Answer: Xanthogranulomatous pyelonephritis

Histology: The lesion consists of a mixture of inflammatory cells with histiocytes, giant cells, granulomas, neutrophils, and plasma cells centered around the renal pelvis and forming a mass. Hemorrhage and hemosiderin are also seen.

Discussion: At first glance, there appear to be a lot of clear cells, raising the possibility of renal cell cancer. However, on higher magnification, the clear cells that are seen are histiocytes admixed with other inflammatory cells. Granulomas are also seen. Care should be given to diagnosing a neoplasm in the presence of abundant inflammation. Xanthogranulomatous pyelonephritis (XGP) is a clinical and pathologic mimicker of renal cell carcinoma. It can form a mass lesion, like renal cell carcinoma, and histologically the abundant histiocytes may be mistaken for clear cell renal cell carcinoma. This can be particularly treacherous on frozen section diagnosis. XGP occurs in the setting of chronic infection with urea-splitting organisms, such as E. coli, Proteus, and Staph. aureus. Classic cases are associated with struvite (magnesium ammonium phosphate) stones formed in the renal pelvis (staghorn calculi).

The inflammatory process also raises the possibility of malakoplakia. Malakoplakia consists predominantly of histiocytes, and characteristic targetoid bodies are seen in the stroma and cells (Michaelis-Gutmann bodies). These structures are thought to represent calcified partially digested bacteria. Michaelis-Gutmann bodies were not seen in this case.