Which marker is included in the IHC panel for primary RCC?

• A. CK7
• B. PAX8
• C. CD10
• D. CAIX

Answer: (B)

Immunohistochemistry for renal tumors relies on markers that reveal renal lineage. PAX8 is a nuclear transcription factor expressed in renal tubular epithelium and is positive across the major subtypes of renal cell carcinoma. Because of its broad and robust expression in RCC, PAX8 is a reliable marker to confirm that a tumor is of renal origin, which is especially helpful when distinguishing RCC from metastases from other sites or from non-renal primaries.

CAIX is strongly characteristic of clear cell RCC but isn’t universal across all RCC subtypes. CK7 expression varies by subtype (often positive in papillary and chromophobe RCC, variable or negative in clear cell). CD10 can be positive in RCC but is less specific, as it appears in other tumor types as well. So, while the other markers provide useful information, PAX8 serves as a versatile, broadly applicable renal lineage marker that best supports identifying primary RCC across its main subtypes.