KEY FACTS
Terminology
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Chronic renal inflammation usually associated with longstanding urinary calculus obstruction (75%)
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Renal parenchyma is gradually replaced by lipid-laden macrophages
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Diffuse (> 80%) and focal (< 20%) forms
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3 stages of xanthogranulomatous pyelonephritis: Intrarenal → perirenal → perinephric ± retroperitoneal involvement
Imaging
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Diffusely enlarged kidney with hypoechoic round masses replacing normal parenchyma on US
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“Staghorn” calculus with renal enlargement and perirenal fibrofatty proliferation
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Renal sinus fat obliterated with large central “staghorn” calculus
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Perinephric extension ± adjacent organs or structures may include sinus tracts or abscesses
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Multiple focal, low-attenuating renal masses with rim enhancement on CT
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Ultrasound ideal at initial investigation; CT good for assessing excretory function and retroperitoneal involvement
Pathology
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Lipid-laden, “foamy” macrophages, diffuse infiltration of plasma cells and histiocytes
Clinical Issues
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Flank pain, fever, palpable mass, and weight loss
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Rare complications: Hepatic dysfunction, extrarenal extension, fistulas
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Long-term chronic process with good prognosis if treated and rare mortality
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Antibiotic treatment is sometimes effective
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Severe disease or perinephric extension usually requires nephrectomy
, causing replacement of parenchyma by collections of foamy macrophages 
.
