Teaching Case

A middle-aged obese woman with a history of chronic obstructive lung disease, hepatitis C virus infection and urolithiasis including previous left-sided pyelo-lithotomy for staghorn lithiasis 15 years earlier.
Currently she was hospitalized with left-sided flank pain and tenderness, and laboratory signs were consistent with a presumptive diagnosis of acute pyelonephritis.

Ultrasound (Fig. 1) showed extensive replacement of the left kidney by large hypo-anechoic regions with poorly perceptible residual parenchyma.
Unenhanced CT (Fig. 2a...c) was requested to investigate the markedly abnormal but inconclusive sonographic appearance, and to rule out obstructive calculi. Multiplanar CT images confirmed renal parenchymal thinning with sizeable, confluent water-attenuation cavities and "staghorn" lithiasis of the renal pelvis, upper and lower calyces. Mild ipsilateral fascial thickening and peripelvic fat stranding suggested the presence of active urinary infection. The patient underwent ureteral stenting (Fig. 2d) to relieve pyonephrosis.
Six weeks later, after stent removal, contrast-enhanced CT (Fig. 3) confirmed findings of staghorn calcific lithiasis, thinned and poorly enhancing renal parenchyma, and water-attenuation cavities representing uneven calyceal dilatation. The enlarged left kidney occupied and compressed the perirenal and pararenal spaces.
After prolonged antibiotics to eradicate infection, laparoscopic nephrectomy (postoperative status in Fig.4) was performed. Histopathology confirmed the radiologist's preoperative suspicion of xanthogranulomatous pyelonephritis.

Xanthogranulomatous pyelonephritis (XGP) is a rare form of a granulomatous inflammatory process affecting the kidney, collecting system and surrounding retroperitoneum, leading to progressive destruction of the renal parenchyma characteristically replaced by lipid-laden “foamy” macrophages (xanthoma cells). The hypothesized pathogenesis involves an incomplete immune response to subacute bacterial infection, superimposed on long-standing urinary obstruction. Almost invariably unilateral, XGP initially develops in the renal pelvis and then diffuses to the medulla, cortex, and often to the perinephric space [1, 2].
XGP mostly occurs in middle age, with a predilection for perimenopausal women with history of urolithiasis and long-standing urinary tract infections and obstruction. The clinical manifestations are nonspecific, often insidious compared to the entity of the imaging abnormalities. Symptoms include low-grade fever, flank pain and tenderness, malaise, weight loss, lethargy, leukocytosis and pyuria. Sometimes a mass is clinically palpated. Escherichia coli and Proteus mirabilis are the commonest identifiable micro-organisms [1, 2].
Ultrasound initially shows extensive replacement of the normal renal architecture by hypoanechoic masses corresponding to dilated calyces and abscesses filled with pus and debris, and amorphous central echogenicity with acoustic shadowing representing a staghorn calculus. These sonographic findings invariably require multidetector CT for correct characterisation, including intravenous contrast medium unless contraindicated by renal dysfunction. Cross-sectional imaging findings include a non-functioning enlarged kidney with poor, heterogeneous contrast enhancement. The majority (75-90%) of cases have obstructing pelvis or ureteral lithiasis, often in a central “staghorn” configuration. The destroyed parenchyma is replaced by multiple rounded hypodense cavities representing dilated, pus-filled calyces. Fatty xanthomatous deposits with negative CT attenuation are detected in approximately 30% of cases and highly specific for XGP. Contrast excretion is rarely seen at the time of diagnosis. Furthermore, the majority (two-thirds) of cases have perinephric fat inflammation or extrarenal extension of XGP into adjacent structures such as posterior pararenal space and psoas muscle. The diagnosis is challenging in atypical cases, such as in absence of calculi (10% of cases), and the rare (below 10% of cases) focal XGP which appears as a minimally enhancing renal mass and is commonly misinterpreted as bacterial abscess or tumour [1-8].
The diagnosis of XGP is generally made at histopathology after nephrectomy. However, the radiologist aware of this disease and of its combination of characteristic findings may suggest the correct preoperative diagnosis. CT is the mainstay technique, since it provides comprehensive anatomic and functional assessment, thus allowing correct surgical planning [1-8].