Useful immunohistochemical panel for differentiating clear cell papillary renal cell carcinoma from its mimics.

Clear cell papillary renal cell carcinoma (CCPRCC) is a recently described low-grade renal cell tumor. In this study, we investigated the expression of paired box 8 (PAX-8), carbonic anhydrase IX (CA IX), CK7, and α-methylacyl-CoA-racemase (AMACR) in this tumor by immunohistochemistry in a group of 20 cases of CCPRCC. Clear cell papillary renal cell carcinoma showed diffuse (70%) or intermediate (30%) nuclear positivity for PAX-8 in each case, with predominantly moderate intensity (50%). Ninety percent of the cases showed some degree of cytoplasmic staining for CA IX, predominantly with moderate intensity (50%). In addition, each case of CCPRCC also showed diffuse membranous staining for CK7. Most CCPRCCs (95%) were negative for AMACR. PAX-8, CA IX, CK7, and AMACR comprise a concise panel for distinguishing CCPRCC from its mimics. PAX-8 positivity helps to confirm the renal origin of this tumor. Positivity for CA IX and CK7 differentiate CCPRCC from conventional clear cell renal cell carcinoma, which is usually CA IX positive while CK7 negative. The CK7-positive and AMACR-negative pattern seen in CCPRCC differentiates it from papillary renal cell carcinoma, which is usually positive for both AMACR and CK7.