyping of papillary RCC is of value and that the oncocytic variant of papillary RCC should not be considered as a distinct entity. The hybrid oncocytic chromophobe tumor, which is an indolent tumor that occurs in 3 settings, namely Birt-Hogg-Dubé Syndrome, renal oncocytosis, and as a sporadic neoplasm, was placed, for the time being, within the chromophobe RCC category. Recent advances related to collecting duct carcinoma, renal medullary carcinoma, and mucinous spindle cell and tubular RCC were elucidated. Outside of the epithelial category, advances in our understanding of angiomyolipoma, including the epithelioid and epithelial cystic variants, were considered. In addition, the apparent relationship between cystic nephroma and mixed epithelial and stromal tumor was discussed, with the consensus that these tumors form a spectrum of neoplasia. Finally, it was thought that the synovial sarcoma should be removed from the mixed epithelial and mesenchymal category and placed within the sarcoma group. The new classification is to be referred to as the International Society of Urological Pathology Vancouver Classification of Renal Neoplasia.

The classification of renal epithelial neoplasia has undergone significant change over the last 3 decades. Advances in our understanding of basic morphology, immunohistochemistry (IHC), cytogenetics, and molecular pathology have led to an expansion in the number of distinct tumor entities that we currently recognize.^1–3 Two important international consensus conferences (Heidelberg 1996, Rochester 1997) provided the basis for much of the last World Health Organization (WHO) renal tumor classification, which appeared in 2004.^2–4 Over the last decade several new tumor entities have emerged, and there have been refinements in many existing WHO categories.⁵ The classification working group of the International Society of Urological Pathology (ISUP) consensus conference on renal neoplasia was entrusted with the responsibility of reviewing available literature and making recommendations regarding additions, changes, and refinements to the current renal tumor classification system. The background and logistical considerations related to this consensus conference are dealt with in an introductory report.⁶ The literature review, results of a preconference survey, and deliberations at the consensus conference yielded recommendations relating to proposed new epithelial neoplasms, emerging/provisional new entities, and new concepts and clarifications regarding the existing WHO renal tumor entities. The proposed new epithelial neoplasms and emerging/provisional new entities are shown in Table 1. Emerging/provisional new entities, while appearing distinctive, are so rarely reported that the working group reserved judgment regarding their place in the classification system pending publication of additional reports and/or further refinements in diagnostic criteria.

TABLE 1: Proposed New Renal Epithelial Tumors and Emerging/Provisional Tumor Entities

The ISUP-recommended modifications of the WHO 2004 renal tumor classification are displayed in Table 2.

TABLE 2: ISUP Vancouver Modification of WHO (2004) Histologic Classification of Kidney Tumors

PROPOSED NEW EPITHELIAL NEOPLASMS

Tubulocystic Renal Cell Carcinoma

The concept of tubulocystic renal cell carcinoma (TC-RCC) evolved from the literature on putative low-grade collecting duct carcinoma (CDC).^7,8 A similar-appearing tumor was depicted in an early textbook by Pierre Masson,⁹ who used the term “Bellinian epithelioma.” In recent years, 3 large series of TC-RCC have been published and provide the basis for recognizing this distinctive tumor.^10–12 In total, <70 cases have been reported; patients have a mean age of about 60 years (range, 18 to 94 y) with a strong male predominance (≥7:1).^10–13 Patients are usually asymptomatic, and on imaging TC-RCC is usually seen as a complex cyst, often type 3 or 4 in the Bosniak classification system. These tumors are generally low stage and amenable to either partial or radical nephrectomy.

Tubulocystic carcinomas typically have a cortical or cortical-medullary epicenter and are defined by characteristic macroscopic and microscopic appearances. These tumors are well circumscribed, usually encapsulated, and have a white or gray “bubble wrap” cut surface. Histologically, they are composed of well-formed small-sized to medium-sized tubules and cystically dilated larger tubules in varying proportions (Fig. 1). The luminal spaces are lined by atypical cells with abundant eosinophilic cytoplasm and often have, at least focally, a hobnail configuration. The nuclei are enlarged and have prominent nucleoli (nucleolar grade 3). Mitoses are inconspicuous. The intervening stroma is generally hypocellular and fibrotic. Occasional chronic inflammatory cells may be present.

FIGURE 1: TC-RCC. Note variably sized simple tubules, some of which are cystically dilated. The tubules are lined by cuboidal cells showing nuclear enlargement and prominent nucleoli.

By IHC, the tumor cells stain positively for cytokeratins CK8, CK18, and CK19 and less frequently for CK7.^10–12 A small percentage of cases stain positively for high–molecular weight keratin (34βE12). Nearly all cases show positivity for CD10 and racemase (AMACR). Less than half of the cases will show positivity for PAX2 and carbonic anhydrase IX (CA-IX).

Ultrastructural studies show glandular epithelial cells with short microvilli having a brush border configuration and complex cytoplasmic interdigitation. These features along with IHC results, show mixed features of proximal convoluted tubules and distal tubules.¹²

There have been limited molecular studies of TC-RCC because of its rarity. A gene expression study of 1 case showed molecular clustering with papillary renal cell carcinoma (PRCC).¹¹ In another study no molecular overlap with clear cell or chromophobe renal cell carcinoma (RCC) was demonstrated.¹³ Using oligonucleotide microanalysis, a unique signature has been identified for TC-RCC, and gains in chromosomes 7 and 17 and loss of the Y chromosome using fluorescence in situ hybridization (FISH) analysis were commonplace, suggesting a relationship between TC-RCC and PRCC.^12,13

The morphology of TC-RCC may overlap to some extent with PRCC and CDC.^11,13–15 Some type 2 PRCCs have areas that are virtually indistinguishable from TC-RCC suggesting a possible relationship. Furthermore, separate TC-RCC and PRCC tumors may be found in the same kidney.¹³ These observations suggest a relationship between these 2 tumors.

Less commonly, CDC may show areas resembling TC-RCC, a finding which is not surprising given the fact that TC-RCC evolved out of the concept of low-grade CDC.¹⁶ Conversely, rare cases of TC-RCC can have areas that overlap with CDC.¹⁶ However, CDC typically shows distinct histologic features including a medullary/hilar epicenter, an infiltrative growth pattern, and a desmoplastic stromal reaction. Both CDC and TC-RCC show high-grade nucleolar features. Gene expression profiling studies show that TC-RCC and CDC are distinctive entities at a molecular level.¹⁷

The term TC-RCC should be restricted to those tumors that display typical macroscopic and microscopic appearance as described above. The term should not be used in situations in which there is a tubulocystic pattern admixed with the usual elements of PRCC or CDC. Some would allow focal areas similar to PRCC in an otherwise typical TC-RCC.¹⁶

The great majority of reported TC-RCCs (>90%) have behaved in an indolent manner. In one report, the role of extended active surveillance has been suggested.¹⁸ The resected tumors are generally low stage (pT1, pT2) with <10% showing pT3 features. A local recurrence has been documented,¹¹ and 4 cases with metastatic disease including pelvic lymph nodes, liver, and bone have been noted.^11–13,16 There is a report of sunitinib being used to treat a patient with metastatic TC-RCC.¹⁹

Acquired Cystic Disease–associated RCC

Acquired cystic disease–associated RCC (ACD-RCC) is the most common subtype of RCC occurring in end-stage kidneys, specifically those with acquired cystic disease. The tumor is characterized by cells with eosinophilic cytoplasm, cribriform/sieve-like architecture, and intratumoral oxalate crystals. At the 2012 ISUP consensus conference, 91% of respondents thought that ACD-RCC should be recognized as a distinct renal cell tumor entity.

ACD-RCC accounts for 36% of the largest tumors present in end-stage kidneys, and 46% of these are in end-stage kidneys with acquired cystic disease.²⁰ Most tumors are diagnosed incidentally on radiologic follow-up in patients with chronic renal disease.^20,21 In some recent publications, a long duration of dialysis (>10 y) has had a stronger association with ACD-RCC compared with other subtypes of RCC arising in this setting,^22–24 although such an association was not identified in another large series.²⁰

The tumors often appear as nodules arising from cyst walls, occasionally completely filling these cysts, or as solid masses separate from the cysts. The noncystic tumors are well circumscribed and may be surrounded by a thick fibrous capsule showing dystrophic calcification. Cut surfaces are tan to yellow, and hemorrhage and necrotic/friable areas are commonplace. Multifocality and bilaterality are seen in >50% and >20% of the cases, respectively.²⁰

ACD-RCC shows acinar, tubular, solid-alveolar, microcystic and macrocystic, papillary, and solid sheet-like architectural patterns in various combinations and proportions.^{5,20–22,24,25} Most tumor cells have abundant granular eosinophilic cytoplasm, with round to oval nuclei showing vesicular chromatin and prominent nucleoli. Foci with clear to vacuolated cytoplasm may also be present. One characteristic feature is the presence of intracellular and/or intercellular microscopic lumina (holes), imparting a cribriform/sieve-like appearance^{5,20,22,24,25} (Fig. 2). Another characteristic feature is the presence of intratumoral oxalate crystals in most, but not all, tumors.^{5,20,22,24–26} Some tumors may also show sarcomatoid or rhabdoid features.^5,20,27

FIGURE 2: ACD-RCC. Note tubules lined by multilayered atypical epithelium showing prominent vacuolation yielding a cribriform-like pattern. Note also the presence of oxalate crystals.

As many tumors show a variable proportion of papillary architecture, misinterpretation as type 2 PRCC is not uncommon, and it is not surprising that PRCC was previously considered to be the most common RCC subtype in end-stage kidneys. Similarly, the presence of cytoplasmic clarity and acinar architecture in some areas of the tumor may lead to the diagnosis of clear cell RCC. However, in the background of acquired cystic disease, careful attention to morphology of the entire tumor is essential. The presence of multiple small sieve-like lumina/cribriform architecture and intratumoral oxalate crystals are diagnostic of this entity.

These tumors by definition are associated with features of end-stage kidney with acquired renal cysts in the background kidney.²⁰ The cysts are usually numerous and mostly unilocular. However, multilocular cysts, and multiple cysts clustered together, are not infrequent. The cysts are predominantly lined by large cells with eosinophilic cytoplasm and large nuclei with prominent nucleoli (morphologically similar to the cells of the tumors). Although most cysts are lined by a single layer of epithelium, multilayering and papillary proliferation of the cyst lining cells identical to what is seen in ACD-RCC is not infrequent.

In end-stage kidney disease and in a sporadic setting, there was no consensus at the ISUP meeting regarding the nomenclature of such cysts with proliferative epithelium; 46% of the respondents favored the nomenclature of “atypical cysts,” and 54% favored calling them “cysts with epithelial proliferation.”

ACD-RCCs stain diffusely positive for AMACR and negative or, at the most, only very focally positive for CK7. Positivity for CD10, RCC, CD57, and GST-α is also described.^{20,21,24,27–29} However, a specific IHC profile is not required to make this diagnosis.

Molecular genetic studies have revealed gains and losses of multiple chromosomes. Although gains of chromosomes 7 and/or 17 in some tumors have been reported, gains of chromosomes 1, 2, 3, 6, 7, 16, and Y were also frequently observed.^28–31 Gains of chromosome 3, in particular, have been among the more consistent findings.^27–30

ACD-RCC has a relatively good prognosis, because most cases are diagnosed early in patients on long-term follow-up for chronic renal disease.^20,21,23,26 However, some rare typical cases, as well as tumors with sarcomatoid or rhabdoid features, can metastasize.^20,27 In addition, among all histologic types arising in the setting of end-stage kidney disease, ACD-RCC is reported to have relatively more aggressive behavior than other RCC subtypes.^20,22

Clear Cell (Tubulo) Papillary RCC

Clear cell (tubulo) papillary renal cell carcinomas (CCPRCCs) are composed of clear cells of low nuclear grade, variable papillary, tubular/acinar, and cystic architecture, and a characteristic linear arrangement of nuclei away from the basal aspect of cells.^32,33 These neoplasms have a distinctive IHC profile of CK7/CA-IX/high–molecular weight cytokeratin positivity and CD10 and AMACR negativity. Eight-five percent of participants believed that this neoplasm should be recognized as a distinctive entity at this time, and 65% of respondents by consensus believed that both morphology and IHC are required to diagnose this neoplasm.

The original term clear cell papillary RCC and a proposed synonym clear cell tubulopapillary RCC have been used interchangeably.^32,33 Approximately half of the participants preferred the former term, whereas the remaining preferred the latter term. Tumors of similar morphology and immunophenotype but with prominent smooth muscle stroma have been reported under the name renal angiomyoadenomatous tumor.³⁴ Clear cell RCC with diffuse CK7 positivity is now considered to be the same tumor as CCPRCC.³⁵

CCPRCC was initially reported in patients with end-stage renal disease²⁰; however, the majority of cases reported subsequently have been sporadic.^{25,32,33,36,37} On the basis of a few reports, CCPRCC comprises about 1% of all renal cell neoplasms.^21,33 The age at presentation is similar to that for RCCs in general (mean 60 y; range, 18 to 88 y), and there is no sex predilection.^33,36,37

Grossly, CCPRCCs are well circumscribed and usually well encapsulated. The cut surface is tan-white to yellow with grossly apparent fibrotic areas and ranges from completely solid to predominantly cystic. These tumors are usually unicentric, unilateral, and small, the largest one in the literature being 6 cm in diameter. Multifocality and bilaterality, however, may be present in some cases^20,22,26; the latter setting raises the differential diagnosis of von Hippel-Lindau–associated renal neoplasia. On microscopy, CCPRCCs have variable tubular/acinar, papillary, and cystic architecture.^{20,21,26,32,33} In some cases papillae are tightly packed giving rise to a solid appearance. Sometimes these papillary structures project into cystic spaces. In other cases, the tubules show branching and in foldings, imparting an incipient papillary architecture. Still other tumors have markedly crowded, very small, “collapsed” acini, containing scant cytoplasm, giving the tumor a solid nested appearance. Fibrous stroma separating tumor nodules within a single tumor mass is frequently evident. Some tumors with grossly apparent fibrotic cut surface show extensive myoid metaplasia of the capsule, with extensions of smooth muscle into the tumor mass and encasing nests of tumor cells. Tumors with “collapsed” acini, variable tubular/acinar architecture, myoid metaplasia, and diffuse CK7 positivity have been considered to be separate tumor entities (renal angiomyoadenomatous tumor/RCC with diffuse CK7 immunoreactivity) by some authors.^34,35 However, these morphologic patterns can be seen in otherwise typical CCPRCC, and most experts believe that they are part of the morphologic spectrum of CCPRCC.^21,33,38

By definition, the neoplastic cells have clear cytoplasm with low nuclear grade (nucleolar grade 1 or 2). The existence of cases of CCPRCC of higher nuclear grade is not well established at this time, although it is certainly possible. One characteristic feature of this tumor is the linear arrangement of the nuclei away from the basal aspect, toward the middle or the apex of the cells^{20,21,31,33,36,37} (Fig. 3). Foamy macrophages, tumor necrosis, and vascular invasion are not seen. Most tumors are small and confined to the renal parenchyma, although rare cases extending into the renal sinus have been described.^36,37

FIGURE 3: Clear cell (tubulo) PRCC. Note bland-appearing tubules and occasional small papillae. Uniform small nuclei are arranged in a linear manner away from the basal aspect of the tubules.

The IHC features of the tumor are quite characteristic.^{20,21,32,33,36} The tumors show diffuse and intense staining with CK7, almost always in 100% of the tumor cells. Tumor cells also express CA-IX diffusely in a membranous distribution; the absence of staining along the luminal borders of the tumor cells is quite characteristic (cup-shaped distribution).²¹ Stains for AMACR, CD10, and RCC are negative in most cases, whereas it is common to see patchy to diffuse immunoreactivity for high–molecular weight cytokeratin (34βE12) in the majority of these neoplasms. Given that the proposed definition of CCPRCC includes typical morphology and IHC findings, cases with typical morphology, but without the typical IHC profile, cannot be definitively placed in this category, although it does seem likely that they do belong.

The main differential diagnosis of CCPRCC is with clear cell RCC. Some clear cell RCCs may have foci resembling CCPRCC with subnuclear clearing causing linear arrangement of the nuclei.^21,39 Some cases may even show CK7 positivity, but such positivity is only focal. Unlike CCPRCC, they are also CD10 positive and 34βE12 negative. The CA-IX staining pattern is also different, with the luminal aspects of the cells also staining positive (box-shaped distribution) in clear cell RCC.

At the molecular genetic level, CCPRCCs lack deletions of 3p25, VHL gene mutations, VHL promoter hypermethylation, or trisomies of chromosomes 7 and 17.^21,29,32,33 However, although the mechanism is not clear, VHL transcripts are underexpressed, and SNP-array and microarray comparative genomic hybridization analyses have confirmed such findings in 2 different series.^36,37 Low copy number gains of chromosomes 7 and 17 have been reported in a few cases.^32,33 Rare cases considered as CCPRCC with other chromosomal aberrations have also been reported.⁴⁰

The number of cases in the literature with extended clinical follow-up information is small; however, published data indicate that these are neoplasms with indolent clinical behavior. No cases with metastasis have been reported.^{5,20,21,25,32,33,36,37} If further studies confirm their apparent indolent course, it is possible that neoplasms meeting the criteria for CCPRCC will subsequently be reclassified as being of “low malignant potential” rather than as a carcinoma.

MiT Family Translocation RCC

The MiT subfamily of transcription factors includes TFE3, TFEB, TFEC, and MiTF. Gene fusions involving 2 of these factors have been implicated in RCC. The Xp11 translocation RCCs, first recognized in the 2004 WHO Renal Tumor Classification, harbor fusions involving TFE3. The t(6;11) RCCs have recently been shown to harbor a gene fusion involving TFEB but have yet to be formally recognized. On the basis of clinical, morphologic, IHC, and genetic similarities to the Xp11 translocation RCC, 69% of respondents by consensus believed that the t(6:11) RCC should be included with the Xp11 translocation RCC under the category of MiT translocation RCC. Recent findings regarding these neoplasms are summarized below.

Xp11 Translocation RCC

Xp11 translocation RCCs are a group of neoplasms distinguished by chromosomal translocations with breakpoints involving the TFE3 transcription factor gene, which maps to the Xp11.2 locus. The result is a TFE3 transcription factor gene fusion with one of multiple reported genes including ASPL, PRCC, NonO (p54^nrb), PSF, and CLTC.^41–45 Variant translocations with no known fusion partner include t(X;3)(p11.2;q23) and t(X;10)(11.2;q23) translocations.

Although RCCs account for <5% of renal tumors in children, Xp11 translocation RCCs likely constitute approximately 50% of these cases. It has been estimated that 1% to 4% of adult RCC are Xp11 translocation RCCs.^46–48 Although Xp11 translocation RCC is relatively rare in the adult population, RCC is overall much more common in adults than in children. Thus, adult Xp11 translocation RCC may vastly outnumber pediatric Xp11 translocation RCC because of the much higher incidence of RCC in the adult population. Up to 15% of patients with Xp11 translocation RCC have had a history of chemotherapy exposure.⁴⁹

The most distinctive histologic pattern of the Xp11 translocation RCC is that of a neoplasm with both clear cells and papillary architecture and abundant psammoma bodies. Xp11 translocation RCCs can also present with unusual morphology mimicking other types of RCCs, including multilocular cystic RCC–like features, pleomorphic giant cells, tubular growth reminiscent of CDC, and well-developed fascicles of spindled neoplastic cells with bland nuclei and focal myxoid stroma.⁴⁶

Xp11 translocation RCCs underexpress epithelial markers such as cytokeratins and EMA. In contrast, Xp11 translocation RCCs do consistently express CD10 and RCC marker, and most express PAX2 and PAX8.⁵⁰ Some Xp11 translocation RCCs with typical morphology express melanocytic markers such as Melan-A and HMB-45.

The most sensitive and specific IHC marker for the Xp11 translocation RCC is strong nuclear TFE3 immunoreactivity, using an antibody to the C-terminal portion of TFE3.⁵¹ One drawback is that the assay is technically challenging, and suboptimal fixation or detection methods can result in detection of native TFE3 protein causing high background staining. Another is that genetic mechanisms other than TFE3 gene fusions, such as TFE3 amplification, can upregulate TFE3 expression.⁵² A TFE3 break-apart FISH assay performed on paraffin-embedded tissue has been reported for molecular confirmation of alveolar soft part sarcoma and Xp11 translocation RCC. Hybridization with probes centromeric and telomeric to TFE3 normally show a fusion signal, but TFE3 rearrangement results in a split signal.^53,54 This assay is very useful for detecting TFE3 gene fusions in Xp11 translocation RCC and suffers less from the technical issues involved with the TFE3 IHC assay.

Expression of cathepsin-K has been shown to be mediated by overexpression of MiTF in osteoclasts. As TFE3 belongs to the MiTF family, Martignoni et al^55,56 hypothesized that overexpression of TFE3 protein in Xp11 translocation carcinoma might also mediate cathepsin-K expression. As suspected, cathepsin-K labels approximately 60% of Xp11 translocation RCCs, almost all t(6;11) RCCs, but no other common RCC subtype.

As these neoplasms have only been recently recognized as a distinct entity by the WHO, outcome data on Xp11 translocation RCC are still premature at this time. Children with regional nodal metastases but without hematogenous spread have a favorable short-term prognosis, even though these cases qualify as high-stage presentation. In a literature review, >90% of these patients remained disease free at last follow-up having a median of 4.4 years and a mean of 6.3 years.⁵⁷ Adults may have a worse prognosis and do poorly when presenting with systemic metastases. Unfortunately, Xp11 translocation RCCs in adults often present with advanced disease and distant metastases. Mean survival after diagnosis is in the range of 1 to 2 years.^46,58 Regardless of the age of the patient, Xp11 translocation RCCs have the potential to metastasize late, as many as 20 or 30 years after diagnosis. Satisfactory long-term follow-up data are necessary before any favorable short-term outcome can be confirmed. Specific therapies for Xp11 translocation RCC are not clear at this time. Because of upregulation of C-Met, these tumors are eligible for clinical trials using Met inhibitors. Overexpression of phosphorylated S6 suggests the potential utility of mTOR pathway inhibitors in therapy.⁵⁰

An unusual malignant melanotic epithelioid renal neoplasm bearing a TFE3 gene fusion has been reported and designated “melanotic Xp11 translocation renal cancer.” Two initial cases occurred in children and presented with widespread metastatic disease.⁵⁹ An additional case report described a specific PSF-TFE3 gene fusion.⁶⁰ These neoplasms are difficult to classify, overlapping with Xp11 translocation RCC, melanoma, and perivascular epithelioid cell neoplasm (PEComa). Although they are immunoreactive to TFE3, all reported cases were negative for renal tubular markers CD10, PAX2, and PAX8, in contrast to the typical Xp11 translocation RCC. The neoplastic cells were not immunoreactive to MiTF, in contrast to melanoma. PEComas represent the most closely associated phenotype. Interestingly, a subset of PEComas in extrarenal sites have recently been shown to harbor TFE3 gene fusions and demonstrate aberrant strong TFE3 expression by IHC. Although the cases are few, distinctive features included young age, absence of association with tuberous sclerosis, minimal immunoreactivity for muscle markers, and prominent epithelioid clear cell morphology.^61,62 Recently, a renal PEComa in a 57-year-old, which labeled for muscle markers and harbored a TFE3 gene fusion, was reported.⁶³ Both melanotic Xp11 translocation carcinoma and PEComas harboring TFE3 gene fusions may represent distinct entities, which overlap with Xp11 translocation RCC and broaden the spectrum of TFE3-associated cancers.

t(6;11) Renal Cell Carcinomas

A lesser well-known member of the translocation RCC family are tumors characterized by the t(6;11)(p21;q12), which results in an Alpha-TFEB gene fusion,^64–70 and approximately 30 genetically confirmed cases of t(6;11) RCC have been reported.^{55,65–67,69,71–73} This neoplasm typically demonstrates distinctive biphasic morphology, comprising larger and smaller epithelioid cells, with the latter often clustered around basement membrane material; however, the full spectrum of the morphologic appearances of the t(6;11) RCC is not known (Fig. 4). Cases without the small cell component and instead dominated by sclerosis, clear cells, or papillary architecture have been reported. The t(6;11) RCC differs from most conventional RCCs in that they consistently express melanocytic IHC markers such as HMB-45 and Melan-A but are either negative or only focally positive for epithelial markers such as cytokeratins. Virtually all cases express the cysteine protease cathepsin-K,⁵⁵ which is expressed in osteoclasts and a subset of Xp11 translocation RCCs but in no other common RCC subtype. The majority of cases express PAX8, supporting renal tubular differentiation.⁷³

FIGURE 4: t(6;11) RCC. Note distinctive biphasic morphology comprising larger and smaller epithelioid cells, the latter of which are clustered around eosinophilic basement membrane–like material.

As a result of promoter substitution, the Alpha-TFEB gene fusion results in overexpression of native TFEB in the t(6;11) RCC. Nuclear labeling for TFEB protein by IHC is a sensitive and specific assay for these neoplasms.⁶⁵ However, IHC is highly fixation dependent and has proven to be particularly difficult for this protein. Recently, a break-apart FISH assay for TFEB gene fusions has been developed for archival material and has allowed the expansion of the clinical and morphologic spectrum of the t(6;11) RCC.⁶⁶

Of the approximately 30 genetically confirmed cases that have been reported, 3 have metastasized and caused death of the patient. Although the initially reported cases occurred in young patients less than 20 years of age, the majority of cases reported to date has occurred in young adults (mean age=28.5 y; median age=25 y). The clinicopathologic spectrum of these neoplasms remains to be determined. Utilization of TFEB IHC and a break-apart TFEB FISH assay in archival paraffin-embedded material should further our understanding of this distinctive neoplasm.

Hereditary Leiomyomatosis RCC Syndrome–associated RCC

Hereditary leiomyomatosis RCC–associated RCC (HLRCC) is not a “new” entity, and indeed it was described in the 2004 WHO classification of renal neoplasms in the section related to hereditary RCC.^4,74–79 Nevertheless, at that time it was not listed as a distinct subtype of RCC, and there was a suggestion that HLRCC was a hereditary counterpart of type 2 PRCC. It is now known that HLRCC, although usually having a papillary pattern, is a tumor with an aggressive behavior and as such should be recognized as a distinctive tumor subtype.^80,81

The HLRCC syndrome is autosomal dominant and associated with germline mutations in the fumarate hydratase gene located at chromosome 1q42. The main features of this syndrome are smooth muscle and renal neoplasms. These patients frequently harbor multiple cutaneous and uterine leiomyomas. The uterine leiomyomas are particularly difficult to manage, and approximately 50% of women require hysterectomy before reaching the age of 30 years. Rare cases of leiomyosarcoma have been reported.

The renal carcinomas associated with this syndrome affect approximately one third of patients and have been reported to resemble sporadic type 2 PRCC or even CDC. Unlike most hereditary RCC syndromes, these syndromic neoplasms are solitary but are highly aggressive. A majority of patients have presented with advanced-stage disease and have died of these cancers.⁸⁰ Morphologically, the most distinctive feature of these neoplasms is cytologic—namely, a distinct prominent eosinophilic nucleolus with a clear halo, similar to the cytology of a cytomegalovirus inclusion⁸¹ (Fig. 5). Interestingly, the distinctive eosinophilic nucleolus is also found in the leiomyomas that these patients typically harbor. The renal neoplasms frequently have papillary architecture, but other architectures such as cribriform and solid may also be present. RCCs and leiomyomas in the HLRCC syndrome demonstrate biallelic inactivation of fumarate hydratase, with germline mutations in one allele and loss of the second allele, typical of a tumor-suppressor gene. Although these neoplasms have been considered to be the hereditary counterpart to type 2 PRCC, it is noted that somatic mutations in fumarate hydratase are in fact rare in sporadic type 2 PRCC. Moreover, type 2 PRCC, as defined by some authors, may be a heterogenous “entity,” which includes multiple distinct renal carcinomas. Although experience with HLRCC is largely limited to the National Institutes of Health (NIH) Group, approximately 80% of the respondents by consensus believed that it was important to recognize HLRCC as a distinctive entity at this time, as the diagnosis carries significant implication for both patient management and follow-up of family members. For example, patients with this syndrome (unlike patients with other hereditary RCC syndromes) are followed more closely for even small (<3 cm) renal masses and are taken to surgery more quickly because of the highly aggressive nature of these RCCs, relative to other hereditary RCCs.

FIGURE 5: HLRCC. Note large tubular-alveolar structures lined by eosinophilic cells with enlarged nuclei and prominent nucleoli.

EMERGING/PROVISIONAL NEW TUMOR ENTITIES

Thyroid-like Follicular RCC

Thyroid-like follicular renal cell (TLF-RCC) is provisionally defined as RCC resembling well-differentiated follicular carcinoma of the thyroid gland. In the fewer than 15 cases reported in the literature, the age range is wide (29 to 83 y), and there is a slight female predominance.^82–87 Macroscopically, these tumors are well circumscribed, solid, and generally homogenous tan-brown in color. At the microscopic level, the neoplasms are typically encapsulated and have a macrofollicular and/or microfollicular architecture with associated dense colloid-like material (Fig. 6). Although cases with a focal papillary architecture have been cited in the literature on TLF-RCC, it is thought that this category should be reserved for tumors with a pure follicular architecture.^88,89 The nuclei are generally round and regular and contain enlarged nucleoli (nucleolar grade 2 or 3). A solitary case without papillary architecture showed clearing and grooves, which raised the differential diagnosis of a metastasis from the follicular variant of papillary carcinoma.⁸⁴ By IHC these neoplasms stain negatively for thyroid transcription factor-1 and thyroglobulin, allowing their distinction from metastatic follicular thyroid cancer to kidney, which has been more commonly described than TLF-RCC.^82,83 Labeling for CK7, PAX2, and PAX8 has been variable. Only limited genetic studies have been performed on this neoplasm without any distinctive pattern emerging. Although the majority of cases have behaved in an indolent manner, 2 examples of TLF-RCC metastatic to regional lymph nodes in 1 example showing metastases to lung have been described.^83,84

FIGURE 6: TLF-RCC. Note microfollicular architecture and dense colloid-like material.

Problematic issues with TLF-RCC at this time include the limited number of reported cases; the issue of whether a papillary component is allowable and the fact that a follicular pattern may be seen focally in other established types of RCC including PRCC and CCPRCC. Furthermore, follicular patterns may be seen in benign renal epithelial neoplasms including oncocytoma and metanephric adenoma. The majority of conference respondents did not think that TLF-RCC should be recognized as a distinctive entity at this time. It is therefore placed in a provisional category.

Succinate Dehydrogenase B Mutation–associated RCC

Renal carcinomas have been reported with germline succinate dehydrogenase B mutations that are associated with the pheochromocytoma/paraganglioma syndrome type 4 (PGL4).^90–95 This syndrome is characterized by a predilection to pheochromocytoma, paraganglioma, so-called type 2 gastrointestinal stromal tumors (similar to those frequently seen in children and in association with Carney syndrome), and an estimated approximately 14% lifetime risk of renal neoplasia. Fewer than 10 cases of RCC associated with germline succinate dehydrogenase B mutations (SDHB RCC) have been reported. Most have affected young adults, and most cases have been associated with an indolent course on limited follow-up. The exceptions are 2 cases that underwent sarcomatoid change, both of which metastasized and 1 of which resulted in death of the patient.

Morphologically, these neoplasms are most usually unencapsulated and composed of compact nests of eosinophilic polygonal cells, with entrapped renal tubules at the periphery. The cells may have vacuolated cytoplasm or distinctive pale eosinophilic cytoplasmic inclusions, which correspond to giant mitochondria by ultrastructural examination (Fig. 7). Loss of SDHB protein by IHC is reported to be a sensitive and specific marker for these neoplasms.

FIGURE 7: SDHB RCC. Note compact nests of eosinophilic polygonal cells with vacuolated cytoplasm. Distinctive pale eosinophilic cytoplasmic inclusions are present.

Although these findings suggest that SDHB RCC may be a distinctive entity, problematic areas remain. First, there is limited experience (<10 cases published), and some renal neoplasms associated with pheochromocytoma/paraganglioma syndrome type 4 have been reported as clear cell RCC and oncocytoma, although these have not been illustrated. Secondly, there is relatively limited experience with SDHB IHC in the kidney (unlike the situation with gastrointestinal stromal tumors). As SDHB normally localizes to the mitochondria, one should see granular mitochondrial-pattern staining in cells with eosinophilic cytoplasm and intact SDHB. However, it has been stated that cells with clear cytoplasm (usually reflecting displacement of mitochondria from the cytoplasm and replacement by glycogen or fat) may give a false-negative result on SDHB IHC. It is not clear whether there is a minimal size of tumor that needs to be evaluated before loss of SDHB labeling can accurately predict syndromic cases. Some authors have suggested that only whole sections should be evaluated and that tissue microarrays may falsely suggest loss of labeling because of the limited tissue size. Finally, mutations in other subunits of succinate dehydrogenase (such as succinate dehydrogenase A, B, C, or D) can abrogate SDHB labeling. At this time, SDHB RCC is considered a provisional entity, which requires further experience before it can be accepted as a distinctive recognized entity by the ISUP. Only approximately half of the respondents with knowledge of this entity felt that the lesion deserved recognition as a distinctive entity at this time, whereas the other half either did not or were uncertain.

ALK Translocation RCC

Two cases of RCC harboring a t(2;10)(p23;q22) translocation resulting in a fusion of the gene for the cytoskeletal protein vinculin (VCL) with the anaplastic lymphoma kinase (ALK) gene have been reported.^96,97 It is noteworthy that both cases occurred in young patients (ages 6 and 16 y) with sickle cell trait (raising the differential diagnosis of renal medullary carcinoma [RMC]) and demonstrated distinctive morphology characterized by polygonal to spindle cells with abundant eosinophilic cytoplasm and frequent intracytoplasmic lumina (Fig. 8). Although the case numbers are small, the findings suggest that VCL-ALK RCC have distinctive clinical and pathologic features. A recent study from Japan⁹⁸ identified 2 further RCCs in adults (ages 36 and 53 y) associated with ALK fusions involving partner genes (TPM3, EML4) other than VCL. Interestingly, neither of these cases was associated with sickle cell trait, and morphology (papillary and unclassified) was different from that reported for the VCL-ALK fusion. The Mayo Clinic Group recently reported 2 additional ALK-positive cases (fusion partner not determined) in adults, both of which demonstrated clear cells and papillary architecture and behaved aggressively.⁹⁹ The majority of respondents did not think that ALK translocation–associated RCC should be recognized as a distinctive entity at this time given the paucity of cases reported.

FIGURE 8: ALK-translocation RCC. Note large irregular polygonal cells with pleomorphic nuclei, abundant eosinophilic cytoplasmic, and focal vacuolation.

NEW CONCEPTS AND REFINEMENTS TO EXISTING WHO (2004) RENAL CELL TUMOR CATEGORIES

Clear Cell Renal Cell Carcinoma

Multilocular Cystic Renal Cell Neoplasm of Low Malignant Potential (Multilocular Cystic RCC)

Multilocular cystic RCC accounts for approximately 4% of all clear cell RCCs and affects mid-age adults with a male to female ratio of 1.2 to 2.1:1. Up to 90% of cases are discovered incidentally on radiologic evaluation for other causes^100,101 and usually present as a unilateral solitary lesion.¹⁰² Macroscopically, multilocular cystic RCC consists exclusively of variably sized cysts separated by thin septa and filled with clear, serous, or gelatinous fluid. The 2004 WHO defines multilocular cystic RCC as a tumor composed entirely of numerous cysts, the septa of which contain groups of clear cells indistinguishable from grade 1 clear cell carcinoma (Fig. 9). The clear cells can be focal and mistaken for lymphocytes or histiocytes, although their prominent associated vascularity is an important diagnostic clue. The cysts themselves are often denuded, but may be lined by a single layer of flat to cuboidal epithelium, which may have clear cytoplasm. Occasional small papillae may be seen.

FIGURE 9: Multilocular cystic renal cell neoplasm of low malignant potential. High-power image of septa containing clusters of clear cells indistinguishable from grade 1 clear cell carcinoma.

Genetic studies have clearly linked multilocular cystic RCC and clear cell RCC.^103,104 Similar to clear cell RCC, chromosome 3p deletions have been found in 74% of multilocular cystic RCC, and VHL mutations have been identified in 25% of cases. Like clear cell RCC, the neoplastic cells in most cases are strongly reactive to PAX2 and CA-IX.

In line with the minimal tumor burden present in these neoplasms, their prognosis is excellent. Multiple publications spanning >200 patients, with a follow-up time of >5 years, have shown no recurrence or metastases in patients whose tumor was defined according to the definition adopted by the WHO (see above). On the basis of the excellent outcomes, 65% of the participants at the RCC Consensus Conference in Vancouver by consensus support redesignation of these lesions as multilocular cystic renal cell neoplasms of low malignant potential.¹⁰⁵ Furthermore, the majority of participants (78%) by consensus believed that cells displaying nucleolar grade 2 are acceptable in multilocular cystic RCC.

The main differential diagnosis is clear cell RCC with extensive cystic change. Expansile solid nodules of clear cells, which alter the natural septal configuration, serve to differentiate extensively cystic RCC from multilocular cystic RCC, although it is clear that these 2 entities exist in a spectrum. RCC with extensive cystic necrosis also enters the differential diagnosis.^106–108 The latter is typically composed of multiple cysts filled with hemorrhagic and necrotic debris and separated by irregularly thick shaggy walls composed of variable mixtures of fibrous tissue and neoplastic cells. This distinction is highly relevant, as even extensively necrotic cystic RCCs (99% necrotic) have been shown to be capable of aggressive clinical behavior. An unresolved issue is when one sees areas of hyalinization that form a mass; these areas may reflect regression of RCC, and their significance is unknown. Other issues relate to whether multilocular cystic RCC should be staged and whether complete submission of the tumor is required before the diagnosis can be rendered. Finally, the differential diagnosis also includes translocation RCC, which can closely mimic multilocular cystic RCC.¹⁰⁹ Attention to patient age and the presence of psammoma bodies are clues to suggest this diagnosis.

Papillary Renal Cell Carcinoma

RCC with a papillary architecture was recognized in early studies and was considered by Grawitz to be a true renal malignancy, different from tumors with alveolar architecture, which he considered to be of adrenal origin.¹¹⁰ The publication of a series of cases by Mancilla-Jimenez and colleagues in 1976 established PRCC as a specific tumor morphotype, and this was formalized through its inclusion in the Mainz Classification and the Heidelberg and Rochester consensus conferences recommendations.^1–3,111

PRCC is now recognized as the second most common type of RCC, comprising 6% to 18% of tumors in reported series ,and detailed studies have shown these tumors to have distinctive clinical and genetic features.^112,113

Approximately 20% of PRCCs are discovered as incidental findings, and unlike clear cell RCC, spontaneous hemorrhage is a presenting feature in 8% of cases.¹¹⁴ Additional differentiating clinical features from both clear cell and chromophobe RCC (CHRCC) are that these tumors are more likely to be multifocal, whereas calcification is seen in 30% of cases on imaging.^111,115 In contrast to the main morphotypes of RCC, PRCC is most commonly associated with trisomy/tetrasomy of chromosome 7, trisomy of chromosomes 12, 16, 17, and/or 20, and loss of the Y chromosome.¹¹³ Loss of heterozygosity at both the VHL and FHIT loci, but no loss of chromosome 3p, has also been reported.^116–118

Papillary RCC is characterized by the presence of a papillary or tubulopapillary architecture, with neoplastic cells overlying a delicate fibrovascular core or forming compact tubules. Two subtypes of PRCC have been described.^119,120 Type 1 tumors are characterized by a simple cuboidal or columnar covering of tumor cells on papillary stalks, with nuclei aligned in a linear manner. Other features frequently associated with type 1 tumors are scanty and pale tumor cell cytoplasm, as well as the presence of psammoma bodies and aggregates of foamy macrophages. Type 2 tumors show pseudostratification of tumor nuclei with cells often having voluminous cytoplasm and moderate to marked nuclear pleomorphism usually with prominent nucleoli.⁴ The subtypes of PRCC differ in IHC staining with type 1 tumors showing expression of CK7, vimentin, and MUC1, whereas CK20 and E-cadherin expression is more frequently seen in type 2 tumors.^121–124

Several studies have shown type 1 and 2 tumors to differ in genotype, with type 1 tumors showing chromosome 7p and 17p gains and type 2 tumors showing allelic imbalance of one or more of chromosomes 1p, 3p, 5, 6, 8, 9p, 10, 11, 15, 18, and 22.^125–128 In gene profile studies, high-grade type 2 tumors have been differentiated from a mixed group of PRCC consisting of type 1 tumors, low-grade type 2 tumors, and tumors showing a mixed type 1 and low-grade type 2 morphology.¹²⁹

It has been noted that in published series of PRCC, there is considerable variation in the proportion of type 1 and 2 tumors, and this highlights the importance of assigning tumors into their correct subtype according to established criteria.¹³⁰ Subtyping of PRCC has been further compounded by the recent identification of PRCC mimics, with clear cell tubulopapillary RCC, TC-RCC, and mucinous tubular and spindle carcinoma now being recognized as novel or emerging forms of RCC.⁵

In the preconference survey, participants were asked to specify the classification they utilized to subtype papillary RCC. In all, 59% of respondents noted that they classified tumors according to types 1 and 2, whereas 10% also incorporated oncocytic papillary RCC as an additional type (type 3) in their classification of these tumors. Of the remainder, 16% utilized Fuhrman grading only, whereas 10% utilized other criteria, including the option of not subtyping these tumors according to morphologic characteristics. Only 3% of respondents assigned subtype on the basis of molecular classification (type 1 vs. type 2a vs. type 2b).¹²⁹

At the conference there was consensus (75%) that tumors should be classified as type 1 or type 2 and that oncocytic tumors should not be identified as a specific subtype (Fig. 10). In addition to this, a further 14% of participants considered that tumor should be classified as either type 1 or type 2 without taking into account those tumors identified as oncocytic papillary RCC. Retention of oncocytic papillary RCC as a specific subtype was recommended by only 3% of the participants, whereas 7% preferred to grade tumors rather than formally subtyping them. No respondent was in favor of the use of the subtyping classification based on molecular characteristics.

FIGURE 10: Oncocytic PRCC. Note tightly packed papillary structures lined by large cells with abundant granular eosinophilic cytoplasm resembling oncocytes.

CHRCC and Hybrid Oncocytic/Chromophobe Tumors

Hybrid oncocytic/chromophobe tumors (HOCTs) are described as tumors having a mixture of cells with the morphologic features of those seen in CHRCC and renal oncocytoma (RO). HOCTs occur in 3 clinicopathologic situations, being found sporadically, in association with renal oncocytosis/oncocytomatosis, or in patients with the Birt-Hogg-Dubé syndrome (BHD). From published data it would seem that tumors from all 3 groups share similar morphologic features but that they have a differing molecular genetic background.^131–139

HOCTs, regardless of the associated clinicopathologic situation, occur in adult patients, with no sex predilection,^134,136,137 although HOCTs in oncocytosis/oncocytomatosis are occasionally associated with long-term hemodialysis.^{133,137,139–141} There are no specific clinical signs and symptoms for sporadic tumors and those associated with oncocytosis/oncocytomatosis. Patients with BHD syndrome–associated HOCT usually show signs of the syndrome consisting of adnexal skin tumors (fibrofolliculomas, trichodiscomas, acrochordons), pulmonary cysts, spontaneous pneumothorax, bronchiectasis, colonic tumors, medullary thyroid carcinoma, and multiple lipomas.¹³¹ Clinically, sporadic HOCTs are mostly unilateral and solitary, whereas HOCTs in individuals with BHD or oncocytosis/oncocytomatosis are often bilateral and multiple. The majority of HOCTs are category pT1 or pT2 at presentation (TNM seventh Edition).

Tumors are usually well circumscribed and nonencapsulated with homogenous tan to brown cut surface. Although necrosis is not frequently seen, central fibrotic strands/scars may be present.^{131–139,142}

Sporadic HOCTs are composed of neoplastic cells predominantly arranged in a solid-alveolar pattern, with nuclei showing mild nuclear pleomorphism and abundant granular eosinophilic to oncocytic cytoplasm (Fig. 11). Frequently, neoplastic cells have a perinuclear halo and are occasional binucleate. No raisinoid nuclei of the type seen in classic CHRCC are present. Usually tumor cells resemble cells of RO with perinuclear cytoplasmic clearing, and, occasional, small tubules may be present.^{132,134,136,138}

FIGURE 11: Sporadic HOCT. Note tightly packed nests of eosinophilic cells with minimal nuclear pleomorphism. The nuclei are regular without raisinoid features, and small perinuclear halos are seen.

HOCTs in oncocytosis/oncocytomatosis are almost identical to those tumors that occur sporadically, being composed of sheets of cells separated by a delicate vasculature. The cells are round to polygonal with finely granular cytoplasm, and the nuclei are slightly pleomorphic and irregular with visible nucleoli. Again no typical raisinoid nuclei are seen, and mitotic figures are inconspicuous. There has been consensus that oncocytosis-related HOCT are distinct tumors and that they do not represent a stage of morphologic progression between RO and CHRCC.^{133,137,139,141,142}

HOCTs associated with BHD typically show 3 morphologic patterns: (1) an admixture of areas typical of RO and CHRCC; (2) scattered chromophobe cells in the background of a typical RO; and (3) large eosinophilic cells with intracytoplasmic vacuoles (Fig. 12). In these tumors, the nuclei are often more pleomorphic than other forms of HOCT and occasionally have a “raisinoid” morphology.^{131,135,143,144}

FIGURE 12: HOCT associated with BHD syndrome. Note admixture of chromophobe and oncocytic-type cells. Many large eosinophilic cells have prominent intracytoplasmic vacuoles.

The IHC profile of HOCT differs slightly according to clinicopathologic groups The majority of the tumors express parvalbumin, antimitochondrial antigen, and CK7. CD117 is invariably positive.

Ultrastructurally, neoplastic cells from sporadic HOCTs consistently show numerous mitochondria of varying sizes. In addition, sparse microvesicles with amorphic lamellar content and abundant microvesicles in the cytoplasm are present.¹³⁶ In HOCT in patients with oncocytosis/oncocytomatosis, the cytoplasm of tumor cells shows RO-like areas with numerous mitochondria containing lamellar cristae, whereas the cytoplasm in the CHRCC-like areas show a significantly diminished number of mitochondria with lamellar cristae, increased amounts of glycogen, and no cytoplasmic microvesicles.¹⁴² To date there has not been any study dealing with the ultrastructure of HOCTs in BHD in the English language.

At the molecular level, sporadic HOCTs are characterized by multiple monosomies and polysomies of chromosomes 1, 2, 6, 9, 10, 13, 17, 20, 21, and 22, with lack of mutations in the VHL, c-kit, PDGFRA, and FLCN genes. Monosomy of chromosome 20 is the most frequent finding, and this is of importance as this is highly unusual for both RO and CHRCC.¹³⁶ HOCTs associated with oncocytosis/oncocytomatosis are characterized by variable genetic profiles. The majority of tumors show no losses of chromosomes 1, 2, 6, 10, and 17; however, losses of chromosomes 1, 14, 21, and Y have been documented.^133,140,141 HOCTs in BHD show a similarly variable profile. Rarely, multiple abnormalities are seen in chromosomes 2, 3, 4, 5, 6, 13, and 18, although, to date, loss of chromosome 1 or translocation of 11q13 has not been reported. Loss of heterozygosity of 3p is rarely seen in this type of tumor.^131,135,143 The most prominent molecular feature of HOCT in BHD is an elevated expression of mitochondria and oxidative phosphorylation (OXPHOS)–associated genes and germline mutations in the FLCN gene, the latter being easily detected in formalin-fixed, paraffin-embedded material.

HOCT, regardless of clinical association, seems to behave indolently and no evidence of aggressive behavior has been documented. At worst, these tumors may exhibit a low malignant potential, although this will require longer follow-up of cases for confirmation.

The majority of ISUP conference participants (73%) reported that they recognize HOCT as a subcategory of CHRCC, with some preferring the designation oncocytic neoplasia of uncertain malignant potential. At the present time it was considered that it was not possible to determine whether each of the 3 subtypes of HOCTs should be recognized as separate entities. From a morphologic perspective, there are similarities between HOCT, RO, and CHRCC. However, all 3 subtypes of HOCT have molecular genetic profiles that differ from both RO and CHRCC.^{131,133,135,140,141,143}

Collecting Duct Carcinoma

At the consensus conference it was agreed upon that for a diagnosis of CDC to be made a tumor should show the following features: (1) at least some of the lesion involves the medullary region; (2) there is a predominant formation of tubules; (3) a desmoplastic stromal reaction should be present; (4) cytologic features are high grade; (5) growth pattern is infiltrative; and (6) there is an absence of other typical RCC subtypes or urothelial carcinoma.^145,146

Several controversial issues were addressed at the consensus conference. The first was related to those tumors in which >95% has the morphology of CDC, but focal urothelial carcinoma is also present. By consensus 68% of the respondents considered that these cases should be diagnosed as urothelial carcinoma with prominent glandular differentiation. An argument supporting this is that the stroma of the bladder and kidney are very different and may account for the different morphologic appearance of a urothelial carcinoma when it involves the kidney. A contrary argument is that the variable morphology in these tumors represents divergent differentiation from the site where the collecting ducts joined the urothelium, so that a minor component of urothelial carcinoma is acceptable within what is otherwise a typical CDC. Second, in the setting of an undifferentiated carcinoma, the consensus (85%) was that if there was any component that met the criteria for CDC, then the diagnosis should be poorly differentiated CDC as opposed to “unclassified carcinoma.” The majority opinion was that CDCs are, by definition, high grade and as a consequence should not be assigned a grade.

Immunohistochemically, there is clear overlap between CDC and urothelial carcinoma. PAX8 labeling is seen in virtually all CDCs, with the majority showing moderate to strong immunoreactivity, and in 17% to 20% of upper tract urothelial carcinomas.¹⁴⁷ Similarly, p63 positivity, which is seen in almost all urothelial carcinomas, is also seen in a minority (14%) of CDCs.¹⁴⁷ Although the distinction of CDC from urothelial carcinoma can be difficult and is somewhat controversial, it does have significant clinical implications, as patients with urothelial carcinomas of the kidney usually undergo evaluation of their bladder for additional tumors, whereas patients with CDC do not.

Complete loss of INI1 expression was observed in 3 of 20 (15%) cases of CDC, and another 15% of cases showed focal and weak staining. Despite this no significant differences were found in the clinicopathologic and outcome features relating to INI1 status.¹⁴⁸ As a consequence INI1 immunoexpression was considered to be of limited value in the differential diagnosis of CDC versus RMC (see below).

The 3-year relative survival rates for localized, regional, and distant disease have been reported to be 93%, 45%, and 6%, respectively (P<0.001), although most patients present with T3 and T4 disease.¹⁴⁹ Some patients with CDC do respond to combination chemotherapy.¹⁵⁰

Renal Medullary Carcinoma

RMC was recognized by Davis et al in 1995 and occurs almost exclusively in children and young adults with sickle cell trait.¹⁵¹ It is highly invasive and almost uniformly lethal. These tumors often have rhabdoid cytology, and inflamed desmoplastic stroma.

RMC, both with and without rhabdoid histology, shows loss of INI1 labeling by IHC, similar to pediatric rhabdoid tumors. In contrast, most RCCs or urothelial carcinomas, including those with histologic rhabdoid features, express INI1.¹⁵² However, CDC, which can be difficult to distinguish morphologically from medullary carcinoma,¹⁵³ can, in a minority of cases, show a loss of INI1.¹⁴⁸

RMCs in patients with or without sickle cell disease show involvement of genes important in hypoxia-induced signaling pathways and, in particular, show increased hypoxia-inducible factor-1α expression. Expanded whole-genome expression analysis also shows increases of TopoII in all cases. There is also overall deregulation of DNA remodeling and repair and an ontological association between RMC and urothelial carcinoma.^154,155 However, there is not a single consistent chromosomal or molecular abnormality in medullary carcinoma.¹⁵⁶ There is accumulating evidence that chemotherapy can prolong life and offer palliation to patients with RMC.¹⁵⁷

Mucinous Tubular and Spindle Cell RCC

Reports of a distinctive low-grade renal neoplasm composed of tubules lined by bland cuboidal cells together with spindle cells and intercellular mucin first emerged in the 1990s, and this tumor was included in the 2004 WHO Classification under the rubric of mucinous, tubular, and spindle cell RCC (MTS RCC).^4,158–163 This rare tumor affects adults over a wide age range and shows a female predominance (3 to 4:1). Some patients with MTS RCC have nephrolithiasis, but most are asymptomatic.¹⁶¹ The tumors are generally of low-staging category (pT1, pT2) at diagnosis and are amenable to partial or complete nephrectomy.

MTS RCCs are usually found in the cortex, and they have a solid cut surface. Their coloration can vary from white or gray to yellow, tan, or even pink. Histologically, there are bland tubules, many of which are elongated and merge into cord-like structures. Transitions between elongated tubules and spindle cells are seen, and in some tumors the spindle cell pattern is dominant, at times resembling a mesenchymal neoplasm such as leiomyoma. A lightly basophilic mucin is present at least focally in most tumors (Fig. 13). Sometimes large lakes of mucin are seen, and this may be highlighted by the Alcian blue stain. Occasional other features include collections of chronic inflammatory cells and foam cells, foci of clear cells, and small foci of necrosis.

FIGURE 13: Mucinous tubular and spindle cell RCC. Note admixture of small tubules and elongated spindle cells. A small amount of mucin is present focally. Some mucinous tubular and spindle cell carcinomas may be mucin poor and resemble type 1 PRCC.

By IHC the tumor cells stain positively for low–molecular weight keratins (CK8, CK18) and CK7.^158–162 Stains for high–molecular weight keratins show variable expression, and Ulex europeaus and other markers of lower nephron differentiation are often negative but may occasionally be positive.

Subsequent to the description of MTS RCC in the WHO 2004 Classification, additional series of tumors have been published, including ones showing “mucin-poor” patterns.^163,164 Cases with no mucin display a prominence of small tightly packed tubules, some of which are elongated and spindled, resembling type 1 PRCC.^163,165 Some authors have suggested that MTS RCC itself may represent a variant of papillary RCC.¹⁶⁵ In fact, some cases of MTS RCC may be indistinguishable from type 1 papillary RCC on routine microscopy and IHC, and the distinction between the 2 may require molecular genetic studies. In a FISH-based study of 10 MTS RCCs, none showed the typical gains of chromosomes 7 and 17 and losses of chromosome Y, which are found in PRCC.¹⁶⁶ Other investigators have disputed this point and suggest that some typical MTS RCCs show gains in chromosomes 7 and 17, suggesting a close relationship with PRCC.¹⁶⁵ Other molecular genetic analyses have suggested that MTS RCC is a distinctive neoplasm.^160–167

Most cases of MTS RCC behave in a rather indolent manner, although examples of local recurrence in the renal bed and metastases to regional lymph nodes have rarely been described.¹⁶⁰ In one remarkable case, a large (18 cm) tumor metastasized not only to abdominal lymph nodes but also to the liver where it displayed the typical morphologic pattern of MTS RCC.¹⁶⁸ In addition, recent examples of MTS RCC with sarcomatoid transformation have been recorded, and in some cases metastatic disease and tumor-associated death have been described.^169–173 In one case of metastatic MTS RCC, a response to sunitinib has been documented.¹⁷³

RECOMMENDATIONS REGARDING OTHER RENAL TUMOR CATEGORIES

Angiomyolipoma Including Epithelioid Variant

PEComa lesions, including those composed predominantly of fat and those almost exclusively composed of spindle-shaped smooth muscle cells, classic triphasic angiomyolipomas (AMLs), microscopic AMLs, intraglomerular lesions, oncocytoma-like AMLs, epithelioid AMLs; and the recently-described AMLs with epithelial lined cysts, all strongly express cathepsin-K.¹⁷⁴ Although some AMLs express TFE3 immunohistochemically,¹⁷⁵ especially when using automated IHC stainers with short antibody incubation times, only a distinctive subset of PEComas harbor TFE3 gene rearrangements.⁶¹ The latter are typically not associated with tuberous sclerosis, are purely epithelioid, label minimally for muscle markers, and tend to affect young patients.

There was consensus (86%) that AMLs with epithelioid morphology should be divided into “epithelioid AML with atypia” and “epithelioid AML without atypia,” although the distinction is somewhat subjective (Fig. 14). Two large retrospective studies have assessed the issue of epithelioid AML. In 1 study with 33 cases of pure epithelioid AML, the clinicopathologic parameters associated with disease progression (recurrence, metastasis, or death due to disease) in univariate analysis included associated tuberous sclerosis complex or concurrent AML (any metastasis, P=0.046), necrosis (metastasis at diagnosis, P=0.012), tumor size >7 cm (progression, P=0.021), extrarenal extension and/or renal vein involvement (progression, P=0.023), and a carcinoma-like growth pattern (progression, P=0.040) (the 5 adverse prognostic parameters for pure epithelioid PEComas).¹⁷⁶ Tumors with <2 adverse prognostic parameters (13 cases) were considered to be low risk, with 15% having disease progression. Tumors with 2 to 3 adverse prognostic parameters (14 cases) were considered to be “intermediate risk,” with 64% having disease progression. Tumors with ≥4 adverse prognostic parameters (6 cases) were considered to be high risk, with all patients having disease progression. In 80% of tumors with ≥3 adverse prognostic parameters, patients had disease progression. An exact logistic regression analytic model showed that a only carcinoma-like growth pattern and extrarenal extension and/or renal vein involvement were significant predictors of outcome (P=0.009 and 0.033, respectively). The other large series specifically analyzed epithelioid AMLs with at least moderate atypia.¹⁷⁷ Follow-up information was available for 34 cases. A predictive model of 4 atypical features included: (1) ≥70% atypical epithelioid cells; (2) ≥2 mitotic figures per 10 hpf; (3) atypical mitotic figures; and (4) necrosis. The presence of 3 or all of the features was highly predictive of malignant behavior. This model accurately categorized 78% of clinically malignant and 100% of the clinically benign epithelioid AMLs with atypia.

FIGURE 14: Epithelioid AML. Note presence of large eosinophilic epithelioid cells showing nuclear pleomorphism and mitoses.

It should be noted that both studies are retrospective and suffer from the biases inherent in consultation material. Prospective studies of epithelioid AML outcome are in progress but have yet to be published, although it is suggested that the worrisome histologic features reported above do not necessarily predict aggressive behavior.¹⁷⁸ At the Vancouver Consensus Meeting, there was near consensus (64%) that the prognosis of epithelioid AMLs should be based on published criteria being divided into low, intermediate, and high risk of malignant behavior, as opposed to benign or malignant; only 25% favored diagnosing tumors as benign or malignant, and the remaining 11% were uncertain.

Interestingly, epithelioid AMLs tend to be immunoreactive for estrogen receptor (ER) more than the classic triphasic AML.¹⁷⁹

An unusual variant of AML is oncocytic AML with the same IHC profile as usual AML.¹⁸⁰ A single case has been reported of liposarcoma arising in AML.¹⁸¹ It is now recognized that lesions that have in the past been designated as “capsular leiomyomas” are immunohistochemically identical to AML and are considered as variants of AML.¹⁸² Rare cases of intraglomerular AML have also been described.¹⁸³

AML with epithelial cysts¹⁸⁴ have 3 components: (1) epithelial cysts lined by cuboidal to hobnail cells, which label for PAX8 and PAX2 and likely represent entrapped renal tubules; (2) a compact subepithelial “cambium-like” layer of cellular, Müllerian-like AML stroma with prominent admixed chronic inflammation; and (3) muscle-predominant AML with associated dysmorphic blood vessels exterior to the cellular subepithelial stroma. Immunohistochemically, the stromal components label with HMB-45 and Melan-A most intensely in the cellular subepithelial stroma, whereas smooth muscle actin and desmin demonstrate the opposite pattern, with greatest intensity in the peripheral muscle–predominant AML areas. Immunoreactivity for ER and progesterone receptors (PR) and CD10 shows the strongest and most diffuse staining in the subepithelial AML cells.

Cystic Nephroma/Mixed Epithelial and Stromal Tumor

Cystic nephroma (CN) (also known as multilocular renal cyst) and mixed epithelial and stromal tumor (MEST) are each benign mixed mesenchymal and epithelial neoplasms of the kidney. In most cases, these entities can be readily distinguished from one another. CN lacks solid areas and is composed of simple cysts lined by a single layer of flat, low cuboidal, or hobnail epithelial cells (Fig. 15). Fibrous septa may be paucicellular or cellular with a density resembling that of ovarian stroma. In contrast, MEST is variably cystic and solid and demonstrates a complex architecture of cysts, tubules, and stroma of variable cellularity^185–190 (Fig. 16). The epithelial elements comprise glands, cysts, and papillae of variable sizes and architectures. They are lined by flatted, cuboidal, or columnar cells or by urothelium, particularly when the lesion projects into the renal pelvis. In rare cases, the lining cells are ciliated.¹⁹¹ The stroma is variably cellular and ranges from markedly hyalinized fibrous tissue to smooth muscle, fat, or nondescript cellular spindle cell stroma. In both lesions, the epithelial elements label for renal lineage markers such as PAX2 and PAX8,¹⁹² whereas the stroma labels for hormone receptors ER and PR.

FIGURE 15: CN. Note paucicellular fibrous septae covered by attenuated and cuboidal cells.

FIGURE 16: MEST. Note complex admixture of tubules, cysts, and stroma. Tubules are lined by bland cuboidal cells.

Although many believe that the epithelium in MEST represents altered entrapped renal tubules, a recent study presented molecular evidence that the epithelium and stroma are clonally related in at least some cases.¹⁹³ Rare cases of malignant MEST have been reported.^194,195

In recent years, multiple observers have noted clinical, pathologic, and genetic similarities between CN and MEST.^196–201 First, both are associated with a strong female sex predilection, with a female to male ratio of approximately 5:1. Second, the lesions have a similar age distribution, occurring predominantly in premenopausal women. Third, cases overlapping morphologic features of CN and MEST are well recognized and have been reported. Fourth, the stromal cells of both lesions label similarly for ER and PR. Fifth, in a small study comparing 3 CNs and 3 MESTs, these 2 lesions had a similar gene expression profile relative to renal carcinomas.¹⁹⁹ The majority of respondents (72%) in the consensus conference believed that CN and MEST are variations of the same lesion. However, a minority of respondents (16%) believe that CN and MEST are not the same entity. These respondents point out that the predilection to involve adult women is not at all specific in renal neoplasia, as AML, metanephric adenoma, and mucinous tubular and spindle cell carcinoma also preferentially involve this demographic. Further, many other renal neoplasms beside MEST can have areas that mimic CN, including multilocular cystic RCC, cystic partially differentiated nephroblastoma, and renal synovial sarcoma. They also note that ER/PR labeling of the renal stroma is not specific, as it is seen in AML and as a reaction to obstruction. Fourthly, it is not clear whether MEST was ever reported before 1950, suggesting that there may have been a relatively recent environmental trigger for this neoplasm (such as oral contraceptive use or other hormonal therapy), whereas CNs have been documented well before this time. Finally, most classic CNs and MESTs are, as mentioned above, readily distinguished from one another.

Primary Renal Synovial Sarcoma

Primary renal synovial sarcoma had previously been included under the category of mixed mesenchymal and epithelial tumors.^4,202 However, it is evident that the epithelial components of these tumors represent entrapped native renal tubules. Therefore, primary renal synovial sarcoma has been moved to the mesenchymal tumor category.