The Application and Pitfalls of Immunohistochemical Markers in Challenging Diagnosis of Genitourinary Pathology

Morphological Overlaps: Genitourinary pathology features significant morphological overlaps, complicating diagnosis especially with limited diagnostic materials.
Role of Immunohistochemical Markers: These markers help when morphological features are insufficient for diagnosis.
2022 WHO Classification: The article updates immunohistochemical markers relevant to newly classified genitourinary neoplasms, focusing on difficult differential diagnoses and potential pitfalls.

Diagnostic Review: Discusses use of immunohistochemical markers for lesions in the kidney, bladder, prostate, and testis, emphasizing challenges in differential diagnosis for various tumor types.
New Entities and Markers: Emerging markers from the updated WHO classifications and diagnostic panels for challenging cases are addressed.

Definitive Tool: A vital tool for diagnosing complex lesions in the genitourinary system, provided interpretations are contextualized with morphological findings.
Data Sources: The review is informed by current literature and authors' experiences.

Common Tumors: Majority are epithelial neoplasms such as Clear Cell RCC, Papillary RCC, Chromophobe RCC, and Oncocytoma.
Rare Tumors: Include Clear Cell Papillary RCC, Mucinous Tubular Carcinoma, etc.
New Classifications: New entities introduced in the 2022 WHO classification (e.g., low-grade oncocytic tumor (LOT), eosinophilic vacuolated tumor (EVT)).

Markers to Support Diagnosis: Certain IHC markers are more specific to renal tumors, aiding differentiation from non-renal neoplasms.
Common Markers: Includes Cytokeratins, Vimentin, CD10, PAX2, and PAX8.
Negative Markers: Although some IHC markers like TTF1 and CDX2 are negative in RCC, their presence strongly indicates non-renal origin.

Metastasis Detection: Identifying the origin of metastatic renal cell carcinoma can be difficult; PAX8 is highly recommended for confirmation.
Differentiating Tumors: Differentiation between renal tumors can be complicated when limited samples such as needle biopsies are used, where IHC plays a critical role.

PAX8 and PAX2: High sensitivity for confirming renal origin, frequently expressed in various subtypes of renal cell neoplasms.
RCCMa: Expressed in up to 80% of renal neoplasms, but lacks specificity as it may also be seen in non-renal tumors.
CD10: Present in most clear cell and papillary renal cell carcinomas, but also found in multiple non-renal tumors.
KIM-1: A sensitive marker for clear cell and papillary renal cell carcinoma, but shared expression in ovarian clear cell carcinoma limits its utility.
Vimentin and Cytokeratins: Used in conjunction with other markers to assess renal origin and differentiate tumor types.

Fumarate Hydratase–Deficient RCC: Identified by alterations in FH gene and includes specific IHC markers, with a characteristic morphology and staining pattern.
Succinate Dehydrogenase-Deficient RCC: Requires loss of SDHB staining for diagnosis, careful interpretation needed to prevent false conclusions.
Eosinophilic Solid and Cystic RCC: Display specific IHC patterns and unique morphological features; positive for CK20.
Low-Grade Oncocytic Tumors: Resemble oncocytomas with a specific IHC profile that differentiates them from other tumors.

Optimized Panels: Recommended initial IHC panels include markers like CK7, CAIX, and AMACR depending on the suspected tumor type.
Use of Ancillary Studies: Additional molecular tests may be warranted for precise diagnosis when morphology and IHC yield ambiguous results.

Urothelial Carcinomas: Diverse morphologies can lead to challenges in histological diagnosis.
Role of IHC: IHC is beneficial for confirming urothelial origin and differentiating between variants of urothelial carcinoma.

GATA3 and CKs: Sensitive markers for urothelial differentiation, but need caution due to expression in other tumors.
p63 and Thrombomodulin: Used for distinguishing squamous and urothelial cell neoplasms.

Markers for Differentiation: Absence of basal markers indicates malignancy; p63 and HMWCK are crucial.
Lumen Cell Markers: AMACR is significant for identifying adenocarcinoma but has sensitivity issues in some variants.

GCT Identification: Different GCT subtypes can be diagnosed through a combination of H&E morphology and IHC (e.g., SALL4, OCT4).
Challenges: Potential for overlap between testicular tumors and other malignancies requires careful analysis.

Morphology vs. IHC: While most lesions can be diagnosed based solely on morphology, IHC is increasingly necessary in complex cases to establish accurate diagnoses and avoid misinterpretation.
Knowledge of IHC Pitfalls: Understanding the limitations and potential misinterpretations of various immunohistochemical markers is critical for pathologists.