LAUSR

Renal cell carcinoma is now increasingly treated with checkpoint inhibitor therapy, predominantly in the metastatic setting or occasionally in the adjuvant setting; however, there is little published histopathology data demonstrating the post-therapy features of renal cell carcinoma tumor tissue. We report a middle-aged man, who was undergoing treatment with pembrolizumab for locally recurrent cutaneous basal cell carcinoma and was incidentally found to have a renal mass. Radical nephrectomy demonstrated a 5.0 cm renal mass with extensive xanthogranulomatous features, mimicking xanthogranulomatous pyelonephritis. However, rare clusters of cells demonstrated gland-like lumina, clear cytoplasm, and slightly increased atypia, suggesting scant residual renal cell carcinoma cells. Immunohistochemistry demonstrated focal positivity for pan-keratin and keratin 8/18, EMA, PAX8, CD10, and carbonic anhydrase 9 (CA9), supporting a diagnosis of renal cell carcinoma with extensive treatment response, to the point that it mimicked xanthogranulomatous pyelonephritis. This report demonstrates the histologic features of post-immunotherapy renal cell carcinoma, aiding pathologists in recognition of this phenomenon, which may be misdiagnosed as an inflammatory process, if immunotherapy was performed for other reasons.