Xanthogranulomatous pyelonephritis (XGP), first described by Schlagenhaufer in 1916,  is a rare, serious, chronic inflammatory disorder of the kidney characterized by a destructive mass that invades the renal parenchyma.  XGP is most commonly associated with Proteus or Escherichia coli infection. Pseudomonas species have also been implicated. The kidney is usually nonfunctional. Most cases of XGP involve a diffuse process; however, up to 20% are focal. (See Etiology and Pathophysiology.)
Pathologically, XGP is characterized by lipid-laden foamy macrophages. XGP shares many characteristics with true renal neoplasms in terms of its radiographic appearance and its ability to involve adjacent structures or organs. XGP is often associated with urinary tract obstruction, infection, nephrolithiasis, diabetes, and/or immunocompromise. (See Workup.)
The treatment of XGP is almost universally extirpative and can pose a formidable challenge to the surgeon. Although open extirpative therapy has historically been the hallmark of management, an increasing number of reports have described successful laparoscopic intervention. (See Treatment.)
Most cases of XGP are unilateral, but bilateral disease has been reported. Bilateral nephrectomy and long-term dialysis is a treatment option in such cases. Although Perez et al described successful treatment with partial nephrectomies in 1 patient,  bilateral XGP is usually fatal. (See Treatment.)
The overall prognosis for XGP is good. Death from this entity is exceedingly rare, although morbidity is substantial.
The kidneys are paired organs in the retroperitoneum. They are covered in a thin, fibrous capsule that is in turn covered by Gerota’s fascia. On the right, the liver and adrenal gland abut the superior pole of the kidney. The second part of the duodenum is medial to the right kidney and may be reflected medially to gain exposure to the inferior vena cava (Kocher maneuver). On the left, the superior pole of the kidney is attached to the spleen superolaterally (splenorenal ligament) and is very near to or in direct contact with the tail of the pancreas medially.
The colon is anterior to the kidneys and is easily reflected anteriorly and medially when transabdominal exposure is required. The renal hilar structure, from anterior to posterior, includes the vein, the artery, and the collecting system. The left renal vein has 3 draining tributaries—namely, the adrenal, gonadal, and posterior lumbar veins.
Etiology and Pathophysiology
The exact etiology of xanthogranulomatous pyelonephritis (XGP) is unknown, but it is generally accepted that the disease process requires long-term renal obstruction and infection. As previously stated, XGP is most commonly associated with Proteus or Escherichia coli infection; Pseudomonas species have also been implicated. 
Stones (frequently of staghorn proportions) develop in 80% of patients with XGP but are not required to make the diagnosis. XGP is often observed in patients with diabetes, immunocompromise or both. Abnormal lipid metabolism has also been hypothesized as an etiologic factor in individuals with XGP.
XGP displays neoplasmlike properties capable of local tissue invasion and destruction and has been referred to as a pseudotumor. Adjacent organs, including the spleen, pancreas, or duodenum, may be involved. Malek and Elder have proposed the following stages of XGP involvement  :
Adjacent retroperitoneal structures
The gross appearance of XGP is that of a mass of yellow tissue with regional necrosis and hemorrhage, superficially resembling renal cell carcinoma. The pathognomonic microscopic feature is the lipid-laden foamy macrophage accompanied by both chronic- and acute-phase inflammatory cells. Focal abscesses may be observed.
The primary mechanisms involved in XGP include nephrolithiasis, collecting system obstruction, and infection. No single factor can explain the process; rather, an inadequate host response to the acute inflammatory response occurs in the obstructed, necrotic kidney. 
XGP occurs in approximately 1% of all renal infections. It is 4 times more common in women than in men and is usually noted in the fifth and sixth decades of life. XGP affects both kidneys with equal frequency.
Although XGP is rare in the pediatric population, it is found in approximately 16% of pediatric nephrectomy specimens. In children, XGP is more common in boys and usually affects those younger than 8 years.