COW-2021-50

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Xanthogranulomatous pyelonephritis

Xanthogranulomatous pyelonephritis (XPN) is the consequence of chronic bacterial infections in an obstructed kidney with calculi (1, 2, 3, 5 and 6). Gram negative bacterial infections are the main cause with E.coli being the most common culprit organism (2, 4, 5, 6). Most patients are female and usually present with flank pain, fever, fatigue, and lower urinary tract symptoms with gross hematuria (5, 6). However, patients may present with a renal mass (4, 6).

XPN is more commonly a diffuse process but may be segmental or focal. On gross exam, the pelvic-calyceal system maybe dilated with thickened mucosa and may contain yellow material. Yellow, irregular mass-like tissue may involve the renal medulla and extend into the cortex. Calculi are usually present. Microscopically, the disease process begins as areas of abscess formation and necrosis in the pelvis and sinus fat. As the disease progresses, there is adjacent accumulation of foamy lipid-laden macrophages (xanthoma cells) which can form larger nodules. The inflammatory infiltrate may also contain plasma cells and lymphocytes. Peripherally, there is fibrosis. The process can extend to involve the renal parenchyma and peri-nephric adipose tissue (1, 2, 5, 6). Treatment is based on extent of renal involvement. Initially it involves antimicrobial therapy before surgical intervention (2, 3).

The differential diagnosis for xanthogranulomatous pyelonephritis includes clear cell renal cell carcinoma (CCRCC), spindle cell neoplasms including sarcomatoid renal cell carcinoma (2,3,5) and other granulomatous processes such as malakoplakia (6), acid fast and fungal infections and sarcoidosis (2,3,5). Usually, careful microscopic evaluation can differentiate XPN from these other entities. CCRCC tumors cells have clear to eosinophilic cytoplasm that is not foamy and these tumors are not usually associated with a prominent inflammatory reaction (2). When necessary, immunohistochemical stains can be performed. Foamy macrophages are positive for CD68 but not for epithelial markers, while carcinoma is positive for keratin (5, 6). The fibroblastic proliferation in XPN is bland; sarcomatoid carcinoma more often has atypical spindle cells (2). Co-existing CCRCC in XPN can occur and thus careful attention on histological examination is required (3). Special stains such as AFB and GMS may assist in the diagnosis if infection with acid fast or fungal organisms is a concern.

This case serves as a reminder to consider XPN in the differential diagnosis of a renal mass and emphasizing the importance of obtaining patient history.

1. Xi Xie1, Ning Wang, Yuyong Wang, Huadong He, Fanlei Kong and Nin. Non-invasive papillary urothelial carcinoma, low-grade of the renal pelvis mimicking a xanthogranulomatous pyelonephritis in a male patient: A case report and review of literature. International Journal of Immunopahtology and Pharmacology, 2020 Jan-Dec; 34:2058738420925720.

2. Jha SK, Aeddula NR. Pyelonephritis Xanthogranulomatous. [Updated 2020 May 30]. In: Stat Pearls [Internet]. Treasure Island (FL): Stat Pearls Publishing; 2020 Jan-. Available from: https://pmlegacy-ncbi-nlm-nih-gov.ezproxy.library.tufts.edu/books/NBK557399.

3. Moss BF, Potter L, Cliff A, et al Xanthogranulomatous pyelonephritis with associated renal cell carcinoma BMJ Case Reports CP 2019;12:e232097.

4. Zhou Ming, Netto J. Gorge, Epstein I. Jonathan. 2012 High yeild pathology: Uropathology.Elsevier.

5. Yang J. Ximing, Zhou Ming. 2021 Practical genitourinary pathology. Frequently asked questions. Springer.

6. Murphy MW, Grignon JD, Perlam JE. 2004. Tumors of the kidney, bladder and related urinary structures. AFIP Atlas of Tumor Pathology. American Registry of Pathology.

Stephanie J Conrad, Hongzhi Xu, Elizabeth Genega
Department of pathology, Tufts Medical Center, Boston, MA

Kidney

Kidney, xanthogranulomatous pyelonephritis, macrophage