What is xanthogranulomatous pyelonephritis (XGP), and what are its CT and MRI features?
XGP is an uncommon chronic granulomatous process that is secondary to bacterial infection and urolithiasis, in which there is replacement of the renal parenchyma by lipid-laden macrophages. This most often occurs in middle-aged patients and is twice as common in women than in men. Nephrolithiasis is almost always present, often with presence of a staghorn calculus.
On CT and MRI, unilateral renal enlargement is usually seen, which is diffuse in 90% of cases and focal in 10% of cases, along with decreased or absent excretion of contrast material. Multiple cystic-appearing areas secondary to inflammatory infiltrates replace the renal parenchyma, often in a configuration resembling the paw print of a bear, known as the “bear paw” sign. There is commonly extension of inflammatory tissue into the perinephric fat and even into the retroperitoneum or abdominal wall. Fistulae to adjacent structures are common, often to the skin or large bowel