Xanthogranulomatous Pyelonephritis: Bullets

Basics

• Chronic destructive granulomatous process.
• Results from an incomplete immune response to a subacute bacterial infection.
• The renal parenchyma is destroyed and replaced by low-attenuation xanthomatous masses (-10 to 30 HU, depending on the lipid content).
• Calcifications can be seen within these xanthomatous masses (pink arrows).
• Can be focal (10% of cases) or diffuse (90%).
• Focal form can mimic neoplasm.
• Risk factors include diabetes mellitus (10% of patients), female gender (2:1 female:male ratio), and low socioeconomic status.
• Most patients have no specific risk factor.

Imaging:

• Radiography: Large staghorn calculus (nonspecific), enlarged kidney, obscuration of the psoas margin (late finding).
• Intravenous urography: Pronounced decrease in renal function with little or no excretion on delayed imaging (nonspecific).
• Ultrasound: Enlarged kidney with a large shadowing echogenic structure in the renal pelvis (staghorn calculus). Loss of normal renal architecture.
• CT: Combination of the following findings is strongly suggestive of diffuse disease: Nonfunctioning enlarged kidney + staghorn calculus in a contracted renal pelvis + expanded calices (usually filled with a low-attenuation inflammatory exudate) + inflammatory changes in the perinephric fat.
• Focal disease can mimic neoplasm, and should be considered malignant until proven otherwise. Urine culture can be helpful.

The CT images show an enlarged left kidney with multiple low-attenuation parenchymal lesions. One of these in the upper pole has fine calcifications that can be seen on the radiograph (pink arrow). The renal pelvis and the calyces are dilated.

References

• Al-Ghazo MA, Ghalayini IF, Matalka II, Al-Kaisi NS, Khader YS. Xanthogranulomatous pyelonephritis: Analysis of 18 cases. Asian J Surg. 2006 Oct;29(4):257-61.
• Craig WD, Wagner BJ, Travis MD. Pyelonephritis: radiologic-pathologic review. Radiographics. 2008 Jan-Feb;28(1):255-77.
• Fan CM, Whitman GJ, Chew FS. Xanthogranulomatous pyelonephritis. AJR Am J Roentgenol. 1995 Oct;165(4):1008.