Xanthogranulomatous Pyelonephritis (XGP)
XGP is what happens when a chronically obstructed, infected, stone-bearing kidney is slowly destroyed and replaced by greasy yellow inflammatory tissue — the great mimic of renal cell carcinoma.
The big picture
XGP is what happens when a chronically obstructed, infected, stone-bearing kidney is slowly destroyed and replaced by greasy yellow inflammatory tissue. It is the great mimic — on imaging it can look exactly like renal cell carcinoma, and under the microscope its foamy cells have been mistaken for clear-cell carcinoma, leading to unnecessary radical surgery. The essential pattern to recognise: a non-functioning, enlarged kidney with an obstructing stone and chronic infection, treated almost always by nephrectomy.
The framework: chronic obstruction + infection + stone → destructive granulomatous replacement (lipid-laden macrophages) → a non-functioning enlarged kidney that mimics cancer → nephrectomy.
Mechanism pathway
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Diagnostic algorithm
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Board traps
XGP = chronic destructive pyelonephritis → non-functioning, enlarged kidney with an obstructing stone (often staghorn).
Lipid-laden foamy macrophages are the histologic hallmark.
Mimics RCC on imaging and can be confused with clear-cell carcinoma on frozen section → don't over-treat as cancer.
"Bear's paw" CT appearance; CT diagnoses it preoperatively ~90% of the time.
Associated with Proteus, stones, diabetes; more common in women.
Treatment is nephrectomy + antimicrobials (difficult surgery; perinephric extension common).