An enlarged, non-functioning kidney with a central staghorn stone and dilated calyces replaced by low-attenuation material (bear-paw sign), spreading into the perinephric space — the chronic infection that mimics tumour.
Orient first
- Chronic obstruction (staghorn calculus) with infection replaces parenchyma by lipid-laden macrophages.
- Diffuse form is typical; a focal form mimics renal cell carcinoma.
- Extension into the psoas, bowel or skin (fistula) is common.
Acquire the study
- CT with unenhanced and nephrographic phases; coronal reformats.
The manoeuvre
- Unenhanced series: central staghorn calculus in a contracted pelvis.
- Nephrographic phase: dilated calyces with low-attenuation contents and thin enhancing rims (bear-paw pattern).
- Perinephric extension: fat stranding, collections, psoas abscess.
- Fistula to bowel or skin.
- Contralateral kidney function and stones.
What confirms it
- Enlarged non-functioning kidney with staghorn stone and bear-paw calyceal replacement in a patient with chronic infection.
What licenses you to exclude it
- A kidney without a stone or obstruction makes the diffuse form unlikely.
The classic misread
- Calling focal XGP renal cell carcinoma — histology may be needed.