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renal masses and cysts


  • it is common to have an incidental finding of cysts on kidneys on a CT KUB or CT abdo scan
  • some should have a renal USS to further elucidate their nature and determine whether the patient should be referred to urology or renal medicine (nephrology)
  • the Bosniak classification system has been created to help address this:
    • Bosniak I or II ⇒ ignore
    • Bosniak IIF ⇒ follow up - perhaps in 6 months
    • Bosniak III or IV ⇒ excise
  • the following imaging features indicate that a cyst is NOT simple:
    • calcification
    • hyperdense / high signal
    • septations
    • multiple locules
    • enhancement
    • nodularity / wall thickening
  • indications for excision are thus one or more of the following:
    • calcification status: enhancement, nodular and wall thickening
    • hyperdens status: poorly defined heterogeneous enhancement; solid on USS;
    • septations status: thick, irregular, nodular enhancement
    • enhancement level: > 15 Hounsfield Units increase of the mass after contrast injection (indicates vascularity)
    • multiloculated (unless due to infection)
    • all nodules other than very small non-enhancing nodules
    • wall thickening (unless due to infection)1)


Bosniak classification

Bosniak type I

  • simple cyst, imperceptible wall, rounded
  • work up : nil
  • % malignant : ~ 0%

Bosniak type II

  • minimally complex
  • a few thin (< 1mm) septa
  • may contain fine calcifications or a small segment of mildly thickened calcification.
  • non-enhancing high-attenuation (due to to proteinaceous or haemorrhagic fluid) renal lesions of less than 3 cm are also included in this category
  • these lesions are generally well marginated.
  • work up : nil
  • % malignant : ~ 0%

Bosniak type IIF

  • minimally complex but requiring follow up.
  • increased number of septa, minimally thickened or enhancing septa or wall
  • thick or nodular calcium may be present
  • hyperdense cyst that is:
    • > 3 cm diameter
    • mostly intrarenal (less than 25% of wall visible);
    • no enhancement
  • work up : needs ultrasound / CT follow up
  • % malignant : ~ 25 %

Bosniak type III

  • indeterminate, thick or multiple septations, mural nodule, hyperdense on CT (see 2F)
  • treatment / work up : partial nephrectomy or RF ablation in elderly / poor surgical risk
  • % malignant : ~ 54%

Bosniak type IV

  • clearly malignant, solid mass with large cystic or necrotic component
  • all the characteristics of category III lesions but also with enhancing soft tissue components independent of but adjacent to the septa
  • treatment: partial / total nephrectomy
  • % malignant : ~100%

DDx of renal cysts

  • 33% of people older than 50 years develop renal cysts but most are simple cysts
  • multicystic dysplastic kidney (MCDK) - developmental
  • genetic:
    • juvenile nephronophthisis (JNPHP)
    • medullary cystic kidney disease (MCKD)
    • glomerulocystic kidney disease (GCKD)
    • Von Hippel-Lindau syndrome (VHLS) - CNS hemangioblastoma (Lindau tumor), bilateral and multicentric retinal angiomas, renal cell carcinomas, pheochromocytomas, islet cell tumors of the pancreas.
    • tuberous sclerosis (TS)
  • acquired:
    • simple cysts
    • acquired cystic renal disease - bilateral
    • medullary sponge kidney (MSK) - usually benign, asymptomatic, congenital disorder often detected in 20-40yr olds, but may cause UTI, renal stones, haematuria and distal renal tubular acidosis type 1. Causes tubular ectasia and dilated collecting ducts rather than large cysts.
    • renal angiomyolipomas
  • malignant:
renal_cysts.txt · Last modified: 2014/04/03 03:58 by

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