Over the past decades, major technological and scientific breakthroughs have allowed for the development of important clinical tools for better RCC detection, evaluation, and therapeutic decision-making, while also reshaping medical understanding of RCC pathogenesis and progression-driving molecularities. Even so, RCC remains, to this day, one of the deadliest urological malignancies, accounting for ~3% of the total cancer burden in the global adult population 
The widespread integration of ultrasonography (US) and computer tomography (CT) in routine clinical practice has significantly improved RCC detection yields, reflected in the long-standing and still ongoing stable increase in RCC incidence rates globally 
. Concurrently, a significant drop in RCC initial stage at diagnosis has also occurred, with most RCCs being detected incidentally currently, as asymptomatic, localized, small renal masses. Within this subgroup of RCC cases, 5-year survival rates have been significantly improved as a consequence of earlier curative intervention, i.e., partial/radical nephrectomy 
. Conversely, despite important recent progress in systemic RCC therapeutic management, advanced RCC patients remain incurable, with persistently poor clinical outcomes. Thus, overall, RCC-specific mortality rates have been steadily increasing since the 1990s (~1.1%/year) 
To address these pervasive clinical limitations, regarding systemic/recurrent RCC therapeutic management and outcomes, contemporary RCC clinical definitions have been greatly nuanced and refined to better serve in RCC subtyping, prognosis assessment, and therapeutic response prediction. Classically, RCCs were simply defined as malignant parenchymatous renal neoplasms, spawning from tubular epithelial cellularity. However, in light of recent extensive RCC genomic profiling efforts and comprehensive metabolic molecular analyses, RCCs are currently viewed as an extremely heterogenic group of distinctive tumor subtypes, which only share an anatomical origin, while having different cellular progenitors, within the renal parenchyma. In fact, individual RCC subtypes demonstrate relatively specific, yet widely pleomorphic, tumor-driving molecular pathologies and pathognomonic genomics. This emerging intricate molecular amalgamation of novel RCC metabolic profiles, is firmly corroborated by the similarly extensive and well-documented clinical variability in RCC malignant behavior and therapeutic response, both between different RCC cases and disease phenotypes comparatively, but also during the natural evolution of individual RCCs (disease progression, systemic dissemination, and/or metastatic recurrence) 
In lack of a definitive systemic treatment modality for advanced RCCs, integration of the amounting fundamental medical knowledge regarding RCC molecular pathology into RCC clinical management has been paramount to obtaining improved risk stratification and evidence-based therapeutic decision-making. As a result, definitive pathological diagnosis and RCC subtyping have become more nuanced and cumbersome, with the latest guidelines (i.e., the World Health Organization—WHO’s 4th edition of the Urological Tumors Classification, 2016) identifying almost twenty distinct subtypes of malignant renal cell tumors, alongside mesenchymal, neuroendocrine, nephroblastic, and cystic variants 
. Even so, these classifications will only become more comprehensive further on, as additional RCC entities, with distinguishing clinical, morphological, immunohistochemical, and/or molecular traits, have already been identified and are still awaiting formal acknowledgement 
To further complicate the already elaborate and dynamic landscape of RCC clinical management, systemic RCC therapy has been greatly diversified recently, with the advent of personalized oncological therapy and RCC tumor microenvironment molecular/genomic characterization initiatives. Notably, focused evaluation of the biologic implications of various inflammatory pathways regarding RCC metabolomics and proliferation, mainly the Von Hippel–Lindau (VHL), mechanistic target of rapamycin (mTOR), tumor necrosis factor (TNF), and Janus kinase/signal transducer and activator of transcription (JAK/STAT) 
pathways, has proven fruitful, providing multiple clinically relevant RCC-associated-antigen targeted molecules. Moreover, immunotherapy, a novel and promising type of oncotherapy, aiming to reactivate the cytotoxic immune response against tumor cells, has found important applications in advanced/recurrent RCC therapeutic management. Despite clinical difficulties regarding adequate immunotherapy response prediction for RCCs and the extensive therapy-associated costs, immunotherapy represents the only type of oncotherapy which allows, even theoretically, for the elaboration of a definitive cure for disseminated RCCs/cancer in general, as it is the only oncotherapy capable of targeting and annihilating non-dividing, quiescent, stem-like dysplastic cells 
Currently, as clinical definitions have become greatly nuanced, a complete RCC diagnosis requires careful multimodal evaluation, in a well-coordinated and phased approach, involving, at times, very difficult differentials.
2. Conventional Staining and RCC Microscopic Morphological Evaluation
Notwithstanding the long-held, central role of RCC morphological evaluation, using standard microscopy and conventional hematoxylin–eosin (HE) staining, in RCC definitive pathological diagnosis, essential for further therapeutic decision-making and predicting prognosis/risk stratification, in light of recent insights into RCC molecular pathology, the method, although still important, has become insufficient for adequate RCC subtyping. Even within the classical triad of conventional morphologies, namely clear cell (ccRCC), papillary (pRCC), and chromophobe (chRCC), which demonstrate relatively distinctive architectures and cellular features, microscopic evaluation of morphology remains subjective and user-dependent. Additionally, more often than not, meticulous evaluation will encounter, at least focally, overlapping patterns and/or atypical architectures, hindering definitive RCC subtyping. High-grade cellularity and severely dedifferentiated renal tumors lose characteristic morphological traits completely and cannot be subtyped using solely conventional staining microscopy. Determining metastatic cell origin, in advanced RCCs, with the additional difficulty of non-renal origin differentials, will most certainly require ancillary studies 
In fact, as a general consensus among pathologists, no truly reliable, objective, and validated histologic/ultrastructural criteria exist for distinguishing between benign and malignant renal epithelial tumors 
, with the very meek exception of oncocytomas and small low-grade papillary adenomas (≤5 mm) 
. Furthermore, the amounting, emergent RCC clinical entities, have shown, for the most part, unspecific and poorly defined architectures, with heavily disputed morphological growth characteristics. Even within conventional RCC morphologies, absolute homogeneity is extremely rare and non-specific arrangement patterns (solid, papillary, tubular/cystic, sarcomatoid/rhabdoid) and cellular features (cytoplasmic clearing, eosinophilia/basophilia) are routinely identified, particularly in high-grade and/or poorly preserved tumor areas. Clearly, meticulous sampling of gross RCC specimens, especially in those which prove to be difficult to classify, will prove to be much more useful and cost-effective than additional staining. Transitional areas, from well-differentiated/low-grade patterns to more unusual, pleomorphic patterns, should be sought after and evaluated preferentially, as they usually offer the most useful diagnostic information 
All in all, conventional microscopic evaluation of RCC morphology remains an important initial step in tumor pathological analysis. It allows for an essential primary classification into RCC predominant morphological trait subgroups, simplifying the differential diagnosis and guiding further targeted analysis, while greatly reducing financial costs. Even so, it has become insufficient for an accurate definitive RCC diagnosis and prognosis assessment.
3. Definitions and Comprehensive Profiles for RCC Subtyping
Definitive RCC pathological diagnosis, meaning irrefutable demonstration of tumor renal cell origin and accurate RCC subtype identification, as well as the subsequent prognosis assessment, requires careful integration of clinical, pathological, and molecular characteristics, to allow for objective carcinogenic metabolic profiling and characterization of progression-driving tumor biology. Thus, as our understanding of RCC molecular pathology has become more nuanced, ancillary studies, mainly immunohistochemistry (IHC), has become integrated in clinical practice, as essential tools for routine RCC pathological evaluation. Despite the plethora of seemingly promising diagnostic and predictive applications reported, for the dozens of novel tumor-associated molecules and their corresponding IHC targeted-antibodies, due to the lack of validation studies, these data constitute no more than level 2/3 evidence. Moreover, heterogeneity within existing data regarding specific IHC methodology and antibody clone used (monoclonal/polyclonal) further encumbers comparisons between existing results 
. In fact, as an investigative method, IHC has multiple inherent conceptual limitations, as well as significant technical (clone selection, titration, validation, false positives/negatives etc.) and interpretative (subcellular localization and pattern) weaknesses 
. Table 1
provides a comprehensive summary of evidence regarding morphological, IHC, and genetic profiles for all currently accepted RCC subtypes.
A recurring issue is represented by the indiscriminate use of an inordinate number of antibodies, without a reasonable, structured, diagnostic approach, which only serves to generate additional confusion, while simultaneously wasting valuable resources. For this reason, in 2012, the International Society of Urological Pathology (ISUP) reviewed the use of IHC antibodies in adult renal tumors, in order to develop best practice recommendations regarding determining site of origin, typing, and prognosis, with the ultimate goal of establishing formal panels of biomarkers for specific diagnostic difficulties and, implicitly, establishing adequate guidelines for stewardship 
. For a more systematic and practical approach to RCC subtyping, individual, differential diagnosis-driven IHC marker panels have been established (see Table 2
), in order to differentiate among entities within the main RCC subgroups, manifesting specific morphological characteristics, namely predominantly clear cell (cc) population, significant papillary (p) component, extensive cytoplasmic eosinophilia, predominant sarcomatoid growth pattern, and architecture suggestive of distal nephron origin—collecting duct carcinoma (CDC) and renal medullary carcinoma (RMC). Quantitative and qualitative assessments of the staining results are equally important and continuous refinement of antibody panels, taking into account the proven value of new IHC markers and new clones of established markers, as they enter the market, is mandatory 
Overall, Paired box (PAX) 8, a transcription factor (415 aminoacids/48 kDa), involved in kidney, thyroid, and paramesonephric duct-derived, organogenesis, and homeostasis, represents the most useful IHC marker for establishing the diagnosis of metastatic RCC(mRCC) 
, being expressed in all RCC subtypes, including sarcomatoid RCC, mucinous tubular and spindle cell carcinoma (MTSC), and microphtalmia (MiT)-family translocation RCC, with a sensitivity of approximately 95% 
. In healthy kidney tissue, PAX8 normally manifests diffuse staining in the, preferentially distal, renal tubules, and patchy staining of urothelium in the renal pelvis. In accordance with this pattern of developmental expression, in addition to RCCs, PAX8 consistently stains Műllerian neoplasms and thyroid neoplasms, and, in smaller subsets (~20%), urothelial carcinomas of the renal pelvis, Wolffian duct lesions, and thymic neoplasms 
Out of the PAX gene family for tissue specific transcription factors 
, PAX8 is generally the more sensitive marker 
. PAX2 stains similarly to PAX8 
; with the possibly useful difference of negative PAX2 staining in thyroid neoplasms, admittedly only reported in small series 
. When using older, polyclonal preparations, endocrine neoplasms (pancreatic islet cell tumors and gastrointestinal tract carcinoids) are often positive for PAX8; however, cross-reactivity with PAX6 is clarifying 
. Additionally, B-cell lymphomas stain positive on polyclonal PAX8 preparation, while also manifesting cross-reaction with PAX5.
Differential diagnosis-driven IHC panels and profiles for morphological RCC traits 
Novel PAX8 antibodies (PAX8R1), to address the specificity issues of polyclonal PAX8 preparations, have been developed. They bind to the C-terminal of PAX8, targeting amino acids 318–426, which are highly divergent among PAX proteins, thus abolishing cross-reactivity with other PAX species 
Further nuancing of IHC differential diagnosis in mRCC involves markers almost always negative in RCC, such as pulmonary marker TTF-1, the intestinal marker homeobox protein CDX2, p63, prostate-specific antigen, and estrogen receptor, which will be useful in excluding other carcinomas that may manifest cross-positivity for PAX8. Positive staining with any of the aforementioned markers represents a strong argument against the diagnosis of mRCC 
. Another useful distinction, between urothelial and renal epithelial origins, can be made using GATA3, a transcription factor involved in cell differentiation and proliferation in a variety of tissues and cell types, which will be expressed in most urothelial tumors, but not in RCCs 
Other supportive IHC markers of mRCC, currently in common practice—cluster of differentiation (CD)10, RCC marker antigen (RCCm), Kidney-Specific Cadherin (Ksp-cadherin)—manifest inadequate specificity and are not usually indicated or useful, outside of very specific, punctual, diagnostic subtleties.
RCCm stains a proximal tubular antigen and demonstrates focal labeling in approximately 80% of RCC 
, yet with notoriously poor specificity, seeing as it also labels many other carcinomas (breast, lung, colon, and of adrenal origin). It is useful in differentiating clear cell RCC (ccRCC) from ovarian clear cell carcinoma, as PAX8 would be positive in both tumors, whereas RCCm would be positive only in the renal neoplasm. Moreover, PAX8 is negative in adrenal cortical neoplasms, which preferentially stain for steroid factor-1 
Another proximal tubular marker, CD10, also manifests high sensitivity, but again very low specificity for RCC, as lung, bladder, colon, and ovarian carcinomas all label for CD10 
. However, CD10 fairly consistently labels ccRCC, thus CD10 negative metastatic lesions represent an argument against this diagnosis for the primary tumor.
Lastly, Ksp-cadherin, a distal tubular marker, manifests high sensitivity for chromophobe RCC (chRCC), although it is not so useful in the metastatic diagnosis context, as the chromophobe variant rarely disseminates systemically 
. Even so, at least focal staining can also be seen in other renal tumor variants, including high-grade ccRCC.