The utility of PAX-2 in distinguishing metastatic clear cell renal cell carcinoma from its morphologic mimics: an immunohistochemical study with comparison to renal cell carcinoma marker.

PAX-2 is a homeogene expressed during kidney development. Although immunohistochemical expression of PAX-2 has been described in a variety of primary renal cell carcinoma (RCC) subtypes and in metastatic RCC, its specificity as a marker of renal lineage in a metastatic setting has not been fully evaluated. In addition, its utility has not been directly compared with the most widely used antibody in this setting, renal cell carcinoma marker (RCCma). We studied PAX-2 expression in metastatic clear cell renal cell carcinoma (CC-RCC) and in a variety of nonrenal neoplasms with clear cytoplasm that may potentially mimic CC-RCC. Archival material from 27 CC-RCCs metastatic to various organs and 50 close morphologic mimics of CC-RCC were retrieved. Immunohistochemistry with PAX-2 and RCCmapi antibodies was performed on each case. Nuclear staining (PAX-2) or membranous staining (RCCma) was scored semiquantitatively. Twenty-three of 27 (85%) metastatic CC-RCCs showed nuclear immunoreactivity for PAX-2, whereas RCCma reactivity was found in 19 of 27 (70%). The immunoprofiles of the metastatic CC-RCC were PAX-2+/RCCma+: 19 of 27 (70%), PAX-2+/RCCma-: 5 of 27 (19%), PAX-2-/RCCma+: 2 of 27 (7%), and PAX-2-/RCCma-: 1 of 27 (4%). Five of the 50 mimics of CC-RCC (10%) had at least focal nuclear reactivity with PAX-2, including 1 of 3 parathyroid carcinomas (33%), 3 of 7 clear cell carcinomas of the ovary (43%), and the 1 clear cell papillary cystadenoma of the epididymis. Membranous RCCma reactivity was identified in 26 of the 50 mimics (52%). We conclude that PAX-2 is a useful marker for distinguishing metastatic CC-RCC from its potential morphologic mimics, but caution must be used in certain differential diagnostic settings where nonrenal tumors such as parathyroid carcinoma, ovarian clear cell carcinoma, and clear cell papillary cystadenoma of the epididymis were shown to express both PAX-2 and RCCma.