renal masses and inherited disorders
Cystic renal masses are classified by Bosniak criteria — category drives surveillance vs surgery. Solid enhancing masses >1 cm are malignant until proven otherwise. ADPKD is the most common inherited renal disease; tolvaptan slows progression in rapidly progressing disease. Renal biopsy is the definitive diagnostic tool for unexplained glomerular/tubulointerstitial disease.
cystic renal masses — Bosniak classification
Bosniak v2019 classification (malignancy rates from pooled meta-analysis):
| category | features | malignancy risk | management |
|---|---|---|---|
| I | simple cyst, thin wall, no septa/calcification/enhancement | ~0% | no follow-up |
| II | few thin septa, fine calcification, <3 cm hyperdense cyst, no enhancement | ~9% | no follow-up |
| II-F | minimally thickened septa/wall, thick calcification, ≥3 cm hyperdense | ~26% | surveillance: 6 mo, 12 mo, then yearly |
| III | thick/irregular enhancing walls or septa | ~80% | partial nephrectomy or active surveillance if comorbid |
| IV | solid enhancing nodular component | ~88% | partial nephrectomy |
solid renal masses
- <1 cm: serial surveillance with US or CT
- >1 cm: majority malignant (RCC ~85%); if classic angiomyolipoma (AML) on imaging (macroscopic fat), no oncological workup needed
- Otherwise → staging CT chest/abdomen/pelvis, refer for partial nephrectomy if life expectancy >5 years
Biopsy indications: diagnostic uncertainty (lymphoma, metastasis, inflammatory pseudotumour), pre-ablation confirmation, multiple bilateral masses, or patient unfit for surgery where histology would change management.
Fat-containing renal mass on CT = AML until proven otherwise. Classic AML has no malignant potential → no cancer staging (epithelioid AML is the rare exception — aggressive variant with metastatic potential).
- Sporadic AML: active surveillance if small; embolisation or surgery if >4 cm (haemorrhage risk)
- TSC-associated AML: mTOR inhibitors (everolimus) first-line for growing asymptomatic AML — preserves nephron mass; embolisation/surgery reserved for acute haemorrhage or treatment failure
ADPKD
AD inheritance (PKD1 ~85%, PKD2 ~15%). PKD1 → earlier ESKD (~55 years) vs PKD2 (~75 years).
clinical manifestations
- Flank pain, macroscopic haematuria, nephrolithiasis (uric acid and calcium oxalate)
- Cyst infections (treat with lipophilic antibiotics that penetrate cysts — fluoroquinolones, TMP-SMX; 4–6 week course)
- Early hypertension — RAS activation from cyst expansion; treat aggressively
- Hepatic cysts (most common extrarenal manifestation), pancreatic cysts
- Intracranial berry aneurysms (~12%; screen with TOF-MRA if family history of SAH/aneurysm, unexplained sudden death, or high-risk occupation)
US diagnostic criteria (with positive family history)
| age | cysts required |
|---|---|
| 15–39 | ≥3 total (both kidneys combined) |
| 40–59 | ≥2 per kidney |
| ≥60 | ≥4 per kidney |
Genetic testing if US equivocal, no family history, or young living donor candidate.
tolvaptan
V₂ receptor antagonist — slows cyst growth by reducing cAMP-driven fluid secretion.
Indications: rapidly progressing ADPKD (Mayo classification 1C–1E or eGFR decline ≥3 mL/min/yr) with eGFR >25 mL/min/1.73 m².
- TEMPO 3:4 (NEJM 2012) — slowed kidney volume growth and eGFR decline in early ADPKD (eGFR >60)
- REPRISE (NEJM 2017) — extended benefit to later-stage disease (eGFR 25–65)
Adverse effects: polyuria (up to 6 L/d — mandatory aquaresis), thirst, hypernatraemia, hepatotoxicity (monthly LFTs for 18 months, then q3 months). Hold and recheck within 48–72 h if ALT/AST >2× ULN; do not restart if >3× ULN or symptomatic hepatic injury.
LFT monitoring is mandatory. Contraindicated in liver disease. Patients must maintain unrestricted water access — dehydration risk with forced aquaresis.
renal imaging — key indications
- US — first-line: kidney size, echogenicity, hydronephrosis, cysts
- Non-contrast CT — gold standard for nephrolithiasis
- CT urography + cystoscopy — haematuria workup (>40 years or risk factors)
- Renal Doppler / CTA / MRA — renovascular disease
- MRI — cystic mass characterisation (Bosniak), ADPKD total kidney volume
renal biopsy
indications
- Unexplained nephritic or nephrotic syndrome
- Rapidly declining eGFR without clear cause
- Suspected allograft rejection
- Systemic disease with renal involvement (vasculitis, amyloid, lupus)
preparation
| parameter | target |
|---|---|
| BP | <140–160 mmHg systolic |
| ASA | hold 5 days |
| warfarin | stop, INR <1.5 |
| UFH | hold 6 h |
| LMWH | hold 24 h; restart 48–72 h post-biopsy |
| DOACs | hold 48–72 h (drug-dependent) |
| uraemic bleeding | DDAVP 0.3 µg/kg pre-biopsy |
contraindications
Absolute: active UTI, uncorrected coagulopathy, severe uncontrolled hypertension, uncooperative patient, poorly visualised kidneys on US
Relative: solitary kidney, severe hydronephrosis, small echogenic atrophic kidneys (unlikely to yield diagnostic tissue and unlikely to change management)
traps
- Assuming all renal cysts are simple — enhancing septa or solid components on CT require Bosniak classification and may need surgery. Cysts incidentally found on US may need CT characterisation.
- Screening all ADPKD patients for intracranial aneurysms — screen only with family history of SAH/aneurysm or if the patient is in a high-risk occupation (pilot, diver). Universal screening is not recommended.
- Forgetting cyst infection in ADPKD — standard antibiotics with poor cyst penetration (beta-lactams) often fail. Use fluoroquinolones or TMP-SMX for 4–6 weeks.
- Biopsy of a clearly malignant-appearing mass — won’t change management if headed to nephrectomy. Biopsy is for diagnostic uncertainty (needle-tract seeding risk is very low with coaxial technique but adds no value when surgery is already planned).
related: chronic kidney disease · hypertension · nephrolithiasis · approach to proteinuria and haematuria · glomerular diseases