Clear Cell Renal Cell Carcinoma With Borderline Features of Clear Cell Papillary Renal Cell Carcinoma: Combined Morphologic, Immunohistochemical, and Cytogenetic Analysis.

Clear cell papillary renal cell carcinoma is increasingly recognized as a distinct tumor with unique morphology, immunohistochemistry, and cytogenetics. Histopathology often mimics clear cell renal cell carcinoma; however, metastasis has not been reported, emphasizing the clinical value of recognizing these likely nonaggressive tumors. We studied tumors with borderline morphology of clear cell papillary renal cell carcinoma, utilizing immunohistochemistry and fluorescence in situ hybridization or karyotyping. Tumors from 22 patients (ages 33 to 82 y) were analyzed. Clear cell papillary renal cell carcinoma-like morphology varied from 10% to 90% of the tumor (median 25%). Sources of resemblance included: branched glands (95%), nuclear alignment (68%), small papillary tufts (32%), focal branching papillae (27%), and prominent papillary structures (9%). Carbonic anhydrase IX uniformly revealed diffuse positivity. Staining for cytokeratin 7 (CK7) was focal (64%) or negative (18%) in most tumors (82%); however, >50% labeling was present in 4 (18%). Reactivity for both CD10 and α-methyl-acyl-CoA-racemase (AMACR) was usually present (median 80% and 60% of cells). Seven tumors showed reactivity for high-molecular weight keratin (32%). Chromosome 3p loss was confirmed in 15 tumors (68%), including 4/7 with labeling for high-molecular weight keratin or >50% reactivity for CK7. A discordant immunohistochemical pattern typically correlates with loss of material from chromosome 3p in tumors with incomplete morphology of clear cell papillary renal cell carcinoma, supporting classification as clear cell renal cell carcinoma. Diffuse labeling for CK7 can uncommonly be observed in clear cell renal cell carcinomas confirmed to have chromosome 3p loss, although these do not exhibit the expected staining pattern of clear cell papillary renal cell carcinoma, including positivity for CD10 and AMACR.