Primary renal cell carcinomas (RCCs) with both papillary

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1 Renal Cell Carcinoma With Clear Cell and Papillary Features Hillary Ross, MD; Guido Martignoni, MD; Pedram Argani, MD N Context. The diagnosis of primary renal cell carcinomas (RCCs) with both papillary architecture and cells with clear cytoplasm can be diagnostically challenging for practicing pathologists. The 4 main neoplasms in the differential diagnosis are clear cell RCC, papillary RCC, clear cell papillary RCC, and Xp11 translocation RCC. Accurate diagnosis has both prognostic and therapeutic implications. Objective. To highlight the helpful cytomorphologic, immunohistochemical, and cytogenetic features of each of these entities to enable reproducible classification. Primary renal cell carcinomas (RCCs) with both papillary architecture and cells with clear cytoplasm pose a difficult diagnostic challenge for pathologists. The most common RCC, clear cell RCC, only rarely has papillary architecture. The second most common RCC, papillary RCC, only rarely contains clear cells. However, recently described but less-common RCCs, clear cell papillary RCC and Xp11 translocation RCC, characteristically feature both papillary architecture and cells with clear cytoplasm. Accurate diagnosis of these distinct entities has both prognostic and therapeutic implications. 1 3 In most cases, routine hematoxylin-eosin cytomorphologic features are sufficient to make or to strongly suggest the correct diagnosis. In most of the other cases, immunohistochemical markers (such as TFE3 and carbonic anhydrase IX [CAIX]) can establish the correct diagnosis. Finally, although less often available, and hence, less commonly used, cytogenetic and molecular pathology assays are the most definitive methods of establishing the correct diagnosis. This review aims to highlight the helpful Accepted for publication November 16, 011. From the Departments of Pathology (Drs Ross and Argani) and Oncology (Dr Argani), Johns Hopkins Medical Institutions, Baltimore, Maryland; and the Department of Pathology, University of Verona, Verona, Italy (Dr Martignoni). The authors have no relevant financial interest in the products or companies described in this article. Reprints: Pedram Argani, MD, Surgical Pathology, Johns Hopkins Hospital, Weinberg Building, Room 4, 401 N Broadway St, Baltimore, MD ( pargani@jhmi.edu), or Guido Martignoni, MD, Anatomic Pathology, University of Verona, Pz.le L. Scuro 10, 37134, Verona, Italy ( guido.martignoni@univr.it, guidomart@yahoo.com). Data Sources. Published peer-reviewed literature was reviewed, accompanied by the authors personal experiences. Conclusions. Key morphologic clues and a focused immunohistochemical panel, including CK7, a-methylacyl coenzyme A racemase (AMACR), TFE3, cathepsin K, and carbonic anhydrase IX (CAIX), now allow most resected RCCs with papillary architecture and clear cells to be accurately classified. In other cases, cytogenetic and molecular findings can establish the diagnosis. Despite these tools, some RCCs with papillary architecture and clear cells do not fit into any of the described entities and currently remain unclassified. (Arch Pathol Lab Med. 01;136: ; doi: / arpa ra) cytomorphologic, immunohistochemical, and cytogenetic features of each of these entities to enable reproducible classification. CLEAR CELL RCC Clear cell RCC represents the most common type of RCC, comprising approximately 60% of all renal tumors. It is almost -fold more common in males than in females, with a peak incidence in the sixth and seventh decades. 4 The classic clinical triad of hematuria, pain, and palpable mass now occurs in only a few cases. Currently, most cases are asymptomatic and identified on imaging as an incidental renal mass. Imaging demonstrates hypervascularity as compared with papillary RCC, which is frequently hypovascular. 5 Tumor stage and Fuhrman nuclear grade best predict outcome. 6,7 Clear cell RCC usually presents grossly as a solitary, golden yellow mass, which reflects the high lipid content of its cells. Cystification with areas of hemorrhage and necrosis may be prominent in higher-grade tumors, which have less lipid and are thus less distinctly yellow. The predominant histologic architecture is solid or acinar, with clear cells separated by hypervascular, thin, fibrous septa. The neoplastic cells of low nuclear grade clear cell RCC (Fuhrman grade 1 ) typically have water-clear, virtually agranular cytoplasm. However, high nuclear grade clear cell RCC typically has more granular, eosinophilic cytoplasm. Rarely, true cases of low-grade clear cell RCC with welldeveloped papillae do exist (Figure 1, A through D). In other low-grade tumors, small papillations lined by clear cells protrude into cystic spaces. More commonly, high-grade clear cell RCC with necrosis demonstrate pseudopapillae caused by fragmentation of the acinar Arch Pathol Lab Med Vol 136, April

2 Figure 1. Clear cell renal cell carcinoma. A, True papillary formation with clear cells projecting into cystic spaces. B, Membranous CD10 labeling. C, Diffuse complete membranous CAIX labeling. D, CK7 is negative (hematoxylin-eosin, original magnification 364 [A]; original magnifications 364 [B through D]). architecture. A helpful feature is the presence of eosinophilic cytoplasm in high-grade tumors, rather than clear cytoplasm. The immunohistochemical (IHC) profile of clear cell RCC typically is that of strong reactivity to cytokeratin CAM 5., vimentin, CD10, epithelial membrane antigen (EMA), RCC marker, and PAX8 and/or PAX. Additionally, diffuse membranous staining for CAIX is present, reflecting inactivation of the von Hippel-Landau (VHL) gene and constitutive activation of the hypoxia-inducible factor (HIF) pathway (see below). Notably, clear cell RCCs are typically negative for both cytokeratin 7 (CK7) and amethylacyl-coenzyme A racemase (AMACR), although both may be focally positive, especially in higher-grade tumors. CK7 may also label cystic areas.8 Cathepsin-K and TFE3, markers of Xp11 translocation RCC, are consistently negative. Clear cell RCCs have consistent genetic abnormalities. A deletion on chromosome arm 3p, where the VHL gene resides, is present in most sporadic and familial tumors.9 11 VHL gene mutations have been reported in at least half of these tumors.9 Recently, molecular analysis of 05 wellcharacterized clear cell RCCs revealed VHL gene mutations in 8% of cases. An additional 8% showed 39 Arch Pathol Lab Med Vol 136, April 01 hypermethylation in the VHL promoter CpG islands.1 The VHL gene product, pvhl, regulates transcription of genes through HIF. Normal pvhl targets HIF1-a for degradation in normoxemic states. When the VHL gene is mutated or the VHL protein is absent, conditions of hypoxia are simulated and HIF1-a accumulates. HIF1-a activates multiple downstream genes, including vascular endothelial growth factor (VEGF), a glucose transporter (GLUT-1), and carbonic anhydrase IX (CAIX). The latter causes the characteristic diffuse IHC labeling for CAIX in clear cell RCC. Highlighting the utility of cytogenetics in the differential diagnosis of RCC with clear cells and papillary architecture, studies13 14 analyzed the morphology and cytogenetics of cases classified as papillary RCC with extensive clear cell change. The investigators found consistent 3p deletion and lack of trisomy of chromosomes 7 and 17 with Gbanding techniques13 and molecular loss of heterozygosity and fluorescence in situ hybridization (FISH) techniques.14 Despite the dominant papillary pattern, these cases are, in our view, best classified as clear cell RCC, although no immunohistochemistry was performed in these studies to corroborate that classification. Importantly, some of these neoplasms were of low Fuhrman grade (1 and ),

3 Figure. Papillary renal cell carcinoma. A, Delicate papillary fronds lined by clear cells. B, Foamy macrophages in fibrovascular cores and hemosiderin deposition in neoplastic epithelial cells. C, CK7 demonstrates membranous labeling. D, a-methylacyl coenzyme A racemase (AMACR) shows strong cytoplasmic labeling (hematoxylin-eosin, original magnifications 364 [A and B]; original magnifications 364 [C and D]). suggesting that the papillae did not result from degeneration of acini as commonly seen in higher grade neoplasms. In summary, the most helpful morphologic clue for a clear cell RCC is hypervascular septa intimately associated with the neoplastic cells with optically transparent cytoplasm. Features further supporting the diagnosis are diffuse, strong, complete membranous CAIX labeling and the absence of CK7 labeling by IHC, as well as chromosome arm 3p loss by genetics. PAPILLARY RCC Papillary RCC comprises 11% to 0% of renal cortical neoplasms, with an approximately 3:1 male to female ratio. Similar to other types of RCC, more than 50% of cases present as incidental masses discovered on imaging workup for other causes.4 In contrast to other RCC subtypes, papillary RCC is the type most often multifocal (up to 45% of cases) and these typically are of independent origin.15 As mentioned previously, these tumors are hypovascular relative to clear cell RCC. Findings on magnetic resonance imaging have been correlated with hemosiderin deposition within the tumor.16,17 The reported 5-year disease-free survival is more favorable in papillary RCC than it is in clear cell RCC.1, Although Arch Pathol Lab Med Vol 136, April 01 the stage of the tumor has prognostic significance, the value of grading remains controversial. Grossly, papillary RCC is usually well circumscribed and surrounded by a fibrous pseudocapsule. On a cut section, neoplasms with prominent hemorrhage can appear brown, friable, and extensively necrotic, whereas those with abundant stromal foamy macrophages (see below) may appear yellow and mimic clear cell RCC. Histologically, the classic architecture demonstrates discrete papillary fronds with fibrovascular cores lined by neoplastic cells (Figure, A). However, tubulopapillary and solid papillary growth patterns are also common. Foamy histiocytes are characteristically present within the fibrovascular cores. Hemosiderin deposition in neoplastic epithelial cells and hemosiderin-laden macrophages can be prominent (Figure, B). Cytologically, the neoplastic cells can vary from having scant amphophilic cytoplasm with nuclei typically arranged in a single cell layer (so called type 1 papillary RCC) to having abundant highly eosinophilic cytoplasm with pseudostratified nuclei (type papillary RCC). Clear cell change and fine cytoplasmic granulations are typically seen in association with the hemosiderin deposition and/or necrosis. The cytoplasmic clearing may reflect phagocytic activity of the renal 393

4 Figure 3. Clear cell papillary renal cell carcinoma. A, Papillary fronds lined by clear cells with nuclei polarized away from the basement membrane. B, Prominent smooth muscle hyperplasia within the tumor. C, CAIX is positive but shows an absence of staining along the luminal surface. D, CK7 demonstrates complete membranous labeling (hematoxylin-eosin, original magnifications 364 [A] and 340 [B]; original magnifications 3100 [C] and 364 [D]). carcinoma cells in these settings. Clear cell change can become quite extensive in some tumors causing morphologic confusion with clear cell RCC. The IHC profile of papillary RCC typically shows strong membranous positivity for CK7 (Figure, C), moreconsistently positive in type 1 than in type papillary RCC. Staining with AMACR demonstrates diffuse cytoplasmic granular positivity (Figure, D). Staining with CD10, PAX, PAX8, and RCC marker are also usually positive, similar to clear cell RCC. The CAIX stain is either completely negative or may be focally positive near areas of necrosis, reflecting foci of physiologic hypoxia. Cathepsin-K and TFE3 are both consistently negative. Cytogenetic studies show distinctive abnormalities unique to papillary RCC. Most of these tumors are characterized by trisomy of chromosomes 7 and 17 along with loss of Y. Typically the 3p arm is intact, in contrast to clear cell RCC. In summary, one may occasionally see clear cell changes in papillary renal cell carcinomas. The clues for accurate diagnosis are finely granular cytoplasm and fine, pigmented cytoplasmic reticulations representing hemosiderin. The diagnosis is supported by strong diffuse CK7 and AMACR positivity but only focal CAIX expression. 394 Arch Pathol Lab Med Vol 136, April 01 Cytogenetic findings of trisomy 7 and 17 further support the classification of the tumor as papillary RCC. CLEAR CELL PAPILLARY RCC Clear cell papillary RCC is a recently characterized, distinctive renal neoplasm. It was initially described in patients with end-stage renal disease18 but is now known to arise in healthy kidneys as well. These tumors tend to be singular and small (,5 cm), but multifocality can be present, especially in the setting of end-stage renal disease. The patients may harbor additional types of RCC elsewhere in the kidney. Limited outcome data are available because of its only very recent description in the literature. A review by Aydin et al19 of the cases published to date suggests that the tumor is indolent. No evidence of disease recurrence was seen in 40 patients with a mean follow-up of 8 months. Grossly, these tumors have a thick capsule and are often cystic. The architecture can show a wide range, including true papillary structures, branching tubules, and solid acinar nests or ribbons. The papillary structures and tubules are lined by cells with clear cytoplasm and low Fuhrman nuclear grade (Figure 3, A). The nuclei are

5 typically polarized away from the basement membrane, creating a characteristic subnuclear vacuole similar to that seen in secretory endometrium. Intraluminal proteinaceous secretions are often present. Pertinent negatives include lack of foamy histiocytes, psammomatous calcifications, or hemosiderin. Signs of biologic aggressiveness should also be absent, such as renal sinus invasion, vascular invasion, tumor necrosis, and high nuclear grade. The stroma of clear cell papillary RCC not infrequently demonstrates smooth muscle metaplasia (Figure 3, B). The extreme end of this spectrum likely includes tumors reported as renal angiomyoadenomatous tumors to reflect the prominence of smooth muscle. 0 Renal angiomyoadenomatous tumors appear to have the same clinicopathologic and immunohistochemical characteristics as clear cell papillary RCC. Despite vigorous arguments to the contrary, 1 a recent abstract 3 comparing these entities suggests that they can now be considered as a spectrum in the same category of tumors. The smooth muscle stromal metaplasia and proliferation, however, is not entirely specific to this entity. For example, smooth muscle stromal metaplasia has been reported in association with clear cell RCC. 4,5 Although these reported tumors showed prominent angioleiomyoma-like stroma, some have demonstrated chromosome 3 aberrations typical of clear cell RCC. 5 Recent data, however, present questions about the relationship of the latter smooth muscle rich neoplasms to clear cell RCC. 6 In addition, Xp11 translocation RCC may show smooth muscle stroma. 7 Hence, we suggest that smooth muscle metaplasia in the kidney (as in the lung) may be a nonspecific common reaction to a variety of stimuli. Clear cell papillary RCC displays a unique IHC profile (Figure 3, C and D) overlapping with clear cell RCC and papillary RCC. Like clear cell RCC, CAIX is diffusely positive; however, in contrast to clear cell RCC, CD10 is usually negative. CAIX commonly shows an absence of staining along the luminal surface, yielding a cuplike staining pattern. 8 Like papillary RCC, CK7 is diffusely positive, but, in contrast to papillary RCC, AMACR is negative. Cathepsin-K and TFE3 are consistently negative. Patchy labeling for high molecular weight cytokeratin (34bE1) is common and useful in the differential diagnosis. Cytogenetically, clear cell papillary RCC lacks typical abnormalities seen in either clear cell RCC or papillary RCC. Clear cell papillary RCC does not demonstrate 3p deletion or trisomy 7/17. 19,9 VHL gene mutation and promoter methylation are also absent. 19 Interestingly, a recent article 8 demonstrated overexpression of other IHC markers of the HIF-activation pathway in addition to CAIX. These tumors were confirmed by sequencing to lack the VHL gene mutation or copy number alteration as reported in previous studies; in fact, VHL messenger RNA tended to be overexpressed in these tumors. This led to the conclusion that the HIF pathway is activated by a non- VHL dependent mechanism in clear cell papillary RCC. 8 Of note, distinctive genetic alterations have been reported in renal angiomyoadenomatous tumors. Fluorescence in situ hybridization analysis demonstrated 4 cases with monosomy for chromosomes 11 and 16 with 3 of those cases (75%) showing additional monosomy for chromosome One might expect similar findings in clear cell papillary renal cell RCC. However, Adam et al 31 recently performed comparative genomic hybridization on 7 cases of clear-cell papillary RCC and demonstrated a lack of any detectable chromosomal imbalances. In summary, clues to the diagnosis of clear cell papillary RCC include unique subnuclear cytoplasmic clearing, low nuclear grade, prominent smooth muscle stroma, and in some cases an end-stage background kidney. The morphology and typical immunohistochemical pattern of positive staining for CK7 and CAIX but negative for CD10 and AMACR should lead to accurate classification. Absence of 3p loss or trisomy of chromosomes 7 and 17 support the above diagnosis. XP11 TRANSLOCATION RCC Xp11 translocation RCCs are a group of neoplasms defined by chromosomal translocations involving the TFE3 transcription factor gene located at the Xp11. locus. TFE3 belongs to the same family of transcription factors as MiTF, TFEB and TFEC. The translocation fuses TFE3 to one of multiple possible reported partner genes, including ASPL, PRCC, NonO (p54nrb), PSF, CLTC, and unknown genes on chromosomes 3 and All of the gene fusions result in overexpression of the TFE3 protein, making it detectable by immunohistochemistry. The most commonly seen translocation is t(x;17)(p11.q5), which fuses the ASPL and TFE3 genes. 36 Of note, the same ASPL-TFE3 gene fusion is seen in alveolar soft-part sarcoma, a rare soft tissue tumor of uncertain histogenesis. RCCs of any type are rare in children and constitute 5% of pediatric renal neoplasms. However, Xp11 translocation RCC likely comprises the most common subtype seen in this age group. A prior history of chemotherapy is the only known risk factor for Xp11 translocation RCC and is associated with up to 15% of cases. Reportedly, patients received a DNA topoisomerase II inhibitor and/or an alkylating agent, 38 both of which break DNA and, thus, may promote translocation. Recurrence can be seen years after treatment; one report 39 described widespread metastases in a patient with Ewing sarcoma 17 years after treatment. The diagnosis of Xp11 translocation RCC may have been previously underestimated in adults. In a large, single-institution study, 443 consecutive nephrectomies were examined by cytogenetics or TFE3 immunohistochemistry to reveal an Xp11 translocation RCC incidence of 1.6%. 40 Another study that used both IHC and FISH confirmation estimated a 4.% incidence in patients older than 18 years. 41 Although Xp11 translocation RCC may be considered rare in the adult population, RCCs, in general, have a much higher incidence in adults. If one estimates new cases of RCC in adults in the United States each year, 4.% would total 160 cases, which vastly outnumber the estimated 5 new pediatric RCCs each year in the United States. 4 Imaging can sometimes be a helpful diagnostic clue for Xp11 translocation RCCs. Similar to the other neoplasms in the differential diagnosis, a unilateral and unifocal renal mass is typically seen. However, extensive psammomatous calcification may be evident radiographically, especially in those neoplasms bearing the ASPL-TFE3 fusion. Because these neoplasms have been only recently recognized, outcome data are still premature and good long-term follow-up data are necessary. Published outcome series in adults show a poor prognosis. Adults often present with high stage and distant metastases. In a small series, 5 of 6 patients (83%) developed hematogenous Arch Pathol Lab Med Vol 136, April

6 Figure 4. Xp11 translocation renal cell carcinoma. A, Papillary and nested architecture with clear to granular eosinophilic cytoplasm. B, Area of tumor with smaller cells surrounding discrete hyaline material, mimicking tumors with a-tfeb gene fusion. C, PAX8 demonstrates positive nuclear labeling. D, MART-1 is diffusely positive in this case. E, Diffuse immunoreactivity for cathepsin K. F, TFE3 immunostain showing specific, strong nuclear immunoreactivity. Note the appropriately negative stromal cells. This case was confirmed by TFE3 break-apart fluorescence in situ hybridization testing (not shown) (hematoxylin-eosin, original magnifications 340 [A] and 364 [B]; original magnifications 364 [C through E] and 3100 [F]). 396 Arch Pathol Lab Med Vol 136, April 01

7 Clinical, Morphologic, Immunohistochemical, and Genetic Features Useful for Classifying Renal Cell Carcinomas (RCCs) With Clear Cells and Papillary Architecture Immunohistochemistry RCC Type Clinical Clear cell RCC with papillae Papillary RCC with clear cells Rare Often multifocal Clear cell papillary RCC Sometimes ESRD Xp11 translocation RCC Often young patients Morphologic Clue CK7 AMACR TFE3 CAIX Other IHC Genetics Hypervascular septa Clear cells in areas of hemorrhagic degeneration Nuclei polarized away from fibrovascular cores, cystic Voluminous cytoplasm, psammoma bodies 3p loss focal Trisomy 7,17 Often CD10, CK903 Neither 3p loss nor trisomy 7,17 /focal focal Often cathepsin K Xp11 translocation Abbreviations: ESRD, end-stage renal disease; IHC, immunohistochemistry; AMACR, a-methylacyl coenzyme A racemase. Figure 5. Unclassified renal cell carcinoma. A, Nested clear cells with variable papillary formations. B, Fuhrman grade 3 nuclei without subnuclear vacuoles inconsistent with a diagnosis of clear cell papillary renal cell carcinoma. C, Diffuse immunoreactivity for CK7. D, Diffuse membranous labeling for CAIX (hematoxylin-eosin, original magnifications 364 [A] and 3100 [B]; original magnifications 364 [C and D]). Arch Pathol Lab Med Vol 136, April

8 spread during the 1-year follow-up, and patients (33%) died. 33 In another small series of 5 adult cases, all (100%) followed a rapidly terminal course, with a mean survival of 18 months (range, 10 4 months). 43 In contrast, children presenting with regional lymph node involvement and lack of hematogenous spread appear to have a favorable short-term prognosis. Despite this locally high-stage presentation, 14 out of 15 (93%) of these reported patients remained disease free with a mean follow-up of 6.3 years. 44 The gross appearance of Xp11 translocation RCC often resembles clear cell RCC as a tan-yellow tumor that can be both hemorrhagic and necrotic. Some cases are deceptive grossly because they consist of grey, friable papillae, which can simulate necrosis (P.A., unpublished data, February 011). Microscopically, the most distinctive and common pattern is the presence of both clear cells and papillary architecture. Significant overlap exists between the different gene fusions, and heterogeneity can exist within the same tumor. Tumors can show cystic spaces as well as solid or nested growth (Figure 4, A). Nuclei tend to be high grade, and the cytoplasm may be clear to granular and eosinophilic. The ASPL-TFE3 gene fusion, in particular, is associated with voluminous cytoplasm, vesicular nuclei, and numerous psammoma bodies within the stroma. Otherwise typical Xp11 translocation RCC can even have a subpopulation of smaller, bluer cells surrounding discrete hyaline material mimicking RCCs with the t(6:11)(p1;q1) translocation and the resulting Alpha-TFEB gene fusion (Figure 4, B). The IHC profile of Xp11 translocation RCC differs from that of both clear cell RCC and papillary RCC (Figure 4, C through F). Epithelial markers, such as AE1/AE3, CK7 and epithelial membrane antigen (EMA), tend to be underexpressed. Vimentin can be negative. However, similar to both clear cell RCC and papillary RCC, PAX8 is positive. Rarely, Xp11 translocation RCCs can express a melanocytic marker, such as Melan-A and HMB-45. Although they may express HIF-1a like clear cell RCC does, they only focally express the downstream target CAIX. 45 In the setting of RCC, cathepsin K, a lysosomal protease, is a highly specific, but somewhat insensitive, marker for Xp11 translocation RCCs. Similar to what MiTF does in osteoclasts, the TFE3 fusion gene product appears to activate expression of cathepsin K. Other renal neoplasms, including clear cell RCC and papillary RCC, are negative for this marker. 46 Of note, RCCs bearing the APSL-TFE3 gene fusion are more likely to be negative for cathepsin K expression than are RCCs with other TFE3 gene fusions. 47 The most sensitive and specific IHC marker is strong nuclear labeling for TFE3. 48 This assay uses an antibody to the C-terminal portion of the TFE3 transcription factor, which is retained in the fusion product. Although TFE3 protein is ubiquitously expressed, it is normally at levels undetectable by immunohistochemistry. The fusion product brings TFE3 in close proximity to a novel promoter, leading to overexpression of the fusion protein. Unfortunately, this assay can be technically challenging because suboptimal fixation can cause high background native TFE3 staining. Adjacent tubules provide a helpful negative control. A TFE3 break-apart FISH assay can be performed on paraffin-embedded tissue and can be a useful tool to provide molecular confirmation of the diagnosis. 49,50 Our experience is that the FISH assay can resolve cases in which the TFE3 IHC is equivocal. 51 In summary, papillary architecture with clear cells is quite typical of an Xp11 translocation RCC. Specific clues to suspect the diagnosis include the young age of the patient, stromal psammomatous calcifications, and underexpression of cytokeratins by immunohistochemistry. Support for the diagnosis can be achieved with cathepsin K immunoreactivity and TFE3 positivity by IHC or TFE3 gene rearrangement by FISH. RCC, UNCLASSIFIED Despite all the diagnostic tools available, one may occasionally encounter tumors that do not fit neatly into any of the entities described thus far. Renal cell carcinoma with papillary architecture and clear cells has traditionally fallen into this category. However, most resected RCCs with papillary architecture and clear cells can now be appropriately classified using morphologic, immunohistochemical, and cytogenetic features (Table). This is likely also true of core biopsies. A recent study 51 demonstrated that diagnostic accuracy was improved for biopsies obtained ex vivo from 83% using just hematoxylin-eosin morphology to 90% using IHC markers. This study 51 included several cases of RCC with papillary architecture and clear cells. Nevertheless, unclassifiable cases remain. An example of an unclassified renal cell carcinoma diagnosed at our institution is seen in Figure 5, A through D. Microscopically, the tumor displayed both papillary architecture and clear cells. The IHC markers demonstrated focal CD10, diffuse CK7, and diffuse CAIX. Cathepsin K and TFE3 were negative. Although the immunoprofile suggested clear cell papillary RCC, the high Fuhrman grade and absence of characteristic subnuclear vacuoles precluded a definitive diagnosis. The neoplasm was signed out as an unclassified renal cell carcinoma. Similar results have been reported in the literature, even after both IHC and genetics have been performed. Gobbo et al 5 used a combined IHC and cytogenetic approach to evaluate 14 RCCs with papillary architecture and clear cells that had been considered unclassified renal cell carcinomas after review of the hematoxylin-eosin sections alone. CK7, AMACR, and TFE3 were used in the IHC analysis. Fluorescence in situ hybridization was used to detect signal abnormalities in chromosomes 7, 17, and Y and for 3p deletion. These techniques enabled 1 of 14 cases (86%) to be definitively categorized, 9 as papillary RCC and 3 as clear cell RCC. However, cases (14%) remained unclassified: one with neither trisomy 7/17 nor 3p loss and one with both trisomy 7/17 and 3p loss. 5 In another study, Mai et al 53 analyzed more than 60 cases of RCC with clear cells and papillary architecture by IHC and FISH. They were able to divide the cases in 3 groups; 15 (3%) clear cell RCC, 5 (38%) papillary RCC, and 6 (39%) combined tumors. Some of the latter tumors showed trisomy 7/17 in areas typical of papillary RCC and both trisomy 7/17 and 3p loss in areas with clear cells, suggestive of an evolution from papillary RCC to clear cell RCC. 53 Although intriguing, the latter category of tumors should remain unclassified at this time, pending further knowledge. Such unclassified tumors represent a logical area for further study. CONCLUSIONS In conclusion, key morphologic clues and a focused immunohistochemical panel, including CK7, AMACR, TFE3, cathepsin K, and CAIX, now allow most resected RCCs with papillary architecture and clear cells to be 398 Arch Pathol Lab Med Vol 136, April 01

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