Benign Renal Tumors
Not every solid renal lesion is cancer — angiomyolipoma and oncocytoma are benign, but the trap is that fat-poor AML and oncocytoma can be indistinguishable from RCC on imaging.
The big picture
The common benign lesions are simple cysts, angiomyolipoma (AML — fat-containing) and oncocytoma. Macroscopic fat on CT effectively diagnoses AML; oncocytoma has no reliable imaging signature and often needs biopsy or excision to distinguish from RCC.
Macroscopic fat means AML; a fat-poor solid enhancing mass is RCC until proven otherwise — and large AMLs need treatment for bleeding risk.
Symptom sorter
Commonest benign lesion.
Diagnostic algorithm
Each step answers one question. Tap to expand.
Treatment ladder
Confirm benignity where possible; treat AML by size and bleeding risk and treat indeterminate solid masses as RCC.
Procedure chooser
- Nephron-sparing surgery for large symptomatic AML or indeterminate masses
Complications
- Retroperitoneal haemorrhage from large AML
- Missed RCC masquerading as benign
- Embolisation post-infarction syndrome
- Surgical loss of nephrons
- Treat large AMLs proactively
- Biopsy/treat indeterminate masses as RCC
- Embolise bleeding AML
- Excise indeterminate lesions
Red flags
Summary tables
Benign renal tumours
| Lesion | Imaging clue | Management |
|---|---|---|
| Simple cyst | Thin, non-enhancing | Reassure |
| AML | Macroscopic fat | Surveil; embolise/operate if large/bleeding |
| Oncocytoma | Central scar (unreliable) | Often biopsy/excise (RCC mimic) |
| Fat-poor AML | No fat, mimics RCC | Biopsy or treat as RCC |
Memory hooks
Macroscopic fat = AML.
No fat + enhancing = RCC until proven otherwise.
Big AML = bleeding risk → treat.
Oncocytoma: benign but a mimic.
Board traps
Fat-poor AML indistinguishable from RCC — do not assume benign.
Large AML with acute flank pain — Wunderlich (retroperitoneal) haemorrhage.
Multiple bilateral AMLs — think tuberous sclerosis.
Clinical cases
A 45-year-old woman has a 6 cm renal lesion with abundant macroscopic fat on CT and intermittent flank pain.
What is the lesion and the main management concern?