Genitourinary Radiology > Kidneys > Parenchymal > Large Kidney > XGP

Xanthogranulomatous Pyelonephritis (XGP)


XGP develops in patients with chronic urinary tract obstructions and infections that have usually been caused by nephrolithiasis, especially staghorn calculi. With chronic obstruction, lipid-laden histiocytes infiltrate the renal parenchyma, replacing the functional elements of the kidney. XGP commonly spreads to surrounding tissues, including the collecting system, perinephric fat, and through Gerota’s fascia to the psoas muscle, and may result in renal-cutaneous or renal-enteric fistulas.

Clinical presentation: Patients with XGP often appear chronically ill. Symptoms include anorexia, fevers, weight loss, and flank pain. The pain of XGP is usually dull and persistent. Urine typically has both leukocytes and bacteria present.

Classic triad: usually unilateral enlarged kidney, nephrolithiasis, poor or absent renal function (limited to no uptake or excretion of contrast agent with CT or MRI).

    U/S: stones in renal pelvis, reniform enlargement of kidney, diffusely abnormal parenchymal echogenicity, absence of normal corticomedullary distinction.

    CT: enlarged kidney, multiple low-attenuation cystic areas described as a “bear paw print”, central stone. This phlegmonous process may enhance peripherally due to inflammatory rind. This process may be difficult to distinguish from a necrotic renal cell carcinoma.

    Retrograde pyelogram: contracted, irregular collecting system

Plain film (KUB) in patient with renal obstruction. Notice the left renal calculus completely filling the region of the collecting system(arrows). This is not excreted contrast but rather stone material from a staghorn calculus.

Contrast enhanced CT scan in patient with chronic obstruction on the left from staghorn calculus shows staghorn (arrows) filling the collecting system(1) and infiltrated parenchyma which enhances poorly (*) in the classic "bear paw" finding.

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