Radiographic appearance

• Simple cyst (< 20 HU on noncon) - benign
• Complex cyst - use Bosniak criteria
  • Hemorrhagic/hyperdense cyst
  • Cystic RCC
  • Cystic nephroma, mixed epithelial-stromal tumor (MEST) - benign behavior, but radiographically indistinguishable from cystic RCC (or from cystic Wilms in children). Manage with nephron sparing if able in adults. Female predominance.
• Enhancing solid renal mass (> 15 HU subtraction)
  • RCC until proven otherwise
  • Angiomyolipoma (< 10% of renal tumors)
    • Benign
    • A/w tuberous sclerosis (60% of TSc have AML) but only 20% of AML have TSc
      • TSc AMLs (and multifocal AMLs) grow faster - 20%/year vs 5%/year)
    • Can be diagnosed with just imaging (CT/MRI) - HU < -20 = fat; PPV nearing 100%
      • Confusing situations: retroperitoneal liposarcoma; RCC with fat; fat-poor AML (5%; up to 30% in TS)
      • Calcifications rarely a/w AML; likely RCC
    • Usually asymptomatic
    • < 4 cm: surveillance
    • ≥ 4 cm: treat due to increased risk of hemorrhage (Wunderlich syndrome)
      • Selective embolization
        • Preferred if acutely hemorrhaging - surgery in this setting often leads to total nephrectomy
      • Excision (nephron sparing)
      • Ablation - under evaluation
  • Oncocytoma (3-7% of renal tumors)
    • Benign
    • Mahogany/tan grossly; pseudotumor with central stellate scar
    • Histologically similar to chromophobe RCC/eosinophilic RCC (both derived from distal renal tubules)
• Moderately enhancing solid mass
  • Papillary/chromophobe RCC, fat poor AML, sarcoma

Pediatric renal masses

Biopsy

• Non-diagnostic in about 15%
• Overall accuracy ~ 80%
• Complications (pneumothorax, significant perinephric bleeding) < 1%
• RCC unlikely to have needle track seeding - but poorly differentiated upper tract UC can; avoid in infiltrative central masses.
• Don't sample cystic masses - high sampling error; risk of tumor cell spillage

Imaging of renal masses

• US - cheap and no radiation, but operator dependent and low sensitivity for detecting solid masses < 3.5 cm. Helpful for simple cysts; unclear about cystic masses.
  • Follow morphology of cystic mass, not size
  • If not a simple cyst - get CT to follow up
• CT renal mass protocol - mainstay
  • Precontrast (to see if a mass is enhancing)
  • Corticomedullary - can help with arterial anatomy, subtyping RCC (hypervascular = ccRCC, hypovascular = papillary)
  • Nephrographic
  • Excretory (about 5-10 min post injection) - ddx urothelial carcinoma, renal sinus cyst, calyceal diverticulum
  • Enhancement = > 15 HU difference from precontrast. 
• MRI renal mass protocol - problem solving
  • T2W with (fat dark) and without fat sat (fat bright) - fluid sensitive, CSF bright
    • Black on T2 - hemosiderin/hemorrhagic 
  • T1W with fat sat - most similar to CT
    • Precontrast - water only (LAVA flex or Vibe) (kidneys bright, fat dark)
    • In phase - kidneys dark, water and fat look bright together
    • Out of phase - kidneys dark, fat/water cancel (intracellular fat, water-fat interface is dark - India ink artifact). Only microscopic fat drops in out of phase (eg lipid poor AML, or microscopic fat = ccRCC)
    • T1 fat only - kidneys black, fat bright - to look for macroscopic fat
    • Dynamic postcontrast
  • Subtractions - postcon T1W - Precon T1W = see only enhancement
    • Enhancement = increase in 15% signal intensity between precontrast and nephrographic phase
  • ccRCC - hypervascular, bright on inphase, dark on out of phase
  • AML - bright on inphase, bright on out of phase (macroscopic fat)

Imaging of a cystic mass

• CT
  • HU < 20 (non-con) = simple cyst
  • HU > 60 (non-con) and homogeneous = most likely hemorrhaghic cyst
  • Heterogeneous = work it up, even if HU < 20. If suspicious, get US - if simple on US, then is hemorrhagic cyst. If not simple on US, get renal mass protocol CT/MRI

Bosniak criteria

• Updated 2019 to emphasize specificity rather than sensitivity
• Based on CT findings
• MR tends to make septa appear thicker (larger voxels) so more likely to upgrade lesion

• Bosniak II also includes: Non-enhancing high attenuation lesion < 3 cm in diameter (eg hemorrhagic or proteinaceous cyst)
• Bosniak IIF also includes: Non-enhancing, hyperdense cyst > 3 cm diameter, mostly intrarenal (< 25% of wall visible)