Complex Renal Cyst

A complex renal cyst is a cystic renal lesion with features beyond a simple cyst: thickened or irregular wall, multiple or thickened septa, calcification, high density content, or enhancing components.

• It is graded on imaging by categories I to IV, with malignancy risk of 0 percent (category I, simple), 0 percent (category II, thin septa or minimal calcification, hyperdense cysts below 3 cm), about 5 percent (category IIF, more septa, thicker calcification, nonenhancing hyperdense above 3 cm), about 50 percent (category III, thick irregular enhancing septa or wall) and about 90 percent (category IV, enhancing soft tissue nodules).
• Causes: haemorrhagic or infected cyst, cystic renal cell carcinoma (clear cell, multilocular cystic and papillary), mixed epithelial and stromal tumour, hydatid cyst, abscess, multilocular cystic nephroma, arteriovenous malformation, calcified cyst, post-inflammatory scar.

Often asymptomatic and incidental.

• Haematuria, flank pain, palpable mass, fever with infection, weight loss, hypertension, anaemia or polycythaemia with tumours.
• Differential diagnosis: simple cyst, autosomal dominant polycystic kidney disease, renal abscess, hydatid cyst, haematoma, cystic renal carcinoma, metastasis, calyceal diverticulum, tuberculosis, hydronephrosis.
• Complications: rupture, haemorrhage, infection, obstruction, malignant progression and metastasis, renal impairment.

Ultrasound identifies septa, nodules and calcification; contrast-enhanced ultrasound distinguishes avascular from enhancing components.

• Multiphase contrast CT (or magnetic resonance imaging) is the main test: precontrast, corticomedullary and nephrographic phases; enhancement above 15 to 20 Hounsfield units after contrast indicates a vascular component. Magnetic resonance imaging is better for septa and in renal impairment (without gadolinium when eGFR is below 30 mL/min).
• Creatinine, urinalysis and culture, full blood count, echinococcal serology in endemic areas.
• Staging CT chest and abdomen for suspected malignancy; biopsy is seldom used for cystic lesions and risks spillage (reserve for solid nodule).

Supportive: treat symptoms, hydration, blood pressure control, avoid nephrotoxins.

Pharmacological

• Infected cyst: ciprofloxacin 500 mg twice daily (or levofloxacin 750 mg daily) for 4 to 6 weeks; add drainage if no response in 72 hours; ceftriaxone 2 g IV daily for sepsis.
• Hydatid cyst: albendazole 400 mg twice daily for 1 to 3 months in cycles (28 days on, 14 days off, 2 to 3 cycles) before and after surgery or percutaneous drainage.
• Haemorrhagic cyst: observation, analgesia, transfusion if needed.
• Renal cell carcinoma: systemic therapy by oncology for advanced disease.

Procedures

• Category IIF: surveillance imaging with CT or magnetic resonance imaging at 6 months, 12 months, then yearly for 5 years; progression (new septa, enhancement, increase in size) is reclassified as category III.
• Category III: partial nephrectomy (nephron-sparing) or radical nephrectomy, or active surveillance in the elderly or comorbid.
• Category IV: partial or radical nephrectomy (laparoscopic or open); thermal ablation for small tumours in high surgical risk patients.
• Percutaneous aspiration or surgical removal for large symptomatic benign cysts; percutaneous drainage for abscess.
• Hydatid cysts: percutaneous puncture-aspiration-injection-re-aspiration with albendazole cover, or surgery with protoscolicidal agents.

Special groups: elderly and comorbid (surveillance), solitary kidney (nephron sparing), von Hippel-Lindau and tuberous sclerosis (genetic surveillance), pregnancy (ultrasound only).

Red flags: new enhancement, growth, solid nodule, haematuria, weight loss, perinephric spread.

Follow-up: imaging for category IIF cysts at 6 months, 12 months, then yearly for 5 years; renal function yearly; after nephrectomy, chest and abdominal CT at 6 to 12 months then yearly for 3 to 5 years depending on stage.