Xantho-what? Pyelo-who?

NEJM 2000;342:1572.

I came across an unusual specimen this weekend while grossing for our pathology lab. A 62 yo woman had a nonfunctioning right kidney that appeared to have a mass by imaging. Of course, in this age group, a renal mass is worrisome for renal cell carcinoma or other primary renal tumor. However, based on her workup and imaging, this was likely xanthogranulomatous pyelonephritis (XGP). Our frozen section when she had her nephrectomy confirmed the diagnosis. This disease can certainly simulate an invasive malignancy, as seen in the CT image for this post. Also seen is perforation of the renal capsule with extruded contents (read: pus) into retroperitoneal and subcutaneous spaces.

What is XGP, you may ask?

Xanthogranulomatous pyelonephritis is a fairly rare entity but serious. It is essentially a chronic infection of the renal parenchyma (pyelonephritis). Interestingly, it is quite destructive, often destroying the medulla and cortex. Histologically, the cardinal finding is lipid-laden macrophages (xanthoma cells). Also are abscesses and granulomas (tada! there's the story behind the name). As is shown by this case, the disorder more frequently affects women than men, usually in the 6th and 7th decades of life. [why is it so hard to think: "6th or 7th decade...that means age 50s and 60s". it doesn't make sense...] Since XGP essentially destroys the kidney it affects, bilateral disease is generally fatal.

So, my job as a pathologist is to ask "why". Why do women in their 60s get this? As is the case in medicine, the answer isn't entirely known but likely the disease process required long-term obstruction and infection. In many cases, immunosuppression is a prerequisite predisposing factor. In our case, the removed kidney had large, "staghorn" calculi in the renal pelvis (it's a pretty exciting day for a pathologist when staghorn calculi are part of the daily grind...). Proteus and E. coli are often cultured. For the budding pathologists out there, the main differential diagnoses may be renal cell carcinoma, leiomyosarcoma and malakoplakia. The spindle cell component of XGP may not display the usual macrophage-rich areas. Demonstrating spindle cell and epithelial components would suggest a renal cell carcinoma with sarcomatoid elements. CD68 positive lipid-laden macrophages suggest XGP. The PAS-positive "Michaelis-Gutmann" bodies in histiocytes (macrophages) are characteristic of malakoplakia. FYI, macrophages and histiocytes: the same thing? Pretty much. Histiocyte is the more general term (i.e. macrophages, dendritic cells, and Langerhans cells are types of histiocytes). Histiocyte literally means "tissue cell". Of course, we all know that tissues have many types of cells besides histiocytes.

So, yes, often the treatment of XGP is nephrectomy or partial nephrectomy. I imagine it would have been quite difficult for antibiotics to penetrate the kidney I was dissecting and kill all that bacteria that was oozing out(yuck).