Objective: The differential diagnosis of kidney tumors, especially those with eosinophilic cytoplasms, can be problematic due to overlapping morphologic features. CD138 is primarily a plasma cell marker but is known to be expressed in the proximal renal tubular epithelium as well. This study aims to investigate the possible contribution of CD138 expression in the differential diagnosis of kidney tumors with eosinophilic cytoplasm.

Material and Method: The case series consisted of 15 chromophobe (ChRCC), 5 eosinophilic variant (EoRCC), 10 clear cell (CCRCC) and 9 papillary (PRCC) renal cell carcinomas, and 13 oncocytomas. Sections obtained from representative paraffin blocks were stained against CD138 antibody.

Results: All CCRCC and PRCC showed membranous CD138 expression. In some of the other eosinophilic renal tumors, cytoplasmic CD138 labeling in varying degrees was detected. In CCRCC cases, CD138 expression was especially observed in low grade areas and areas showing cystic and pseudopapillary growth patterns. A similar pattern of cytoplasmic staining was seen in 3 of the EoRCC and the most of the PRCC cases (6/9).

Conclusion: Our findings suggest that CD138 may contribute to the differential diagnosis of renal tumors because of the membranous staining pattern in CCRCC and EoRCC cases and the cytoplasmic staining in CHRCC and oncocytoma cases. Its contributory role may be improved by combined usage with markers like Cytokeratin 7 and RCC marker.