Xanthogranulomatous pyelonephritis is characterized by microscopic fat infiltration of the renal parenchyma. Fat cells remain outside the atrophied renal parenchyma in replacement lipomatosis.
Reniform shape of the kidney is maintained in both replacement lipomatosis and xanthogranulomatous pyelonephritis.
Both xanthogranulomatous pyelonephritis and replacement lipomatosis are usually unilateral.
DDs for Replacement Lipomatosis
1.Xanthogranulomatous pyelonephritis (XGP)
Enlarged kidney with preserved shape and decreased function that occurs in the presence of long-standing inflammation and calculi (typically staghorn)
Lipid-laden macrophages actually infiltrate renal parenchyma in contrast to replacement lipomatosis in which fat cells remain outside of the atrophied renal parenchyma
Characterized by renal tissue destruction and replacement with hard, yellow, xanthogranulomatous material
Ultrasound shows hypoechoic areas representing purulent material, as well as medium-amplitude echoes corresponding to the fibrofatty and/or necrotic debris
CT shows hydronephrosis or pyonephrosis along with xanthogranulomatous tissue, which typically has attenuation values close to that of water
XGP and replacement lipomatosis may coexist
2. Lipoma
3. Angiomyolipoma
4. Liposarcoma
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