Renal Masses

• Solid Renal Mass
  • Non-contrast
    • Macroscopic fat
      • AML
        US: Echogenic (isoechoic to sinus fat, DDx: Fat-containing RCC). Posterior shadowing.
        CT: Avid enhancement of non-fat containing portion. Calcifications rare.
        MR: High T1. Signal drop-out on opposed phase.
      • RCC (fat-containing)
        US: Variably echogenicity. May have hypoechoic rim and cystic changes. Calcifications may be seen.
    • No fat
      • Metastases
        CT: Small <3 cm, do not calcify or invade renal vein, usually confined to cortex though can be more infiltrative compared to RCC (exophytic).
  • Post-Contrast
    • Non-enhancement
      • Hyperdense cyst (proteinaceous/haemorrhagic)
        CT: Homogenous density <70 HU
        MR: High T1, non-restricting
    • Equivocal (10-19 HU)
      • Consider MR for better characterisation
    • Definite (>20 HU)
      • Mild Enhancement
        • Papillary RCC
          MR: T2 hypointensity with mild enhancement
          US: Variably but usually isoechoic to renal cortex
        • Oncocytoma
          CT: Segmental enhancement inversion on delayed phase, often with non-enhancing central scar (DDx: RCC with central necrosis).
          DSA: Spoke-wheel enhancment (DDx: Chromophobe RCC)
        • AML (lipid poor)
          MR: Intracellular fat on opposed-phase (DDx: Clear cell RCC)
        • Metastasis (lung, breast, GIT, melanoma)
      • Vivid Enhancement
        • Clear Cell RCC
          US: Variably but usually isoechoic to renal cortex
          CT: Corticomedullary phase: Isoenhancing to renal cortex (slightly obscured), nephrogenic phase: Hypoenhancing.
          MR: Signal drop on opposed-phase imaging
        • AML (lipid-poor)
        • Oncocytoma

AML = Angiomyolipoma, RCC = Renal Cell Carcinoma, ADPKD = Autosomal Dominant Polycystic Kidney Disease, ARPKD = Autosomal Recessive Polycystic Kidney Disease, MCKD = Multicystic Dysplastic Kidney, NPHP = Nephronopthsis

• Cystic Renal Mass
  • Non-contrast
    • Renomegaly
      • ADPKD
        Hepatic fibrosis present.
      • ARPKD
        Hepatic fibrosis absent.
    • Normal size kidneys
      • MCKD
        US: Clusters of non-communicating cysts.
      • NPHP
  • Post-contrast
    • Renal Cyst¹
      • Bosniak I (simple)
        CT/MR: Thin non-enhancing wall
      • Bosniak II
        CT/MR: Thin non-enhancing wall with few thin septa. Fine calcifications.
      • Bosniak II-F
        CT/MR: Minimal thickening of wall and septa with equivocal enhancement. Irregular calcification.
      • Bosniak III
        CT/MR: Irregularly thick wall and septa with measurable enhancement. Variable calcification.
      • Bosniak IV
        CT/MR: Irregularly thick wall and septa with measurable enhancement. Variable calcification. Enhancing solid nodularity.
    • Cystic RCC (clear cell)²
      CT/MR: Water-attenuation mass with enhancing nodularity, thick wall or septa.
    • Multicystic RCC
      CT/MR: Multilocular variably-sized cystic tumour with enhancing wall and septa, may be asymmetric. Nodularity not seen.
    • Cystic Nephroma
      CT/MR: Septate cystic mass with multiple loculations, hairlike septa, peripheral and curvilinear calcifications, irregular borders, and minimal contrast enhancement
    • Mixed Epithelial and Stromal Tumour
      CT: Bosniak III or IV lesion with septa, curvilinear calcifications, and a delayed enhancing solid component
      MR: T2 dark fibrotic regions.
    • Renal abscess
      CT: Complex cyst with heterogenous density. Irregular, thick enhancing wall. Surrounding fat stranding. Presence of gas highly suggestive.
      MR: Heterogenous diffusion restriction.
      US: Well-defined hypoechoic cortical/corticomedullary lesion, internal echoes, diffusely hypoechoic parenchyma (acute pyelonephritis). Perinephric collection may be seen.

Notes

• Renal lymphoma typically demonstrates sheet-like diffuse infiltration of the perirenal tissues or can appear as multiple low-attenuation focal lesions.

References

1. Wood III, C.G., Stromberg III, L.J., Harmath, C.B., Horowitz, J.M., Feng, C., Hammond, N.A., Casalino, D.D., Goodhartz, L.A., Miller, F.H. and Nikolaidis, P., 2015. CT and MR imaging for evaluation of cystic renal lesions and diseases. Radiographics, 35(1), pp.125-141. ↩︎
2. Freire, M. and Remer, E.M., 2009. Clinical and radiologic features of cystic renal masses. American Journal of Roentgenology, 192(5), pp.1367-1372. ↩︎