Benign lesions account for ~20–30% of small renal masses. Simple cysts are ubiquitous and need no follow-up, complex cysts are stratified by the Bosniak system, and the two solid benign tumours that matter most are angiomyolipoma (the only one confidently diagnosed on imaging, by its fat) and oncocytoma (indistinguishable from RCC).
Renal Cysts and the Bosniak Classification
Simple cysts are the commonest benign renal lesion (a third of people >60), with risk factors of increasing age, male sex, hypertension, and renal insufficiency; acquired cystic disease (e.g. in ESRD) raises RCC risk. Anything that is not a simple cyst needs contrast-enhanced cross-sectional imaging and Bosniak (v2019) classification. Most cystic RCCs are multilocular cystic renal neoplasms of low malignant potential — a better prognosis than solid RCC, with no reported metastases.
| Bosniak | Key features | Malignancy risk | Management |
|---|---|---|---|
| I | Thin (≤2 mm) smooth wall, no septa/calcification | ~2% | No follow-up |
| II | Few (1–3) thin septa, any calcification; hyperdense cysts | ~10% | No follow-up |
| IIF | Minimally thickened (3 mm) wall/septa, or ≥4 thin septa | ~30% | Imaging q6–12 months for year 1, then annually to 5 years |
| III | Thick (≥4 mm) or irregular enhancing wall/septa | ~60% | Excise (active surveillance if ≤2 cm; PN preferred) |
| IV | Enhancing soft-tissue nodule | ~90% | Excise (PN preferred when feasible) |
Biopsy is unhelpful for most Bosniak III cysts (no targetable solid component) but may be considered for a Bosniak IV solid component (>1 cm) if it will change management or precede ablation. Active surveillance suits low-risk or high-surgical-risk patients (intervention triggers: growth >4 cm, >0.5 cm/yr, metastasis, or patient choice). Thermal ablation is an option for Bosniak III/IV cysts ≤3 cm in poor surgical candidates.
Oncocytoma
Arises from the collecting duct / distal tubule (like chromophobe RCC) and shows abundant mitochondria; the classic spoke-wheel angiographic pattern and central stellate scar have poor predictive value, so it is radiographically indistinguishable from RCC. Biopsy and even frozen section are unreliable (hard to separate from chromophobe RCC). Management is observation, thermal ablation, or surgery — partial nephrectomy is preferred given its benign nature.
Angiomyolipoma
Composed of blood vessels, smooth muscle, and fat; HMB-45 positive; predominantly in women (hormonal influence). Most are sporadic, but 20–30% occur in tuberous sclerosis (and 50% of TSC patients develop AMLs, often multifocal). It is the only benign renal tumour confidently diagnosed on imaging — macroscopic fat (≤−20 HU) is the landmark (calcification with fat instead suggests RCC; ~5% are fat-poor and mimic RCC, where a T2 fat-suppressed MRI helps). The major complication is spontaneous retroperitoneal haemorrhage (Wunderlich syndrome) — AML is its commonest cause — with risk rising with size, aneurysm formation, and pregnancy.
- Asymptomatic <4 cm — observe (imaging at 6–12 months to gauge growth; sporadic AMLs grow ~5%/yr, TSC/multifocal ~20%/yr).
- Symptomatic or >4 cm — intervene (also proactively in women of childbearing age or those with poor access to emergency care): options are partial nephrectomy, selective embolization, thermal ablation, or sirolimus.
Other Benign Tumours
- Papillary adenoma — ≤5 mm, a probable papillary-RCC precursor; an autopsy finding.
- Metanephric adenoma — rare, female 2:1, WT1-positive, polycythaemia in 10%; usually excised for concern of malignancy.
- Cystic nephroma — Bosniak II–III, female 8:1, bimodal age distribution; indistinguishable from cystic RCC (or cystic Wilms in children) → radical nephrectomy in children, partial in adults.
- Mixed epithelial and stromal tumour (MEST) — rare, perimenopausal women on hormone therapy; Bosniak III–IV, usually surgical.
- Juxtaglomerular cell tumour (reninoma) — renin hypersecretion causing hypertension with hypokalaemia, plus polydipsia, polyuria, myalgia, and headache.
Self-Test
1. Describe the Bosniak classification and its malignancy risk. I (simple, ~2%), II (few thin septa/calcification, ~10%), IIF (minimally thick wall/septa or ≥4 septa, ~30%), III (thick enhancing wall/septa, ~60%), IV (enhancing soft-tissue nodule, ~90%).
2. How is each Bosniak class managed? I/II — no follow-up; IIF — surveillance imaging (q6–12 months in year 1, then annually to 5 years); III/IV — excision (active surveillance or ablation in selected cases).
3. Which benign renal tumour shows abundant mitochondria, and where does it arise? Oncocytoma — from the collecting duct / distal tubule.
4. What imaging finding and stain are diagnostic of angiomyolipoma? Macroscopic fat (≤−20 HU) on CT; HMB-45 positivity on pathology.
5. When should an angiomyolipoma be treated? Symptoms or size >4 cm — proactively in women of childbearing age or those with limited access to emergency care.
6. How does a juxtaglomerular cell tumour (reninoma) present? Hypertension with hypokalaemia, plus polydipsia, polyuria, myalgia, and headache.