Ultrasound images demonstrate multiple dilated cystic spaces in the right kidney, as well as multiple stones with posterior shadowing:

On close inspection, the central calyces and renal pelvis do not appear dilated:

CT of the abdomen and pelvis was performed for further evaluation:

This patient presented with classic imaging findings of xanthogranulomatous pyelonephritis (XGP).

This diagnosis is usually made on CT and often seen as an enlarged kidney with staghorn calculus. There is dilatation of the renal calyces with paradoxical normal-sized or contracted renal pelvis. This appearance  has been described as the “bear paw sign“, although it is really just the toes of the paw (represented by the dilated calyces).

Xanthogranulomatous pyelonephritis is a chronic granulomatous destructive process, resulting from immune response to recurrent or subclinical urinary tract infection. The presentation is often indolent with nonspecific symptoms, though occasional flank pain and asymptomatic pyuria (seen in the majority of patients) may be clues to the diagnosis. A staghorn calculus is seen in nearly all cases.

At the time the diagnosis is made, the kidney is often already nonfunctional. A MAG3 renogram was performed in this case which demonstrated markedly decreased renal function on the right. The abnormally shaped right kidney demonstrated delayed gradual accumulation of radiotracer without detectable excretion.

This patient underwent right nephrectomy (which is often the only available treatment) which demonstrated: “End-stage kidney with extensive acute and chronic inflammation composed of foamy macrophages”, the classic pathologic findings for XGP.

Perirenal/pararenal space infiltration, psoas abscess, and cutaneous or enteric fistulas are described complications of this disease.

Craig WD, et al. Radiographics 2008;28:255-276.