Xanthogranulomatous Pyelonephritis Imaging 

Xanthogranulomatous pyelonephritis (XGPN) represents an unusual suppurative granulomatous reaction to chronic infection, often in the presence of chronic obstruction from a calculus, stricture, or tumor. XGPN is characterized histologically by the presence of foamy, lipid-containing macrophages (xanthoma cells), diffuse infiltration with plasma cells, and histiocytes.

XGPN is more common in women than in men; female patients with XGPN usually have a history of recurrent or chronic urinary tract infections.^{[1, 2, 3, 4]} Presenting symptoms may include pyuria, flank pain, fever, dysuria, pyuria, hematuria, proteinuria, or microscopic hematuria.^{[2, 5]} A palpable flank mass may be present, which may be tender or demonstrate costovertebral angle tenderness. XGPN is rare in children.^[3]

Two forms of XGPN are described—a diffuse or global form (83-90% of patients) and a focal form (10-17%). XGPN has been termed the great imitator because it may be misdiagnosed as a renal neoplasm, especially if the lesion is focal. Typically, plain abdominal radiographs demonstrate a large renal calculus (staghorn in 75% of patients), whereas intravenous urograms demonstrate an enlarged, poorly opacified kidney that is associated with a centrally obstructing calculus. See the images below.

Ultrasonographic findings of diffuse disease depict an enlarged kidney that usually preserves the reniform shape. Multiple hypoechoic areas may exist; these represent hydrocalyces, small abscesses, or granulomas. The obstructing calculus may not create shadowing.

XGPN almost always occurs unilaterally, and to the author's knowledge, only 1 patient with bilateral disease has been described. The kidney is involved either globally or focally. Changes of XGPN have been described in kidneys destroyed as a result of pyonephrosis; in renal cell carcinoma; in transitional cell carcinoma; and, rarely, in a renal cyst. These focal pathologic changes are detectable only by using histologic analysis, and they usually do not appear on images.

The treatment of patients with XGPN usually involves intensive antimicrobial therapy, but surgery is invariably required to completely eradicate the infection and the accompanying calculus and/or obstruction. Kidney-sparing surgery may be undertaken in patients with focal disease.

Preferred examination

Ultrasonography is the initial examination in most patients of any age who have urinary symptoms.

Plain radiographic findings confirm the presence of renal calculi.

CT scans demonstrate retroperitoneal involvement more clearly than do other images.

Angiography is occasionally performed to characterize a focal renal mass further and to plan surgery. CT angiography (CTA) and magnetic resonance angiography (MRA) are replacing conventional imaging techniques.

Isotopic renography is useful in demonstrating relative renal functions before surgery.

Limitations of techniques

No radiologic features are characteristic for XGPN, but in the diffuse form, some CT scan and ultrasonographic features may be sufficiently typical to suggest a preoperative diagnosis in the appropriate clinical setting.^[6] Focal disease cannot be diagnosed with confidence radiologically and should be regarded as malignant until proven otherwise. The importance of urine cultures in assessing an unexplained renal mass cannot be overemphasized.

Preoperative differentiation of XGPN and chronic pyelonephritis on the basis of clinical, bacteriologic, and radiologic criteria is problematic.^[6] Patients with XGPN are more likely to be middle-aged women with diabetes mellitus, with frequent history urinary tract infections in both pathologies. Both conditions present with flank pain and tenderness. Anemia, hematuria, and bacteriuria are more frequent in XGPN patients than in those with chronic pyelonephritis. In one study, P mirabilis was detected in over 50% patients with XGPN and in a minority of the chronic pyelonephritis group.^[6] Nephromegaly and renal calculi are more frequently seen in both pathologies. XGPN has a higher rate of postoperative complications.

A plain scout radiograph obtained before intravenous urography typically shows a staghorn calculus (see the image below). Further smaller calcifications may be seen scattered throughout the renal area, and a soft-tissue mass may be identified.

If present, perinephric extension may produce ill-defined renal margins, and a large soft-tissue mass may be seen occupying the renal fossa.

A thickened Gerota fascia is occasionally demonstrated.

After the administration of contrast material, findings depend on the morphologic type of XGPN. An absent nephrogram or focally absent nephrogram may occur.

On nephrotomography, some opacification of the kidney may occur, and a mass may be seen. A central opacified mass may correspond to a xanthoma.

When the inflammatory process extends into perinephric tissues, the renal outline is obscured. Inflammation from XGPN may extend into the perirenal space, pararenal space, ipsilateral psoas muscle, colon, spleen, diaphragm, posterior abdominal wall, and/or skin. This inflammation may obscure the renal outline.

Focal disease may appear as any other renal space-occupying lesion, especially when it is not associated with a calculus. The focal lesion usually shows no function or a patchy nephrogram; this may be associated with splaying and stretching of the calyces.

Retrograde pyelography may show a complete obstruction at the ureteropelvic junction, infundibulum, or proximal ureter.

If a focal, solitary noncystic lesion is seen, the differential diagnosis is that of a solid mass.

Degree of confidence

Although plain radiographs may suggest a renal calculus and a soft-tissue mass, and although an intravenous urogram may suggest no renal function on the affected side, the findings of neither method can confirm the diagnosis of XGPN.

False positives/negatives

Differentiating acute pyelonephritis, other chronic renal infections (eg, tuberculosis), straightforward renal calculus disease, and renal neoplasms from XGPN on plain radiographs and intravenous urograms is not always possible.

CT scanning is the most useful investigation for the preoperative assessment of XGPN.^{[1, 5, 7, 8, 9]} CT scan findings depend on the morphologic type of XGPN.

• In diffuse disease, CT scans may show a staghorn calculus within a nonfunctioning kidney (see image below).Contrast-enhanced computed tomography scan through the mid poles of the kidneys. This image shows a staghorn calculus within the right renal sinus that is associated with mild hydronephrosis, thinning of the cortex, and areas of low attenuation surrounding the calculus. The patient presented with pyrexia and leukocytosis. Ultrasonographic examination revealed a perinephric fluid collection, which was drained percutaneously (not shown). Note the air in the retroperitoneum after percutaneous drainage. At subsequent surgery, xanthogranulomatous pyelonephritis was confirmed.
• Although the reniform shape is maintained, the renal parenchyma is destroyed and is replaced by low-attenuating masses. The attenuation values in these masses vary from -10 to 30 Hounsfield units (HU), depending on the lipid content.
• The renal sinus may not be identifiable.
• Fine calcifications are occasionally discerned within the xanthomatous masses.
• Small gas collections are sometimes seen in intrarenal abscesses.
• The Gerota fascia is invariably thickened and identified.
• Extrarenal extension of XGPN is well depicted on CT scans and has been described both in the diffuse form and the focal form.
• Enhanced CT images show ring enhancement in areas of granulation tissue; the xanthomatous tissue does not enhance.
• The definition of cortical renal abscesses is improved on enhanced images.
• In focal XGPN, a large mass is identified in an otherwise functioning kidney. The mass is usually low attenuating and fails to enhance on CT scans; however, rim enhancement may be seen.
• Calculi are often associated with the central portion of the mass.

Degree of confidence

None of the features described above are characteristic of XGPN, but they are sufficiently typical of the diffuse form of XGPN to suggest a preoperative diagnosis in the appropriate clinical setting, especially by low CT scan values. Focal disease cannot be diagnosed with confidence with radiologic images, and the lesions should be regarded as malignant until proven otherwise. The importance of urine cultures in assessing an unexplained renal mass cannot be overemphasized.

False positives/negatives

Uncomplicated renal calculus disease, acute pyelonephritis with renal abscess formation and perinephric spread, chronic renal infections (eg, renal tuberculosis), and renal neoplasms cannot always be differentiated confidently from XGPN on the basis of CT findings alone.

A few reports describing the MRI appearances of XGPN have been published.^{[5, 9, 10, 11, 12]} Documented findings include the presence of a large mass in the renal fossa that appears multiloculated. Abscesses and calyces demonstrate intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Calcification and renal calculi appear as signal voids. MRI appears to be extremely sensitive in outlining perinephric spread.^[9] Residual parenchyma in the unaffected kidney may appear normal.

Degree of confidence

MRI findings have been inconsistent, with varying degrees of signal change described in areas involved by the inflammatory process compared with normal uninvolved renal tissue. In general, CT scan findings appear to be more helpful than those obtained with other modalities.

False positives/negatives

Similar to CT scan results, MRI features are not reliable in differentiating XGPN from complicated and uncomplicated renal calculus disease, acute pyelonephritis with perinephric spread, and renal neoplasms.

Renal ultrasonographic findings demonstrate an enlarged kidney that tends to maintain the reniform shape.^{[5, 7, 8]} See the images below.

Loss of corticomedullary differentiation occurs.

Parenchymal calcification is uncommon, but when present, it appears as central echogenic foci with associated acoustic shadowing.

The renal pelvis is small because of fibrosis, but hypoechoic dilatation occurs in the calyces, which have an echogenic rim.

The renal parenchyma is replaced by hypoechoic masses, which frequently have low-level internal echoes (see image below).

Sound transmission is usually not enhanced because the hypoechoic areas are not simple fluid-filled spaces.

Renal cortical tissue is markedly thinned and may have scattered cystic collections that represent abscesses.

The obstructing calculus usually does not create shadowing.

Degree of confidence

Although ultrasonographic features are not specific to XGPN, the failure to depict a normal kidney associated with a staghorn calculus suggests the diagnosis. Ultrasonographic results are not routinely helpful in identifying renal calculi, and ultrasonography is less sensitive than other modalities in demonstrating extrarenal spread.

False positives/negatives

The ultrasonographic appearance of XGPN may be mimicked by a simple hydronephrosis associated with a calculus, pyonephrosis, renal tuberculosis, renal cystic neoplasm, and renal lymphoma. In simple hydronephrosis, the dilatated calyces are sharply defined and demonstrate enhanced through-transmission. Pyonephrosis often demonstrates fluid-debris levels.

Radionuclide studies usually do not contribute to the diagnosis, but isotope renography is extremely useful in assessing differential renal function when surgery is contemplated. The authors have demonstrated renal uptake of technetium 99m (^99m Tc)-labeled white cells in XGPN (unpublished data).

The features of angiography described in the global disease include stretching of the segmental and/or interlobular arteries around large avascular masses, similar to those seen in hydronephrosis. Attenuation occurs in major renal arteries, and peripheral vessels lack arborization. Inhomogeneity of the nephrogram may mimic hydronephrosis. In the late arterial phase, hypervascularity or arterial blush may be noted around the periphery of the masses due to granulation tissue. See the images below.

The focal form of XGPN is usually hypovascular. Intrarenal veins are stretched and compressed by inflammatory renal masses. The renal vein may be encased or occluded, but inferior vena cava thrombosis has not been described.

Degree of confidence

Angiography no longer plays a diagnostic role in XGPN, although the authors occasionally use angiography to characterize focal renal masses or to plan surgery. The invasive catheter technique is being replaced with CTA and MRA with reformatting images

False positives/negatives

The angiographic appearance of XGPN may mimic those of hydronephrosis or avascular renal neoplasms.