Anatomic Pathology: Genitourinary Pathology

462) This image demonstrates a section of a large kidney mass (8 cm) of a 57-year-old woman, who presented with back pain. The mass was brown to yellow, and fairly well-circumscribed. The clear cells in this image should be highlighted by which stain?

• Xanthogranulomatous pyelonephritis (XGP) is an inflammatory mass that can mimic renal cell carcinoma (RCC) radiographically and grossly, and sometimes even histologically.

• XGP are usually unilateral, occur more frequently in women, and are generally present in the setting of chronic renal infections in an obstructed kidney, and diabetes. XGP has been associated with bacterial infections, most commonly Escherichia coli.

• Over time the changes can produce a large and destructive mass, typically composed of a mixed population of inflammatory cells. Grossly, a single or multiple masses are seen usually surrounding the pelvicalyceal system. Macrophages often predominate, and confer a yellow color to the mass, which mimics RCC on gross examination.

• Histologically, the mass is predominantly composed of clear/foamy macrophages, scattered multinucleated giant cells, and various inflammatory cells, such as neutrophils, lymphocytes, and plasma cells. The foamy histiocytes can be mistaken for clear cells of clear cell RCC and it is important to heed the cytologic features of the clear cells, with their small and inconspicuous nuclei, without any variation in size or irregularity of their nuclear membranes. Also, the cytoplasm is foamy, in contrast to the clear cytoplasm of conventional RCC.

• Included in the differential diagnosis is extra-nodal Rosai-Dorfman disease, which is composed of sheets of pale histiocytes with emperipolesis. The histiocytes are positive for S100 and CD68. Involvement of the regional lymph nodes is usually present.

Goodman M, Curry T, Russell T: Xanthogranulomatous pyelonephritis (XGP): a local disease with systemic manifestations. Report of 23 patients and review of the literature. Medicine (Baltimore) 1979 Mar;58(2):171–181.

Parsons MA, Harris SC, Longstaff AJ, Grainger RG: Xanthogranulomatous pyelonephritis: a pathological, clinical and aetiological analysis of 87 cases. Diagn Histopathol 1983 Jul–Dec;6(3–4):203–219.

 
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