Xanthogranulomatous pyelonephritis (XGP) is a rare and destructive form of chronic renal infection. The renal parenchyma is replaced by lipid-laden macrophages (foamy histiocytes) forming low-density collections. Typically presents as a unilateral, diffusely enlarged non-functioning kidney. Strongly associated with staghorn calculus (70-80% of cases) and obstruction. More common in middle-aged women (F:M = 3:1). Proteus mirabilis and E. coli are the most frequently isolated pathogens. Perinephric fat involvement and fistulization to adjacent structures can develop. Accurate preoperative diagnosis is critical as it can mimic renal cell carcinoma on imaging.
Age Range
40-70
Peak Age
55
Gender
Female predominant
Prevalence
Rare
XGP develops as a consequence of chronic obstruction and infection. Staghorn calculus or other obstructive causes lead to urinary stasis and create an environment for chronic bacterial infection. Macrophages migrate to the infection site and accumulate lipids from phagocytosed bacterial membrane debris, acquiring a foamy (xanthomatous) appearance. These lipid-laden macrophages progressively destroy normal renal parenchyma, which is replaced by yellow-orange granulomatous tissue. On imaging, this manifests as low-density collections because lipid-containing macrophage aggregates show water/fat mixture density. Inflammatory infiltration of perinephric fat appears as thickening and stranding. In advanced cases, fistulae to the psoas muscle, colon, diaphragm, or skin can develop.
On contrast-enhanced CT, multiple round low-density collections surrounding the central calculus correspond to lipid-laden macrophage aggregates within dilated calyces, with radial arrangement resembling a bear paw. Considered pathognomonic for XGP.
Hyperdense staghorn calculus within an enlarged renal pelvis. Kidney contour is enlarged and lobulated. Parenchyma has lost normal density, containing heterogeneous hypodense areas. The calculus typically fills the pelvicalyceal system completely or partially, causing obstruction.
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The left/right kidney is diffusely enlarged with a hyperdense staghorn calculus within the pelvicalyceal system and multiple surrounding hypodense collections replacing the parenchyma, consistent with xanthogranulomatous pyelonephritis.
Bear paw sign: multiple round-oval hypodense collections surrounding the central calculus, corresponding to lipid-laden macrophage aggregates within dilated calyces. Each collection is surrounded by a thin enhancing wall. Collections arranged in a radial pattern creating the bear paw appearance. Normal renal parenchyma is virtually absent; any remaining thin parenchymal band shows enhancement.
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In the nephrographic phase, multiple round non-enhancing collections surrounding the central calculus with thin enhancing walls demonstrate a positive bear paw sign, pathognomonic for xanthogranulomatous pyelonephritis.
Marked inflammatory thickening and stranding in perinephric fat. Gerota fascia is thickened and may enhance. In advanced cases, inflammation extends beyond the perinephric space to the psoas muscle, posterior abdominal wall, or adjacent organs (colon, duodenum, diaphragm). Perinephric abscess or phlegmon may have developed.
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Marked inflammatory stranding and thickening in the perinephric fat with thickened enhancing Gerota fascia, consistent with perinephric inflammatory extension.
No contrast excretion is observed in the affected kidney on excretory phase — the kidney has lost function. The contralateral kidney may show compensatory hypertrophy. The ureter on the affected side is not opacified or its proximal segment appears dilated and non-opacified. This finding indicates advanced XGP and irreversible parenchymal damage.
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No contrast excretion observed in the affected kidney on excretory phase indicating loss of function, consistent with irreversible parenchymal damage from xanthogranulomatous pyelonephritis.
Heterogeneous signal in the enlarged kidney on T2-weighted sequences. Lipid-laden macrophage collections show intermediate-to-high T2 signal intensity. Fibrotic wall and septa demonstrate low T2 signal, separating the collections. Signal heterogeneity among collections may be present due to debris and hemorrhagic components. Perinephric inflammatory changes show high T2 signal.
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T2-weighted sequences demonstrate heterogeneous collections of intermediate-to-high signal intensity with low-signal fibrotic septa in the enlarged kidney.
Lipid-laden macrophage collections show restricted diffusion on DWI — high signal on DWI, low signal on ADC map. This finding reflects the high cellularity (dense macrophage accumulation) and viscous debris content of the collections. Diffusion restriction is more pronounced in areas where abscess formation has developed.
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Restricted diffusion is observed in the intrarenal collections on DWI, consistent with high cellularity and viscous inflammatory debris content.
On US, the kidney is diffusely enlarged with loss of normal corticomedullary differentiation. Central hyperechoic calculus with posterior acoustic shadowing is observed. Multiple hypoechoic-anechoic areas (collections) are present in the parenchyma. Normal renal architecture has become unrecognizable. Hypoechoic thickening in the perinephric region may be visible.
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On US, the kidney is diffusely enlarged with a central hyperechoic calculus with acoustic shadowing and surrounding multiple hypoechoic collections; normal renal architecture is lost.
Criteria
Affects the entire kidney (85-90%). Kidney is diffusely enlarged with complete loss of function. Staghorn calculus is almost always present.
Distinct Features
Classic bear paw sign, all calyces involved, nephrectomy required. Kidney does not opacify with IV contrast due to loss of function.
Criteria
Affects one segment or pole of the kidney (10-15%). Presents as a focal mass and can be confused with renal cell carcinoma.
Distinct Features
Calculus may not always be present, appears as focal hypodense mass, differential diagnosis from RCC is difficult. Partial nephrectomy may be performed.
Criteria
Spread of the inflammatory process beyond the kidney. Perinephric abscess, psoas abscess, cutaneous fistula, or colonic fistula has developed.
Distinct Features
Perinephric collection, psoas thickening, fistulization to adjacent organs is observed. Surgery is more complicated requiring wide exploration.
Distinguishing Feature
RCC typically shows heterogeneous enhancement with marked contrast uptake; XGP collections do not enhance. Central staghorn calculus is not expected in RCC. Inflammatory clinical findings (fever, leukocytosis, recurrent UTI) are prominent in XGP.
Distinguishing Feature
Renal abscess is typically a single collection with rim enhancement; XGP is characterized by multiple collections within dilated calyces + staghorn calculus. Abscess is focal, XGP is diffuse. Abscess treatment is antibiotics + drainage while XGP usually requires nephrectomy.
Distinguishing Feature
Urothelial carcinoma creates filling defects in the pelvicalyceal system with soft tissue enhancement; in XGP parenchyma is replaced by collections. Calcification is rare in urothelial carcinoma, staghorn calculus is common in XGP. Clinically, infection signs predominate in XGP.
Distinguishing Feature
Focal pyelonephritis shows wedge-shaped hypodense area with striated nephrogram and preserved kidney size; in XGP the kidney is diffusely enlarged with complete parenchymal replacement. Focal pyelonephritis resolves with antibiotics, XGP is a progressive destructive process.
Urgency
urgentManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralWhen XGP is diagnosed, standard treatment is nephrectomy (diffuse form) or partial nephrectomy (focal form). Preoperative antibiotic therapy is initiated. Perinephric extension and fistula presence complicate surgery. Biopsy is usually unnecessary as CT findings (bear paw sign + staghorn calculus + loss of function) are diagnostic. In focal form where differentiation from RCC is impossible, surgery is indicated regardless. Early urology consultation is recommended.
Nephrectomy is usually required for XGP treatment as the kidney is non-functioning. Correct preoperative diagnosis is important as it can mimic malignancy. A focal XGP form exists and can be treated with partial nephrectomy.